An Ecological Assessment of Secondary Poisoning Risk in the Australian Sugarcane Industry

An ecological assessment of secondary poisoning risk in the Australian sugarcane industry

Daniel John Ward

BAppSc (Hons) (QUT)

School of Natural Resources Sciences

A thesis submitted for the degree of Doctor of Philosophy of the Queensland University of Technology.

2008

Abstract

Rodenticide use in agriculture can lead to the secondary poisoning of avian predators. Currently the Australian sugarcane industry has two rodenticides,

Racumin® and Rattoff®, available for in-crop use but, like many agricultural industries, it lacks an ecologically-based method of determining the potential secondary poisoning risk the use of these rodenticides poses to avian predators. The material presented in this thesis addresses this by: a. determining where predator/prey interactions take place in sugar producing

districts; b. quantifying the amount of rodenticide available to avian predators and the

probability of encounter; and c. developing a stochastic model that allows secondary poisoning risk under

various rodenticide application scenarios to be investigated.

Results demonstrate that predator/prey interactions are highly constrained by environmental structure. Rodents used crops that provided high levels of canopy cover and therefore predator protection and poorly utilised open canopy areas. In contrast, raptors over-utilised areas with low canopy cover and low rodent densities, but which provided high accessibility to prey. Given this pattern of habitat use, and that industry baiting protocols preclude rodenticide application in open canopy crops, these results indicate that secondary poisoning can only occur if poisoned rodents leave closed canopy crops and become available for predation in open canopy areas. Results further demonstrate that after in-crop rodenticide application, only a small proportion of rodents available in open areas are poisoned and that these

rodents carry low levels of toxicant. Coupled with the low level of rodenticide use in the sugar industry, the high toxic threshold raptors have to these toxicants and the low probability of encountering poisoned rodents, results indicate that the risk of secondary poisoning events occurring is minimal. A stochastic model was developed to investigate the effect of manipulating factors that might influence secondary poisoning hazard in a sugarcane agro-ecosystem. These simulations further suggest that in all but extreme scenarios, the risk of secondary poisoning is also minimal.

Collectively, these studies demonstrate that secondary poisoning of avian predators associated with the use of the currently available rodenticides in Australian sugar producing districts is minimal. Further, the ecologically-based method of assessing secondary poisoning risk developed in this thesis has broader applications in other agricultural systems where rodenticide use may pose risks to avian predators.

Contents

LIST OF TABLES ...... 7 LIST OF FIGURES ...... 8 CHAPTER 1...... 11 LITERATURE REVIEW ...... 11

INTRODUCTION ...... 11

h e P r o b l e m ...... 1 1

e c o n d a r y p o i s o n i n g h a z a r d t o a v i a n p r e d a t o r s ...... 1 3

i n c p h o s p h i d e ...... 1 6

o u m a t e t r a l y l...... 1 8

c o l o g i c a l l y - b a s e d a s s e s s m e n t o f s e c o n d a r y p o i s o n i n g ...... 1 9 E STUDY AIMS ...... 30 CHAPTER 2...... 33 THE INFLUENCE OF DIFFERENTIAL HABITAT USE BY AVIAN PREDATORS IN ASSESSING SECONDARY POISONING RISK IN AGRICULTURAL SYSTEMS...... 33

INTRODUCTION ...... 33 MATERIALS AND METHODS ...... 36

RESULTS ...... 42

i s t r i b u t i o n o f C r o p H a b i t a t s ...... 4 2 D Rodent Captures ...... 42 Diurnal Raptor Observations ...... 43 DISCUSSION ...... 50 CHAPTER 3...... 56 POTENTIAL SECONDARY POISONING HAZARDS POSED TO AVIAN PREDATORS FROM THE USE OF RACUMIN® (0.037 % COUMATETRALYL) AND RATTOFF® (2.5 % ZINC PHOSPHIDE) RODENTICIDES IN THE AUSTRALIAN SUGARCANE INDUSTRY ...... 56

INTRODUCTION ...... 56

MATERIALS AND METHODS ...... 58

i n c p h o s p h i d e a n a l y s is ...... 6 2

o u m a t e t r a l y l a n a l y s is ...... 6 3

s s e s s m e n t o f s e c o n d a r y p o i s o n i n g h a z a r d ...... 6 5 A

RESULTS ...... 66

o d e n t T r a p p i n g ...... 6 6

o d e n t i c i d e c o n c e n t r a t i o n s ...... 6 8

s s e s s i n g s e c o n d a r y p o i s o n i n g h a z a r d ...... 7 2 A DISCUSSION ...... 75 CHAPTER 4...... 81 FACTORS INFLUENCING SECONDARY POISONING RISK OF AVIAN PREDATORS IN AGRICULTURAL SYSTEMS: A CASE STUDY IN SUGARCANE...... 81

INTRODUCTION ...... 81

MATERIALS AND METHODS ...... 83

o d e l o v e r v i e w ...... 8 3

o d e l i n p u t s ...... 8 4 M District characteristics ...... 84 Rodenticides investigated and the quantities of these available to predators ...... 85 Avian predators modelled ...... 86 Impact of varying LD 50 levels ...... 88 Model description...... 88 5

RESULTS ...... 92 DISCUSSION ...... 103 CHAPTER 5 ...... 107 GENERAL DISCUSSION ...... 107 REFERENCES ...... 118 APPENDIX 1 ...... 137 APPENDIX 2 ...... 139

INSTALLATION ADVICE ...... 139

List of Tables

Table 2.1: Percentage deviation from expected (a) of diurnal and nocturnal raptors observed (b) and the crop canopy class they were associated with on a 25 km transect through an extensive sugarcane growing area in the Herbert River District, North Queensland...... 47

Table 2.2: Percentage deviation from expected (a) of diurnal raptor hunting episodes observed (b) and

a t t u s s o r d i d u s R caught in available crop canopy classes and in open adjacent habitats in the Herbert River District, North Queensland ...... 48 Table 2.3: Percentage deviations from expected (a) by diurnal and nocturnal raptor species associated with crop canopy classes available in the Herbert River District. Figures in parenthesis are the total number of each raptor species associated with crop canopy classes available...... 49 Table 3.1: Study design showing treatments and replicates ...... 60 Table 3.2 : Trap success (rodent captures per 50 trap nights) before and during baiting with 2.5% zinc phosphide (Rattoff®) and 0.0375% coumatetralyl (Racumin®) with different baiting strategies in sugarcane crops. Numbers in parentheses are individual captures/trap nights...... 67 Table 3.3: Number a of Rattoff® baits and Racumin® bait stations which had some portion of bait consumed at the end of each study under different baiting strategies...... 69 Table 3.4: Mean quantity (milligram) of rodenticide residue recovered from tissue from rodents trapped in crops baited with 2.5% zinc phosphide (Rattoff®) and 0.0375% coumatetralyl (Racumin®) and open adjacent areas...... 70 Table 3.5: Mean quantity (milligram) of Coumatetralyl residue recovered from Gut and Liver tissues from rodents trapped in-crop and in open adjacent areas...... 71 Table 3.6: Quantities of rodenticide sold, areas baited and the estimated amount of zinc phosphide and coumatetralyl residue available in open adjacent habitats in the Herbert River District after bait application...... 74 Table 4.1: Avian predator bodyweights and conjectured number of prey consumed per day used to investigate secondary poisoning risk in the Australian sugarcane industry...... 87

List of Figures

Figure 2.1: Map depicting the spatial configuration of remnant vegetation and sugarcane crops where the study area and transect were located ...... 38 Figure 4.1: Screen shot of User Input screen in PoisonV7 model...... 89 Figure 4.2: Screen Shot of Sample Area of PoisonV7 model...... 90 Figure 4.3 (a – d): The effect of increasing crop area under rodenticide on mortality of four predator species. The number of poisoned rodents was held constant at 20%. Data represent quantity of toxicant (mg) as per legend...... 95 Figure 4.4 (a – d): The effect of increasing the number of poisoned rodents available to four predator species. The crop area under poison was held constant at 50%. Data represent quantity of toxicant (mg) modelled as per legend...... 97 Figure 4.5 (a – d): Effect of increasing the area of district under bait on the average numbered days to predator death. Data represent quantity of toxicant (mg) modelled as per legend. Note that birds reaching 30 days remain alive...... 99 Figure 4.6 (a - d): The effect on increasing the number of poisoned rats on the average numbered days to predator death. Data represent quantity of toxicant (mg) modelled as per legend. Note that birds reaching 30 days remain alive...... 101

Figure 4.7 (a and b): The effect of varying LD 50 (10 mg/kg and 60 mg/kg) on a suite of avian predators. Crop area under poison and the number of poisoned rodents was held constant at 50%...... 102

Statement of Original Authorship

The work contained in this thesis has not been previously submitted to meet requirements for an award at this or any other higher education institution. To the best of my knowledge and belief, the thesis contains no material previously published or written by another person except where due reference is made.

Signature: Date:

Acknowledgments

An undertaking like this is never completed in isolation. The success of this project has been contributed to by many people in many different ways. First, thanks to Diana, Nat and

Indigo for accepting the inevitable absences from home (both physical and mental) and for their continued support and understanding.

John Wilson provided guidance and optimism throughout this project. His passion and commitment to research were infectious and without his help this project would not have reached such a positive outcome. Sadly, John passed away before this thesis was finished.

His counsel on all things is sorely missed.

Ian Williamson bravely stepped into the role of supervisor at a very late stage in the project.

His guidance, dedication and good humour is greatly appreciated.

Staff of Herbert BSES and HCPSL provided a lot of support for this project and made fieldwork both easy and enjoyable. Thank you. In particular thanks to Greg Shannon,

Lawrence Di Bella, Mark Poggio (Hebert BSES) and Ron Kerkwyk and Leanne Venturato

(HCPSL). I also thank Brendan Dyer (Tully BSES) who contributed greatly to this project and to the many growers who allowed access to their properties.

This project was undertaken with support provided by the Australian Government and the

Australian Sugar Industry through funding provided by the Sugar Research and

Development Corporation. Additional funding was provided by the Bureau of Sugar

Experiment Stations (BSES). I thank them for their support.

Finally, to those people who helped out along the way but have not been named, thank you.

Chapter 1

Literature Review

Introduction

The Problem

The canefield rat ( Rattus sordidus ) and the grassland melomys ( Melomys burtoni ) are responsible for significant economic damage to sugarcane crops in North

Queensland, Australia. Over the last 15 years, damage caused by these rodents has been limited in most years by the adoption of a rodent integrated pest management strategy (IPM) which focuses on delaying reproduction via the removal of protein- rich in-crop weeds and by minimising recruitment from adjacent donor habitats

(Wilson and Whisson 1993). The success of the ecologically-based management strategy in most years resulted in little interest in the development of strategic- baiting protocols that may be necessary in years when normal management strategies fail to contain damage. Such a situation occurred in the 1999 / 2000 growing season when, due to widespread unfavourable weather patterns, crop maintenance (weed control), adjacent habitat management and harvesting were impeded, leading to a prolonged harvest season and extremely high crop losses due to rodent damage valued at $25 million (Smith et al . 2002).

This event prompted the Australian sugarcane industry to develop strategic-baiting protocols for use within the established rodent (IPM) strategy. Currently, the sugarcane industry has two poison baits available for use, the chronic, multi-dose first generation anticoagulant Racumin® (0.0375 % coumatetralyl, Bayer 11

International) and the acute, single-dose rodenticide Rattoff® (2.5 % zinc phosphide

(Zn 3P2), Animal Control Technologies Australia).

The inclusion of baiting into the rodent IPM raises questions concerning the potential risk of secondary poisoning to avian predators. Secondary poisoning occurs when a predator or scavenger consumes a prey item which has consumed toxic bait, thereby ingesting the toxicant. To date, no formal investigation has been undertaken to quantify the occurrence of secondary poisoning cases or the potential for secondary poisoning incidents to occur in Australian sugar producing areas. One paper has been published (Young and De Lai 1997) which suggested that owl populations were affected by the use of Klerat ® (brodifacoum – a now de-registered rodenticide) to control rodents. While this paper highlighted the issue of secondary poisoning, it provided no detailed information on secondary hazard such as the possibility of an event occurring, and relied on anecdotal knowledge of population densities and changes therein. In short, Young and De Lai (1997) did not provide any evidence-based link between the application of this rodenticide and actual or potential avian predator mortality. Although there are a number of species which inhabit sugar producing areas which could be impacted by secondary poisoning risk, such as various elapid and colubrid snakes and in the more northerly cane growing regions, Australia’s largest carnivore, the spotted-tailed quoll ( Dasyurus maculates gracillis ), this thesis focuses on potential raptor secondary poisoning interactions because this group is highly visible to the broader community and are considered an important environmental issue which requires addressing (e.g. Young and De Lai

1997).

Given the real or perceived risk of secondary poisoning, there is a need to understand the ecological interactions that might lead to secondary poisoning in agricultural systems in general, and in rodent/ raptor interactions in sugarcane in particular. In this chapter I aim to:

1. Review the secondary poisoning literature relating to avian predators

(raptors) and rodent prey generally and with respect to the two rodenticides

(zinc phosphide and coumatetralyl) currently available for use in Australian

sugarcane crops; and

2. Determine the key ecological processes associated with secondary poisoning

and provide justification for the ecological approach used to assess secondary

poisoning hazard in this thesis.

Secondary poisoning hazard to avian predators

Secondary poisoning can be defined as an incident where a predator or scavenger consumes a target or non-target animal which has consumed a toxic bait (after

Colvin et al . 1988). This definition can be extended to include undigested or residual bait occurring in the gastrointestinal tract of prey (e.g. Howald et al. 1999)

(sometimes referred to as primary poisoning) or limited to tissue residues (Sterner and Mauldin 1995). To avoid confusion, in this thesis I take the broader view and include cases where, if a predator or scavenger has ingested a poisoned prey item, then secondary poisoning has occurred regardless of a lack of obvious symptoms

(e.g. haemorrhaging associated with anticoagulant baits).

Broadly, secondary poisoning has been confirmed for a suite of predator and scavenger / prey systems worldwide including mustelids feeding on rats targeted with rodenticides in England (McDonald et al. 1998), raptors preying on insects controlled by anticholinesterase pesticides in the Sahel in Africa (Keith and Bruggers

1998 and references therein), raptors scavenging on coyotes poisoned with anticholinesterase pesticides (Wobeser et al . 2004), raptors scavenging on cattle treated with the anti-inflammatory drug diclofenac in India and Pakistan (Green et al. 2004) and foxes preying on a suite of rodent species in France (Berny et al .

1997). While it is acknowledged that secondary poisoning is a broader problem incorporating a range of predators and their prey, this thesis focuses predominately on secondary poisoning in raptor/rodent systems.

Worldwide, studies have demonstrated in both laboratory (e.g. Bell and Dimmick

1975; Mendenhall and Pank 1980; Newton et al . 1990, 1994; LaVoie 1990; Tkadlec and Rychnovsky 1990 and see Godfrey 1985, Johnston and Fagerstone 1994 and

Joermann 1998 and references therein) and field trials (Hegdal and Colvin 1988;

Eadsforth et al. 1996; Berny et al . 1997; Brown and Lundie-Jenkins 1999; Hosea

2000; Howald et al . 1999; Brown et al. 2002; Stone et al . 2000; Eason et al . 2002 and references therein; Stone et al . 2003) that secondary poisoning can be a real hazard when rodenticides are used to kill or control rodent pest species . For example, in a laboratory experiment, Mendenhall and Pank (1980) fed a suite of owls on rodents killed with different anticoagulant rodenticides. Results demonstrated that owls died of haemorrhage (a classic sign of anticoagulant poisoning) after consuming rodents fed on bromadiolone, brodifacoum and

diphacinone. These results indicate that once ingested, rodenticides occurring within prey can be assimilated by the predator and this assimilation can result in death.

Likewise, field studies have demonstrated that avian predators in natural environments and agricultural ecosystems come into contact with rodenticides used for controlling pest populations. Hegdal and Colvin (1988) reported that brodifacoum (a second generation anticoagulant rodenticide) baits used to control voles ( Microtus spp .) in orchards resulted in the secondary poisoning deaths of 6 of

32 radio-tracked eastern screech-owls ( Otus asio ) exposed to brodifacoum-treated orchards. Recently, Walker et al . (2008) reported that approximately 20% of tawny owls ( Strix aluco ) collected through the Predatory Bird Monitoring Scheme in the

United Kingdom tested positive for anticoagulant rodenticides. In comparable studies (e.g. Newton et al . 1999) between 40% and 70% of raptors have been identified as containing anticoagulant residues. As these studies relate to raptors that feed specifically on small mammals, it is unlikely that they ingested the poison directly (usually presented on cereal-based baits or vegetables) (e.g. Lambert et al.

2007). Therefore, acquisition of rodenticides is likely to have been via secondary poisoning.

In summary, worldwide, secondary poisoning of raptors has been demonstrated to be a real rather than perceived hazard associated with the use of rodenticides to control rodent pests (particularly second generation anticoagulants). In Australia limited secondary poisoning studies have been undertaken with the notable exception of some work in cereal crops (Brown et al . 2002). And unfortunately, no studies have been conducted in Australian sugarcane crops, the topic of this thesis. Therefore, 15

there is currently no assessment available concerning these risks in and around

Australian sugarcane crops. This thesis aims to rectify this knowledge gap.

Currently, the Australian sugarcane industry has two rodenticide baits, zinc phosphide and coumatetralyl, available for in-crop use. These baits differ significantly in mode of action, and hazards associated with their use are described below.

Zinc phosphide

Zinc phosphide, is an acute rodenticide which has been in use since the 1940s (Smith et al . 2002). It is an inorganic compound which acts as a non-specific toxicant in vertebrates. It has been widely studied and has been used in a variety of agricultural systems overseas including wheat, orchards and rice fields (Sterner et al. 1998). In

Australia, it is currently used only in cereal crops and sugarcane. Its mode of action involves hydrolysis of Zn 3P2 to form phosphine (PH 3) in the presence of acids (e.g. stomach acid). After ingestion, death results from reduced electron transport and failed respiration within cells (see Sterner et al. 1998).

The potential for secondary poisoning associated with zinc phosphide has been extensively studied in laboratory trials against a range of avian predators including great-horned owls ( Bubo virginianus ) (Bell and Dimmick 1975) golden eagles

(Aquila chrysaetos ) (Evans et al. 1970) American kestrel ( Falco tinnunculus )

(Tkadlec and Rychnovsky 1990) and spotted eagle owls ( Bubo africanus ) (Siegfried

1968). Results from these studies suggest that zinc phosphide has a low potential for

secondary poisoning in avian predators partly due to the fact that zinc phosphide does not accumulate in muscle tissues (Johnston and Fagerstone 1994). For example,

Bell and Dimmick (1975) fed great-horned owls zinc phosphide poisoned prairie voles ( Microtus ochrogaster ) carcasses over a three day period and closely observed for signs of atypical behavior. Although deaths did not occur, the researchers did observe signs of behavioural changes in experimental owls including changes in roosting patterns and a reluctance to fly. This suggests that although zinc phosphide did not result in mortality, sub-lethal effects did occur. In a similar experiment,

Tkadlec and Rychnovsky (1990) fed kestrels on zinc phosphide poisoned common vole (Microtus arvalis ) over a three day period. All three kestrels survived the experiment and did not exhibit any signs of toxicosis. Interestingly, these kestrels did not consume the gastrointestinal tracts of poisoned voles where approximately 97% of zinc phosphide remained post-mortem.

Based on an extensive literature review, Johnston and Fagerstone (1994) offer four observations / reasons for the low hazard associated with zinc phosphide. These are:

1. Mammalian predators appear less susceptible to zinc phosphide than other

species;

2. Zinc phosphide has a strong emetic action which reduces secondary

poisoning risk to mammals, birds and reptiles;

3. Zinc phosphide decomposes quickly in the gastrointestinal tract; and

4. When given a choice, most animals refuse to consume gastrointestinal tracts

of poisoned animals.

Coumatetralyl

In stark contrast to zinc phosphide, few studies have been undertaken to evaluate the secondary poisoning hazard associated with coumatetralyl. One possible reason for this is that coumatetralyl is not registered for use in the United States of America where many secondary poisoning examinations are conducted.

Coumatetralyl is a first generation multi-dose anticoagulant rodenticide which has been in use as a rodenticide since the 1950s (O’Connor et al. 2003). The mode of action of coumatetralyl is to interfere with the Vitamin K cycle and prevent the formation of blood-clotting factors. The predominant difference between first generation anticoagulants and the more potent second generation rodenticides is that second generation anticoagulants have a greater affinity for binding sites in the liver of poisoned vertebrates and hence greater accumulation and persistence within the animal leading to a higher secondary poisoning potential (Parmar et al. 1987;

Atterby et al . 2005).

Of the few secondary poisoning studies conducted using coumatetralyl, results suggest that hazard is low (Joermann 1998; Burn et al. 2002). For example, Fisher et al . (2003) fed captive barn owls ( Tyto alba ) one coumatetralyl poisoned rodent per day over six successive days and monitored their behavior over the following one month period. The researchers observed no visible effects associated with the ingestion of coumatetralyl and all owls appeared healthy at the end of the trial.

Similarly, O’Connor et al. (2003) fed weka ( Gallirallus australis – a common and endemic New Zealand rail) on coumatetralyl poisoned rodents ( Rattus norvegicus )

over a three day period. No weka died during the experiment and no visible indications of toxicosis were detected. There are no field-based studies of the possible secondary poisoning risk of coumatetralyl.

In summary, the literature suggests that the use of rodenticides can result in secondary poisoning events in avian predators. In Australia, limited secondary poisoning studies have been undertaken and, no assessment has been undertaken in

Australian sugarcane crops. Currently, two rodenticides, zinc phosphide and coumatetralyl, are available for use in Australian sugarcane crops and research has suggested that both of these have a low toxicity to avian predators. Although toxicity is a significant component of hazard, as Moore (1966) stated, pharmacological susceptibility does not necessarily indicate ecological susceptibility. As secondary poisoning results from a foraging interaction between a predator and its prey, it is important to examine the ecological aspects of this interaction.

Ecologically-based assessment of secondary poisoning

A suite of methods have traditionally been employed in order to investigate field occurrences of secondary poisoning. These techniques include carcass searches

(Berny et al. 1997; Brown et al . 2002; Brown and Lundie-Jenkins 1999; Lambert et al. 2007; Walker et al. 2008), indirect (Hegdal et al . 1986) and direct counts (Hegdal and Gatz 1976; Brown et al . 2002) of individuals of species present before and after rodenticide treatment, and monitoring of nest and den sites (Hegdal et al . 1986;

Hegdal and Blaskiewicz 1984). A number of criticisms have been levelled at these techniques. For example, carcass searches have been demonstrated to be unreliable

(Stutzenbaker et al. 1986), with carcasses generally difficult to find in field conditions (e.g. Colvin et al . 1988) and, if raptors are the focus of study, searching must be conducted over large areas given the mobility of these species (Brown et al .

2002). While criticisms such as these call into question the technical problems related to these techniques, one of the major problems associated with using these techniques is that they provide little information on the reasons why the secondary poisoning event occurred.

To illustrate, a number of studies have assessed secondary poisoning by analysing specimens submitted to laboratories as part of wildlife monitoring networks (e.g.

Berny et al . 1997; Walker et al. 2008). Although the analysis of submitted carcasses may provide evidence that rodenticide has been ingested and perhaps the magnitude of exposure, they provide no information about what factors lead to the incident occurring. In contrast, an ecological approach to studying secondary poisoning aims to resolve the critical factors that lead to its occurrence. For example, Colvin et al .

(1988) advocated an ecological approach to assessing secondary poisoning hazard and highlighted the need to investigate non-target species’ use of habitats and their foraging behaviour in explaining any changes in non-target populations. Colvin et al .

(1988) provided two examples demonstrating the utility of this approach. The first study evaluated the potential hazard to barn owls ( Tyto alba ) from the use of brodifacoum baits (a second generation anticoagulant rodenticide) to control rats

(Rattus norvegicus ) and house mice ( Mus musculus ) in and around buildings on farmsteads (see Hegdal and Blaskiewicz 1984).

In their study, radiotelemetry demonstrated that although owls roosted and nested on farmlands and that some farms had substantial populations of rats, owls spent little time hunting on farmsteads and concentrated foraging efforts in fields and marshes which had high populations of the meadow vole ( Microtus pennsylvanicus ), their dominant prey. The researchers concluded that brodifacoum bait (50 parts per million (ppm)) had a low potential secondary poisoning hazard, because rarely were predators recorded as feeding in a habitat or on a rodent species targeted for control.

In contrast to the first study, Hegdal and Colvin (1988) reported that brodifacoum

(10 ppm) was the most probable cause of death in six radio-tracked eastern screech- owls ( Otus asio ) after rodenticide was used to control voles ( Microtus spp .) in orchards. The study concluded that secondary poisoning can be substantial when the rodent species and habitat targeted for rodenticide baiting are also a foraging habitat for predators.

These studies also provide a neat illustration of the pharmacological / ecological susceptibility described by Moore (1966). For example, in the first study, the preparation of brodifacoum applied was five times the strength (50 ppm) than that of the second study (10 ppm). Simply focussing on toxicant strength (or pharmacological susceptibility) would suggest that the former study would pose a greater hazard to avian predators than would the latter. However, an assessment of the ecological susceptibility demonstrates that raptors in the second study were at significantly greater risk of secondary poisoning.

These studies have demonstrated that to adequately understand secondary poisoning hazard in the field, particularly between mobile predators such as raptors and their prey, a solid understanding of those factors that influence risk, such as habitat use and foraging strategies, is fundamental. Therefore, in order to make any assessment of hazard in Australian sugar producing areas, a review of raptor foraging must be undertaken.

In Australian sugarcane producing areas, little research has been conducted into the foraging habits of the major avian predators which include a suite of diurnal raptors such as black kites ( Milvus migrans ), brahminy kites ( Haliastur indus ) and black- shouldered kites ( Elanus notatus ) all of which are recognised small mammal predators (e.g. Marchant and Higgins 1990). Owls are also represented with populations of barn owls ( Tyto alba ), eastern grass owls ( Tyto capensis ) and barking owls ( Ninox connivens ) identified. As with diurnal raptors, these owls are also recognised small mammal predators. Although limited information is available on avian foraging in sugar producing systems, there is a wealth of literature examining avian foraging behaviours, particularly in patchy habitats such as agricultural systems.

In many natural environments, resources are patchily distributed. As a consequence so too is the prey of a forager. The rise of optimal foraging theory in the 1960’s and

1970’s (Owens 2006) saw the development of models that might explain decisions made by foraging individuals in relation to this patchy distribution of resources.

While optimality theory has its proponents (Maynard Smith 1978; Stephens and

Krebs 1986; Stearns and Schmid-Hempel 1987), and its critics (Gould and Lewontin 22

1979; Lewontin 1983; Gray 1987; Pierce and Ollason 1987), simple adaptive optimality models have proved accurate in describing complex behaviours (Owens

2006), and their use in behavioural research has provided a solid framework on which to base empirical studies (Taborsky 2008).

One important model that examined foraging behaviour in relation to prey patchiness was developed by Charnov (1976). This model aimed to determine how long a forager should stay in an area before leaving to search another for prey. To forage effectively, predators must assess this patchy distribution to maximise energy gain and minimise energy expenditure on travel time between patches. This model, known as the marginal value theorem (see Nonacs 2001; Taborsky 2008), proposed that a forager would allocate time in a patch until the instantaneous (marginal) rate of net energy gain decreases to a level equal to the overall rate of the group of available patches.

An example of the marginal value theorem was provided by Pyke (1978) who studied the behaviour of hummingbirds ( Selosphorus sp .) foraging for nectar at flower inflorescences. Each individual inflorescence was considered a unique patch.

The aim of the study was to determine the point at which an individual should leave a patch to exploit another. According to the marginal value theorem, individuals should remain in a patch if, in their estimate, the nectar concentration of the next inflorescence is not less than the overall nectar concentration in the current foraging area. A series of departure times was developed which predicted the probability that individuals would leave an inflorescence of a certain size after a specific number of flowers had been sampled. The foraging behaviour of the hummingbirds conformed 23

to that predicted by the marginal value theorem and hummingbirds were found to be allocating foraging efforts to maximise net food intake.

The marginal value theorem provides an example of a fixed rate mechanism of foraging. Other mechanisms have been proposed such as fixed time foraging where a predator leaves (or gives-up in) a patch after a fixed amount of foraging time and the fixed number mechanism where a predator departs a patch after a number of prey are captured. The choice of which patch departure strategy to use appears dependent on the distribution of prey. For example, a fixed time mechanism has been reported as the best strategy to use when prey distribution varies greatly among patches while a fixed number strategy however appears best when prey distribution is similar among patches (Iwasa et al. 1981). This implies that a foraging animal has some knowledge of the level of resources in a habitat as an animal which does not have this knowledge would leave a patch before resource depletion occurs. These foraging models demonstrate that predators respond to patchy distributions of prey abundance or density. These models however do not account for one well-researched and demonstrated theme in raptor foraging, which is the compounding influence of environmental (or habitat) heterogeneity.

Typically, foraging models have been considered primarily organism-organism interactions. However, resource heterogeneity is a compounding factor in predatory interactions as the efficiency of a predator is not merely dependent on the characteristics of the predator or the prey, but is also influenced by the structure of the environment (e.g. Eklov and Diehl 1994). Therefore, attempts to relate predator behaviour simply to the density of prey without consideration of environmental or 24

habitat heterogeneity are simplistic as prey may be differentially available to predators in different patches and prey availability may not be proportional to prey density.

For example, Bechard (1982) investigated the effect of vegetative cover on foraging site selection by Swainson’s hawk ( Buteo swainsoni ). Using radiotelemetry, hawk foraging site selection was tracked in areas containing varying quantities of cultivated (high plant cover areas) and non-cultivated (predominately low plant cover habitats) land. Results demonstrated that while cultivated fields were the most abundant habitat type available and contained the greatest proportion of prey, they were not utilised for hunting by B. swainsoni until harvest had reduced crop canopy cover. After harvest, these fields were hunted heavily. This study demonstrated that vegetative cover was a more important mediating factor in foraging site selection than was the abundance of the prey item, probably because canopy cover offered better concealment for prey and limited the foraging efficiency of the predator.

Therefore, prey accessibility or availability was the factor which limited the predatory use of these habitats. It further demonstrated that predator / prey models which directly link predator foraging decisions with prey density are overly simplistic. This effect of canopy cover on raptor foraging has been consistently described in the literature for a suite of raptors including Bonelli’s eagle (Hieraaetus fasciatus ) (Ontiveros et al. 2005), tawny owls ( Strix aluco ) (Southern and Lowe

1968), ferruginous hawks ( Buteo regalis ) (Wakeley 1978).

In a recent study, Garcia et al . (2006) demonstrated that foraging by the lesser kestrel

(Falco naumanni ) was focussed in crops with less vegetation cover and which was lower in height than would be expected given random habitat utilisation. Although the reason for this habitat use was not investigated, it is likely mediated by reduced access to prey caused by vegetation structure, to obstruction of aerial hunting ability

(after Shrubb 1980) or a combination of both. These results support the argument that prey availability is more important than prey abundance (Drennan and Beier

2003) and that prey availability is a major determinant of habitat quality for a predator (Widen 1994).

In contrast to the current limited understanding of habitat use by predators foraging in sugarcane producing areas, the spatial and temporal dynamics of the canefield rat

(Rattus sordidus ), the main prey in these areas, has been comprehensively researched

(Wilson and Whisson1993; Whisson 1996).

Wilson and Whisson (1993) documented the spatial dynamics of R. sordidus and demonstrated that crop use was strongly correlated with canopy cover. This study also demonstrated that crop colonisation was an annual event that coincides with canopy closure. Prior to colonisation, canopy density is low and few rodents utilise crop habitats at this stage. After colonisation when crop canopy forms, rodent densities increase through the growing season and peak prior to harvest. Harvest removes the structural cover provided by mature cane crops and rodent densities sharply decline, with animals being redistributed from harvested crops into refuge

areas which provide temporal stability. This cycle continues as the crop re-grows and provides sufficient cover for recolonisation in the following season.

This cover dependant pattern of habitat use is consistent with antipredator behaviour which has been associated with a suite of prey taxa (see Lima and Dill 1990 for a review) and particularly for small mammals (Kotler 1991; Kotzageorgis and Mason

1997; Gonnet and Ojeda 1998; Arthur et al . 2004, 2005; Orrock et al . 2004). In essence, these studies demonstrate that the use of cover by prey species provides refuge from predators (e.g. raptors) which forage preferentially in open areas.

Arthur et al. (2005) investigated the effect of predation and habitat structure on the population dynamics of house mice ( Mus domesticus ). Large outdoor enclosures with differing habitat complexities were studied. These enclosures allowed access to free ranging predators including raptors. Results demonstrated that in enclosures with greater complexity, mouse numbers achieved higher densities than in low complexity habitats.

The use of refuges by prey is arguably the best documented result of the influence of environmental structure. Classically, a refuge can be considered as a spatial component of a habitat which provides protection from a predator (see Lima and Dill

1990). Two types of refuge have been proposed, namely a) areas of an environment where it can be demonstrated that predation risk is reduced with no effect on prey fitness and b) areas were predation risk is reduced but in the absence of predators, fitness is also reduced due, for example, to vegetation cover, which while providing

protection from predatory attack, also limits food availability (Hart and Merz 1998).

For example, individual snowshoe hares ( Lepus americanus ) that foraged in open, resource-rich but predation risky habitats were found to be in better body condition than hares foraging in safe but resource poor habitats (Rohner and Krebs 1996). The trade-off generally in the use of refuges is between the availability of food and the risk of predation which must change with the internal state of an animal (Lima and

Dill 1990).

Predation is typically seen as having direct mortality consequences for prey species

(Sih et al. 1985). However, evidence suggests that the indirect or sub-lethal effects predators have on prey behaviour may be more important than direct effects (Kotler

1997; Orrock et al . 2004). Direct effects usually affect only a small proportion of the population (Kuhara et al. 1999) whereas indirect effects can influence the behaviour

(Fraser and Huntingford 1986; Orrock et al . 2004) and morphology of the population at large (Harvell 1990).

On bright, moonlit nights desert rodents favoured foraging under shrubs which offered a high degree of aerial cover, accepting lower foraging returns for increased safety from avian predators (Price et al. 1984). During summer however, when avian predation risk was low, the intensity of moonlight did not affect patch use. Rodents avoided shrubby areas during summer as the risk from snake attack under shrubs was greater than the risk of foraging in the open (Price et al. 1984). The risk of predation therefore has profound effects on the behaviour of foraging individuals.

In summary, the literature demonstrates that secondary poisoning can be a real hazard to avian predators when rodent prey are targeted for rodenticide control.

Given the complexity of the organism/organism and organism/environmental interactions that shape foraging behaviour, it is obvious that any study attempting to assess secondary poisoning hazard within an ecological framework must account for the critical ecological processes that influence predator-prey interactions in heterogenous environments. Therefore, an understanding of the spatial and temporal utilisation by both avian predators and rodent prey within crops and adjacent habitats is essential to understanding the determinants of the predator/prey interactions within the system and will underpin the ecological-basis of secondary poisoning hazard in this system.

Study Aims

This research was undertaken to investigate the perceived problem of secondary poisoning risk to avian predators associated with the use of rodenticides in the

Australian sugar industry. Although rodenticides are a minor component of the rodent integrated pest management strategy, and are used at relatively low levels, there is widespread public concern that the use of poisons can significantly impact raptor populations. At present, only anecdotal evidence of secondary poisoning events occurring in raptor populations in Australian sugarcane producing regions are available. This relates to the use of Klerat® (active constituent: brodifacoum), a now de-registered rodenticide for use in sugarcane.

There are no studies examining the possibility of secondary poisoning for the two rodenticides currently available for use in sugarcane crops (Rattoff® and

Racumin®).While laboratory trials indicate that raptors have a high toxic threshold to these rodenticides, trials conducted under laboratory conditions cannot reflect the complex processes which lead to secondary poisoning events occurring in the field.

Given this, the objective of this thesis is to provide an ecologically-based field assessment of the risk that the use of rodenticides poses to avian predators in

Australian sugar producing areas.

The specific objectives of this research were to:

1. Determine how avian predators and rodent prey use the changing habitat

provided by sugarcane crops;

2. Determine the availability of poisoned rodents to avian predators and

quantify the amount of toxicant available for ingestion; and

3. Develop an exploratory model to enable the industry to assess future

secondary poisoning risk under differing levels of rodenticide application.

The chapters that form the basis of this thesis meet these specific objectives and ultimately demonstrate that when used in accordance with industry baiting protocols, there is limited risk associated with the use of either Rattoff® or Racumin® in the

Australian sugar industry.

Chapter 2 documents the influence of canopy cover on crop use by rodents and raptors. This chapter demonstrates that rodents are closely associated with crops that provide high canopy levels and therefore high levels of avian predator protection while raptors over-utilise open canopy crops that have low rodent densities but offer high levels of prey accessibility. This chapter demonstrates that predatory interactions between avian predators and their prey take place exclusively in open canopy crops and associated open habitats and therefore, should be the primary focus of secondary risk assessment studies.

Chapter 3 investigates the potential for secondary poisoning events to occur by documenting the availability of poisoned rodents to avian predators in open areas and by quantifying their toxicant load. Results demonstrate that while poisoned 31

rodents were trapped in high accessibility, open canopy adjacent habitats, their numbers were low, as was the quantity of toxicant recovered from them. Based on the low probability of encounter and the low quantity of toxicant they contain, results suggest that the risk of secondary poisoning to raptors is minimal when rodenticides are applied following industry protocols.

The fourth Chapter investigates secondary poisoning scenarios through the use of an exploratory model specifically designed and programmed for this study and, based in part on information derived from the preceding chapters. This chapter demonstrates that in all but extreme cases, secondary poisoning is an unlikely event when rodenticides are applied in-crop following industry protocols.

Chapter 2

The influence of differential habitat use by avian predators in assessing secondary poisoning risk in agricultural systems.

Introduction

Evaluating the risk of non-target secondary poisoning events is a major concern when rodenticides are applied to agricultural crops. Many studies infer this risk based on the toxicity of rodenticides in laboratory studies (e.g. Sterner 1996; Sterner et al. 1998; O’Connor et al. 2003) or from observations of non-target population changes conducted before and after field efficacy trials (e.g. Hoque and Olvida 1988;

Brown et al. 2002). Rarely however, are the complex ecological organism/organism and organism/resource interactions that result in secondary poisoning incorporated into these studies. Those that do, demonstrate that understanding these issues are a critical component of understanding hazard.

For example, Hegdal and Blaskiewicz (1984) evaluated the potential hazard to barn owls ( Tyto alba ) from the use of brodifacoum baits (a second generation anticoagulant rodenticide) to control rats ( Rattus norvegicus ) and house mice ( Mus musculus ) in and around buildings on farmsteads. Radiotelemetry demonstrated that although owls roosted and nested on farmlands and that some farms had substantial populations of rats, owls spent little time hunting on farmsteads and concentrated foraging efforts in fields and marshes which had high populations of the meadow vole ( Microtus pennsylvanicus ), their dominant prey. The researchers concluded that, in this study, brodifacoum bait (50 parts per million (ppm)) had a low potential

secondary poisoning hazard, because rarely were predators recorded as feeding in a habitat or on a rodent species targeted for control.

Hegdal and Blaskiewicz (1984) demonstrate that any realistic estimate of secondary poisoning risk must be underpinned by an understanding of how both predators and prey utilise the various components of the agricultural system in question. If this type of interaction is ignored, secondary poisoning risk based solely on toxicity and field efficacy data may be inaccurate. In this chapter I address this issue by considering the organism/resource interactions between avian predators and rodent prey using Australian sugarcane ( Saccharum spp .) producing areas as a model system.

Sugarcane producing districts provide an ideal model system within which to investigate interactions between crop cycle and habitat use because the growth stage of the crop is easily quantified and the spatial and temporal dynamics of the rodent prey are well understood. In addition, observations by the Bureau of Sugar

Experiment Stations suggest that avian predation rarely occurs in the mature crop components of the system and is restricted to open areas such as road side verges, fallow paddocks and crops in early stages of growth. This possible restricted use of mature crop components occurs even though Wilson and Whisson (1993) have demonstrated that rodent densities are highest in the mature crop components.

Sugarcane is grown as an annual crop in Australia. After a district-wide harvest extending from June to December, sugarcane crops grow through a number of easily characterised stages based on canopy cover, and reach maturity in approximately 6 – 34

8 months. Mature crops, standing approximately two metres tall, dominate the district prior to harvest. Harvesting removes all above-ground crop biomass, changing the district from one dominated by closed canopy vegetation to one of vast, open plains. This cropping cycle has major consequences for rodents that colonise canefields.

The main rodent pest in North Queensland sugar producing districts is the canefield rat, Rattus sordidus . Wilson and Whisson (1993) documented the spatial dynamics of R. sordidus and demonstrated that crop use was strongly correlated with canopy cover. They also demonstrated that crop colonisation was an annual event that coincides with canopy closure. After colonisation, rodent densities increase through the growing season and peak prior to harvest. Harvest removes the structural cover provided by mature cane crops and rodent densities sharply decline, with animals being redistributed from harvested crops into refuge areas which provide temporal stability. This cycle continues as the crop re-grows and provides sufficient cover for recolonisation in the following season.

This cover-dependant pattern of habitat use is consistent with antipredator behaviour which has been associated with a suite of prey taxa (see Lima and Dill 1990 for a review) and particularly for small mammals (Kotler et al . 1991; Kotzageorgis and

Mason 1997; Gonnet and Ojeda 1998; Arthur et al . 2004, 2005; Orrock et al . 2004).

In essence, these studies demonstrate that the use of cover by prey species provides refuge from predators (e.g. raptors) which forage preferentially in open areas.

In contrast, limited information is available on how raptors use the various components of the sugarcane system, and how this influences the interactions between rodent prey and avian predators. Further, there are also no general studies of diet for avian predators found in sugar producing areas and therefore limited information which has documented prey preference. The aims of this study were to:

1. Assess how avian predators utilise sugarcane crops through the growth cycle

and identify how this might influence secondary poisoning risk; and

2. Assess the need to incorporate information on ecological interactions into

studies designed to assess secondary poisoning risk.

Materials and Methods

This study was conducted between January 2002 and June 2003 in the Herbert River

District of North Queensland, Australia. Sugarcane is the dominant crop grown in this district and remnant patches of natural vegetation occur throughout the district and along watercourses (Figure 2.1). The Stone River area of the Herbert River

District was chosen primarily because the occurrence of vehicular traffic is low thus providing a safe environment to conduct vehicular-based surveys.

Sugarcane is grown as an annual crop in north Queensland. The effect of the harvest season is to create a mosaic of growth stages throughout the district which have different levels of canopy closure. The growth stages identified in this study ranged from newly emerged plants that provide no canopy cover through to mature crops that provide a very high level of canopy cover. These six growth stages, and the percentage cover they provide, were quantified by taking 30 digital photographs within each of three randomly chosen crops representative of each growth stage on

one occasion during the study. Photographs were taken at random within each crop from a standard height of approximately 1.2 m with the camera facing either skyward or towards the ground depending on the height of the crop. Photographs were then downloaded to a computer and a one hundred point grid overlaid on each photograph. Canopy cover was determined by the frequency that canopy vegetation was recorded on one hundred grid intersection points. This identified 5 five growth stages based on canopy cover from fallow/recently harvested crops (0 % cover), individual plants (5% cover), individual rows (24 % cover), 1.5 m high crops (32 % cover) and >2.0 m crops (84 % cover) (Table 1). The 0% canopy cover class was divided into two categories, a 0% recently harvested class and a 0% bare earth

(fallow) crop.

Figure 2.1: Map depicting the spatial configuration of remnant vegetation and sugarcane crops where the study area and transect were located

This distinction was made because Wilson and Whisson (1993) demonstrated that fallow paddocks were not utilised by R. sordidus while Whisson (1996) demonstrated that recently harvested crops contained low densities of R. sordidus due to the shelter provided by the ‘trash-blanket’ of green sugarcane tops and dry leaf matter left as a ground cover after harvest.

On each of nine sampling occasions each spanning one week, the spatial distribution of canopy cover classes was determined on a 25 km road transect which bisected an area of extensive sugarcane crops. At each 500 m, canopy cover of crops was determined on both sides of the transect. This resulted in 100 observations of canopy cover for each sampling period.

Once the spatial distribution of canopy cover along the transect was determined, three randomly chosen crops in each of the available canopy classes present and their associated adjacent open habitat were trapped to determine use by R. sordidus .

Trapping was conducted over one night with twenty snap traps spaced five metres apart baited with linseed oil soaked cardboard set inside the crop. Twenty snap traps were also set approximately 1.5 m outside the crop in the associated open habitat at five metre spacing. Traps were set late in the afternoon and cleared early the following morning. Individuals captured were identified to species level and the crop canopy class in which they were caught was recorded.

During each sampling period, diurnal raptor observations were conducted along the

25 km transect. The transect was driven at a constant speed not exceeding 30 km hr –1 over three mornings and three afternoons. Morning transects were started 39

approximately 0.5 to 1 hour after dawn while afternoon transects were started approximately 1.5 hours before dusk. Each time a raptor was sighted, the vehicle was stopped and the species, location along the transect, activity (e.g. hunting, flying, perching) and the canopy level associated with the activity was recorded. An electrical power line ran the entire length of the transect. Therefore, each section of the transect had the same potential for perching. A further three mornings and afternoons were devoted to actively searching for hunting raptors as preliminary results returned a low number of hunting observations. On these days, the transect was driven under the same conditions as described above. When a raptor was sighted, the vehicle was stopped and the raptor observed for a five to ten minute period. The observation was only recorded if the raptor engaged in a hunting activity. For the purpose of this study, hunting was defined as an active behaviour such as quartering (repeatedly searching an area while flying, (Debus 1998)), hovering or diving.

Similarly, on each sampling occasion, owl observations were conducted over three nights on a twenty km section of the transect. This shortened transect was used to avoid spotlighting close to farmhouses. The transect was driven at a constant speed not exceeding 20 km hr –1 and was started approximately two to three hours after dusk. Each time an owl was sighted, the vehicle was stopped and the location along the transect, activity (e.g. hunting, flying, perching) and the crop canopy class associated with the activity was recorded. While every effort was made to identify each owl to the species level, it was not always possible. In this situation, identification was made only to the genus level.

As insufficient raptor observations were available to undertake statistical analysis within each sampling period, raptor/crop associations were tallied in each habitat per

2 sampling occasion and analysed over the entire study period using χ Goodness of fit tests.

In addition, an indication of the magnitude of raptor and crop association, both positive (over-utilised) and negative (under-utilised), was determined as the:

% deviation from expected = 100(O – E) / T

Where:

(O) = The observed number of raptors associated with a particular crop canopy class;

(E) = The expected number of raptors which should be associated with this canopy class given that raptors use habitats in proportion to availability; and

(T) = The total number of raptors sighted.

In cases where low numbers of observations precluded statistical analysis, % deviation from expected was used to indicate the trend in raptor/crop association.

This research was conducted under Queensland University of Technology ethics approval (Ref No. QUT 2021A) and Queensland Parks and Wildlife Services scientific research permit No. F1/000373/00/SAA.

Results

Distribution of Crop Habitats

Over the course of this study, not all six crop classes were available during each sampling period (Table 2.1). However, there were generally four of the six crop classes available except in April 2003 (three crop stages) and April and June 2002

(two crop stages) (Table 2.1). The predominant crop class available over the duration of the study was the 85 % canopy cover class and it occurred at greater than 10 % of the district in all but one sampling period (December 2002). Bare earth was also consistently available over the entire sampling period and it represented approximately 14 % of the district in eight of the nine sampling periods. Crops took approximately two months to grow from one crop canopy class to the next (Table

2.1).

Rodent Captures

A total of 42 R. sordidus were trapped in canefields over the course of this study from 900 trap nights (Table 2.2). In 2002, rodent abundance increased through the growing season (January – June 2002), peaked prior to harvest (June 2002, n = 11) and steadily declined to a low of six captures (December 2002) as harvest progressed through the district. No rodents were trapped in 2003 (Table 2.2).

Overall, rodents were more likely to be trapped within sugarcane crops than in open, adjacent habitats (Wilcoxon test: Z = 2.23, df = 9, p = 0.026). Of the 42 crop rodents captured, 36 (7.5 % trap success) were trapped in the 85 % crop canopy class and six rodents (5.0 % trap success) were trapped in 25 % canopy crop canopy class.

Despite being the most intensively trapped component of the system, only 4 rodents

(0.4 % trap success) were caught in open adjacent habitats over the course of the study. Of these, three (0.6 % trap success) were caught adjacent to 85 % crop canopy class and one capture (0.8 % trap success) was caught adjacent to 25 % crop canopy class in October 2002 (Table 2.2).

Diurnal Raptor Observations

A total of 299 observations of non-hunting diurnal raptors were recorded during the study (Table 2.1). As there was no difference between the numbers of non-hunting diurnal raptors observed during morning (n = 149) or afternoon (n = 150) transects, all observation data were pooled for analysis.

The number of diurnal raptor sightings was not constant over the study period with sightings peaking in August 2002 (78 observations) and declining to a low of seven observations during April 2003 (Table 2.1).

Overall, diurnal raptors were more commonly associated with open canopy classes

(0% bare earth, 0% recently harvested and 5% crop canopy classes) than with closed- canopy crops (25%, 35% and 85% crop canopy classes) ( χ2 = 102.8, df = 5, p

< 0.001; based on pooled data). As Table 2.1 shows, results for most sampling periods consistently reflect this pattern of habitat association.

Three activities describe diurnal raptor behaviour observed during the study. Of the

299 non-hunting diurnal raptor observations, 209 (70%) were of raptors perched on

telegraph poles/ power lines, 65 (22%) raptors were perched on the ground and 25

(8%) raptors were observed flying.

Four diurnal raptor species were commonly recorded on the transect. Sightings consisted of 110 nankeen kestrels ( Falco cenchroides ), 90 black kite ( Milvus migrans ), 53 brown falcons ( Falco berigora ), 44 black-shouldered kites ( Elanus axillaris ) and two incidental sightings of a fifth species, the whistling kite ( Haliastur sphenurus ). As Table 2.3 shows, individual species generally conformed to the overall trend of over-utilising open canopy crops and under-utilising closed canopy crops.

A total of 193 diurnal raptors were observed hunting during the study (Table 2.2). Of these observations, 134 (70 %) were over crops and 59 (30 %) were over open adjacent habitats such as roadside verges and farm roads (Table 2.2). As there was minimal difference between the total number of observations of hunting diurnal predators on morning (87 observations) or afternoon (106 observations) counts, hunting data were pooled for analysis.

The number of hunting observations were not constant over the study period (Table

2.2) and no trend was evident between crop cycle and number of hunting observations. Hunting observations were highly variable in 2002 with peaks in both the growing (April, n = 50) and harvest (October, n = 41) seasons and in 2003 with a low of 2 observations in February and a high of 24 observations in June (Table 2.2).

Overall, there was little difference in crop association between hunting raptors and non-hunting raptors with open canopy crops (Bare, Recently harvested and 5 % canopy cover crops) over-utilised and closed canopy crops under-utilised ( χ2 =

561.2, df = 5, p < 0.001) (Table 2.2). Of the 193 hunting episodes recorded, none was recorded over crops with canopies of 35 % canopy cover or greater (Table 2.2).

In general, individual species over-utilised open canopy crops for hunting (Table

2.3) and most species were observed hunting in both crop and adjacent non-crop components of the system. Black kites (crop = 99, adjacent non-crop = 49) and kestrels (crop = 27, adjacent non-crop = 4) were observed hunting in open canopy crops more than in open adjacent habitats while whistling kites (crop = 3, adjacent non-crop = 3), black shouldered kites (crop = 2, adjacent non-crop = 3) and brown falcons (crop = 3, adjacent non-crop = 0) used both habitats similarly.

A total of 189 observations of owls were recorded during the study (Table 2.1).

Sightings were not constant over the sampling period with the number of observations peaking in April 2002 (n = 52) and declining to a low of two in June

2003 (Table 2.1). Overall, owls did not use crops in proportion to availability ( χ2 =

139.8 df = 5, p < 0.001) being more commonly associated with recently harvested crops and 85 % canopy cover crops than with any others (Table 2.1).

The 189 owl observations in this study consisted of 76 masked owls ( Tyto novaehollandiae ), 40 barn owls ( Tyto alba ), 52 Tyto sp. and sightings of 18 barking owls ( Ninox connivens ) and three grass owls ( Tyto capensis ). Individual species

generally conformed to the overall trend of over-utilising open canopy crops and under-utilising closed canopy crops (Tables 2.1 and 2.3).

Three activities describe diurnal raptor behaviour observed during the study. Of the

189 nocturnal raptor observations, 114 (60%) were of raptors perched on ground, 72

(38%) raptors were perched on signs less than three metres above ground height and three (2%) raptors were observed flying.

No hunting episodes were observed for owls throughout this entire study.

Mean percent canopy cover ( ± SE)

Sampling period 0% 0% 5 ± 1% 25 ± 5% 35 ± 5% 85 ± 1% Total Bare Recently Individual Individual 1.5 m > 2.0 m sightings earth harvested Plants Rows Tall Tall Crops Crops February 02 Diurnal 7 (5) -1(0) -24 (10) 18 (7) 22 Nocturnal -16 (0) -1 (0) -69 (0) 86 (9) 9 % of District c 16 0 0 1 69 14 April 02 Diurnal 4 (8) -4 (32) 40 Nocturnal -12 (2) 12 (50) 52 % of District 16 0 0 0 0 84 June 02 Diurnal 6 (12) -6 (41) 53 Nocturnal % of -16 (0) 16 (25) 25 District 16 0 0 0 0 84 August 02 Diurnal -8 (5) 31 (32) 13 (12) -36 (29) 78 Nocturnal -14 (0) 41 (11) -2 (0) -25 (10) 21 % of District 14 11 2 0 0 73 October 02 Diurnal 20 (15) -31 (5) 12 (18) -1 (12) 50 Nocturnal 49 (19) -41 (0) 7 (10) -15 (3) 32 % of District 0 10 41 24 0 25 December 02 Diurnal 3 (3) 25 (7) -27 (16) -1 (0) 26 Nocturnal -1 (2) 41 (10) -43(10) 3 (1) 23 % of District 10 0 2 87 1 0 February 03 Diurnal 54 (10) -2 (0) -2 (0) -50 (5) 15 Nocturnal -13 (0) -2 (0) -2 (0) 17 (16) 16 % of District 13 0 0 2 2 83 April 03 Diurnal 58 (5) -2 (0) -56 (2) 7 Nocturnal -13 (0) -2 (0) 15 (9) 9 % of District 13 0 0 0 2 85 June 03 Diurnal 75 (7) -5 (0) -2 (0) -68 (1) 8 Nocturnal -13 (0) 45 (1) -2 (0) -30 (1) 2 % of District 13 5 2 0 0 80 Total Diurnal 6 (55) 13 (47) 3 (24) -1 (34) -5 (10) -16 (129) 299 Total Nocturnal -10 (4) 14 (31) 0 (10) -2 (20) -8 (1) 6 (123) 189 Total Rodents crop 0 0 0 6 0 36 42 Total Rodents adjnt 0 0 0 1 0 3 4 a Percentage deviation from expected habitat use based on crop canopy class availability i.e. 100(obs- exp)/total raptors b Diurnal raptors observed twice per day (AM and PM) over a three day period. Nocturnal raptors observed over three nights c Percentage of each available crop canopy class within the district

Table 2.2: Percentage deviation from expected (a) of diurnal raptor hunting episodes observed (b) and Rattus sordidus caught in available crop canopy classes and in open adjacent habitats in the Herbert River District, North Queensland

Mean percent canopy cover ( ± SE)

Sampling 0% 0% 5 ± 1% 25 ± 5% 35 ± 5% 85 ± 1% Open Total period Bare Recently Individual Individual 1.5 m > 2.0 m adjacent sightings/ earth harvested Plants Rows Tall Tall habitat captures Crops Crops February 02 Hunting 72 11 (1) -69 (0) -14 (0) 3 11 Rodents (7) 0 2 0 2 % of District c 16 0 0 1 69 14 April 02 Hunting 69 -69 (0) 9 50 Rodents (41) 8 1 9 % of District 16 0 0 0 0 84 June 02 Hunting 84 -84 (0) 6 9 Rodents (3) 11 0 11 % of District 16 0 0 0 0 84 August 02 Hunting -14 89 (24) -2 (0) -73 (0) 1 25 Rodents (0) 0 8 2 10 % of District 14 11 2 0 0 73 October 02 Hunting 65 (21) -16 (7) -24 (0) -25 (0) 13 41 Rodents 0 0 0 7 1 8 % of District 0 10 41 24 0 25 December 02 Hunting -10 10 (2) 1 (14) -1 (0) 4 20 Rodents (0) 6 0 6 % of District 10 0 2 87 1 0 February 03 Hunting 87 -2 (0) -2 (0) -83 (0) 1 2 Rodents (1) 0 0 0 % of District 13 0 0 2 2 83 April 03 Hunting 85 -85 (0) 10 11 Rodents (1) 0 0 0 % of District 13 0 0 0 2 85 June 03 Hunting -5 (1) 79 (10) 6 (1) -80 (0) 12 24 Rodents 0 0 0 % of District 13 5 2 0 0 80 Total Hu nting 28 (54) 38 (55) 2 (10) -1 (15) -8 (0) -59 (0) 59 193 Total Rodents 0 0 0 6 0 36 4 46 Total Traps 0 180 60 120 60 480 900 1800 a Percentage deviation from expected habitat use based on crop canopy class availability i.e. 100(obs- exp)/total raptors b Diurnal raptors observed twice per day (AM and PM) over a three day period. c Percentage of each available crop canopy class within the district

Table 2.3: Percentage deviations from expected (a) by diurnal and nocturnal raptor species associated with crop canopy classes available in the Herbert River District. Figures in parenthesis are the total number of each raptor species associated with crop canopy classes available. Mean percent canopy cover ( ± SE)

0 % 5 ± 1 % 25 ± 5% 35 ± 5% 85 ± 1 % Total Bare earth Individual Individual 1.5 m > 2.0 m sightings Plants Rows Tall Crops Tall Crops a) Non -Hunting diurnal Raptors

Nankeen kestrel ( Falco cenchroides ) 12 (27) 8 (12) 5 (11) -2 (12) 1 (10) -24 (38) 110 Black kite ( Milvus migrans ) 1 (12) 27 (27) 4 (8) 1 (12) -9 (0) -24 (31) 90 Brown falcon ( Falco berigora ) 3 (8) 10 (7) -1 (1) -7 (3) -8 (0) 5 (34) 53 Black shouldered kite ( Elanus axillaris ) 6 (8) -1 (1) 4 (4) -1 (5) -9 (0) 1 (26) 44 Whistling kite ( Haliastur sphenurus ) 0 0 0 2 0 0 2 Total 6 (55) 13 (47) 3 (24) -1 (34) -5 (10) -16 (129) 299 b) Hunting diurnal Raptors Nankeen kestrel ( Falco cenchroides ) 47 (16) 12 (4) -5 (0) 13 (7) -8 (0) -59 (0) 27 Black kite ( Milvus migrans ) 21 (33) 48 (50) 4 (9) -6 (7) -8 (0) -59 (0) 99 Brown falcon ( Falco berigora ) 1 1 0 1 0 0 3 Black shouldered kite ( Elanus axillaris ) 2 0 0 0 0 0 2 Whistling kite ( Haliastur sphenurus ) 2 0 1 0 0 0 3 Total 28 (54) 38 (55) 2 (10) -1 (15) -8 (0) -59 (0) 134 c) Nocturnal Raptors Masked Owl ( Tyto novaehollandiae ) -12 (0) 9 (9) 0 (4) -1 (9) -8 (0) 12 (54) 76 Unknown Tyto sp. -8 (2) 28 (16) 2 (4) -5 (4) -6 (1) -11 (24) 51 Barn Owl ( Tyto alba ) -10 (1) 2 (2) -3 (1) 2 (6) -8 (0) 17 (30) 40 Barking Owl ( Ninox connivens ) -7 (1) 8 (2) -5 (0) -7 (1) -8 (0) 19 (14) 18 Grass Owl ( Tyto capensis ) -12 (0) 47 (2) 20 (1) -13 (0) -8 (0) -34 (1) 4 Total -10 (4) 14 (31) 0 (10) -2 (20) -8 (1) 6 (123) 189 a Percentage deviation from expected habitat use based on crop canopy class availability i.e. 100(obs-exp)/total raptors 49

Discussion

The differential use by rodent prey and avian predators of components of the agricultural system that varied in crop canopy cover highlights the need to incorporate ecological interactions into assessments of secondary poisoning risk. In the event that each component of the system does not contribute equally to secondary poisoning risk, extrapolation of laboratory toxicology results will lead to erroneous assessment of risk.

Although rodent captures in this study were low, they conform to the observations made by Wilson and Whisson (1993) in their extensive rodent trapping study in sugarcane crops and demonstrate that rodents are more frequently trapped in closed canopy crops than in other available classes. This data also conforms to a solid body of literature which documents the use of high canopy cover areas by prey animals as an anti-predator tactic (e.g. Lima and Dill 1990).

Results from this study demonstrate that diurnal raptors and nocturnal raptors utilised available habitats non-randomly. Diurnal raptors under-utilised mature, closed canopy sugarcane crops even though they contained relatively high rodent prey densities. These raptors focussed hunting efforts in open canopy crops or in open canopy areas adjacent to open canopy crops which are poorly utilised by rodent prey. In contrast, nocturnal raptors were associated more with open habitats adjacent to closed canopy crops throughout most of the growing season.

This non-random use of available habitat is consistent with other studies that have investigated avian predator foraging behaviour in agro-ecosystems (e.g. Simmons

2000; Williams et al . 2000; Sissons et al . 2001; Tome and Valkana 2001; Thirgood et al . 2002; Aschwanden et al . 2005; Garcia et al . 2006). These observations generally support the arguments that prey availability is more important than prey abundance (Drennan and Beier 2003), and that prey availability is a major determinant of habitat quality for a predator (Widen 1994). For example, Garcia et al . (2006) demonstrated that the lesser kestrel ( Falco naumanni ) hunted in crops with less vegetation cover and which was lower in height than would be expected given random habitat utilisation. Although the reason for this habitat use was not investigated, it is likely mediated by reduced access to prey caused by vegetation structure, to obstruction of aerial hunting ability (after Shrubb 1980) or a combination of both.

Interestingly, the differing habitat associations of diurnal and nocturnal raptors in the current study was in contrast to that recorded by Aschwanden et al . (2005) who described the use of habitats for hunting by the common kestrel ( Falco tinnunculus ) and long-eared owl ( Asio otus ) in intensively farmed areas in Switzerland. In this study, both kestrels and long-eared owls hunted in similar habitats (freshly mown meadows and artificial grasslands) which had comparatively low densities of their main small mammal prey (the common vole, Microtus spp.) in comparison to habitats which had higher vole densities and higher cover. Explanations for the difference in habitat use in the current study are probably due to a number of behavioural factors including type of prey consumed, timing and sensory mode of

hunting. For example, the diet of kestrels has been reported as containing insects, small birds, reptiles and mice, with mice becoming a major prey item during mouse plagues (Olsen et al . 1979; Marchant and Higgins 1993; Aumann 2001). Further, this species hunts during the day when rodent activity is expected to be low (Marchant and Higgins 1993). In contrast, owls are significant predators of small mammals in

Australia (e.g. Marchant and Higgins 1993) and hunt at night when rodent activity is highest.

In the current study, diurnal raptors and in particular kestrels conform with the observations of Aschwanden et al . (2005) that, as a visually-oriented hunter, common kestrels utilised open areas with low vegetation structure and this habitat use allows prey detection from a greater distance. Diurnal raptors were predominately seen perching at height on telegraph poles adjacent to open canopy crops and hunting over open canopy crops. In contrast, owls tended to use open, adjacent habitats next to closed canopy crops (which contain the highest density of small mammal prey) more than expected, being recorded perching on the ground in roadside verges or on roadside signs approximately 3 m tall. As an aurally-oriented hunter, hunting close to the ground probably provides owls with faster response to prey noise and a potentially a greater hunting success rate (Aschwanden et al . 2005).

The differential use of perch heights by these these groups of raptors could be best described by the relationship between height and prey capture success proposed by

Rice (1983) who reported that prey capture rates by raptors using visual cues increases quadratically with increasing elevation while capture rates by raptors using auditory cues decreases quadratically with increasing elevation. There is always the possibility that the extent of foraging activity of nocturnal raptors within closed 52

canopy-crops has been underestimated because all observations were made along an open transect that ran between crops. However, given the wingspan of these birds and the closed nature of the crop canopy, it seems unlikely that birds were foraging within these closed canopy crops.

In summary, diurnal and nocturnal raptors showed differing patterns of crop association and this is likely due to behavioural differences. On face value, it would appear that secondary poisoning risk is a greater hazard for owls due to timing of hunting and also dietary niche. However, impacts to diurnal predators cannot be ruled out, particularly relating to the availability of poisoned rodents in the system.

For example, studies have shown that under the influence of rodenticides, rodents have demonstrated aberrant changes in behaviour such as moving sluggishly in open areas in daylight suggesting that they could be selectively preyed on by day-hunting raptors (Cox and Smith 1992). This suggests that understanding the availability of poisoned prey to avian predators is the most critical component of secondary poisoning in this system.

As a component of assessing secondary poisoning, the availability of prey in open areas is a key element in determining risk. Currently, rodenticides can only be applied in sugarcane crops once canopy closure has occurred. As raptors are unlikely to access poisoned rodents in this crop component, secondary poisoning can only occur if poisoned rodents leave the crop and become available to avian predators in open canopy areas. Reliance on more common methods of risk assessment which do not consider the significance of this ecological interaction ( sensu Blus and Henny

1997) could lead to an erroneous assessment of secondary poisoning risk. If poisoned animals in the crop do not move into these open areas and, if these areas are populated by non-poisoned animals from the adjacent habitat, secondary poisoning risk would be greatly overestimated.

The advantage of developing an ecologically-based method of secondary poisoning assessment is that it identifies those factors which influence risk in the field and facilitates their investigation. Results of the current study suggest that risk can be assessed using the following hierarchical framework:

1. An assessment of whether or not poisoned rodents leave mature crops and

become available for predation. If poisoned rodents remain within the crop,

risk will be negligible as results demonstrated that this crop component were

under-utilised by avian predators. In this case, applying common techniques

to assess this problem would grossly overestimate potential secondary

poisoning risk. If however, poisoned rodents do become available to avian

predators, the following warrant further investigation;

2. The proportion of poisoned rodents available to avian predators in open

canopy areas. Results of the current study demonstrate that although open

canopy areas are poorly utilised by rodent prey, they are heavily utilised by

avian predators. These open areas therefore are pivotal in the secondary

poisoning process. Obviously, risk will increase as the proportion of

poisoned rodents increase due to the greater access predators have to

poisoned prey. In the event that the proportion of poisoned rodents available

in open canopy areas is significant then;

3. The amount of rodenticide available in poisoned rodents needs to be

quantified. Because the risk of secondary poisoning occurring is dose related,

any evaluation of secondary poisoning risk must incorporate an assessment

of the number of prey consumed and the quantity of rodenticide available in

those prey. Although laboratory-based toxicology studies may provide worst-

case estimates regarding the quantity of rodenticide recovered from study

animals, their validity in accurately reflecting what occurs under field

conditions needs to be examined.

In order to account for the complex nature of secondary poisoning risk, a field-based method of assessment is important to reflect patterns of habitat use and assess field values of toxicant available to predators. The advantage then of this ecologically- based risk assessment method is that it provides a foundation for identifying the processes which influence risk and in doing so highlights those factors which contribute to risk. Further, this field-based technique is readily adaptable to other broad-scale agricultural systems.

Given the differential habitat use that occurs in the Australian sugarcane system, a large-scale field trial that determines the probability of poisoned rodents becoming available to avian predators in open areas and assesses the level of toxicant likely to be ingested by raptors is required. This field trial is the subject of the next chapter .

Chapter 3

Potential secondary poisoning hazards posed to avian predators from the use of Racumin® (0.037 % coumatetralyl) and Rattoff® (2.5 % zinc phosphide) rodenticides in the Australian sugarcane industry

Introduction

The canefield rat ( Rattus sordidus ) and the grassland melomys ( Melomys burtoni ) are responsible for significant economic damage to sugarcane crops in North

Queensland, Australia. Over the last decade, damage caused by these rodents has been limited in most years by the adoption of a rodent integrated pest management strategy (IPM) which focuses on delaying reproduction via the removal of protein- rich in-crop weeds and by minimising recruitment from adjacent donor habitats

(Wilson and Whisson 1993). The success of the ecologically-based management strategy in most years resulted in little interest in the development of strategic- baiting protocols that may be necessary in years when normal management strategies fail to contain damage. Such a situation occurred in the 1999 growing season when, due to widespread unfavourable weather patterns, crop maintenance (weed control), adjacent habitat management and harvesting were impeded, leading to a prolonged harvest season and extremely high crop losses due to rodent damage.

This event prompted the Australian sugarcane industry to develop strategic-baiting protocols for use within the established rodent IPM strategy. Currently, the sugarcane industry has two poison baits available for use, the chronic, multi-dose first generation anticoagulant Racumin® (0.0375 % coumatetralyl, Bayer

International) and the acute, single-dose rodenticide Rattoff® (2.5 % zinc phosphide

(Zn 3P2), Animal Control Technologies Australia). Racumin® wax blocks are registered for use in bait stations (e.g. agricultural piping) and are applied on a 30x30 m grid at the rate of 0.83 kg/ha for moderate rodent activity or 1.2 kg/ha at extreme rodent activity. Wax blocks are checked on a regular basis and replenished until bait take stops, suggesting rodent activity has ceased. Rattoff® is registered under an emergency use permit and single (10 g) sachets are hand-applied on a 10x10 m grid, resulting in an application rate of 1 kg / ha. The IPM strategy dictates that rodenticide is applied only in closed canopy crops (e.g. 2.0m tall closed canopy crops).

The inclusion of baiting into the rodent IPM raises questions concerning the potential risk of secondary poisoning to avian predators. To date, no information is available concerning these risks in and around Australian sugarcane crops.

Elsewhere, laboratory trials suggest that neither zinc phosphide (Bell and Dimmick

1975; Sterner et al. 1998) nor coumatetralyl (Fisher et al. 2003; O'Connor et al.

2003) pose a significant secondary poisoning risk to avian predators. For example,

Johnston and Fagerstone (1994) suggested a minimum LD 50 of >20 mg/kg of Zn 3P2 for raptors. In a laboratory trial, Sterner et al . (1998) fed six voles (Microtus canicaudus ) 2% Zn 3P2 poisoned oats and reported residual carcass levels which ranged from 0.07 ± 0.01 mg to 1.80 ± 0.12 mg Zn 3P2. Based on their data, a 500 g raptor would need to consume at least 142 and 5.6 voles at the lower and upper residue ranges respectively to ingest a lethal dose. Similarly, Fisher et al . (2003) fed coumatetralyl to adult wood rats ( Rattus tiomanicus ) over a three day period and then fed barn owls ( Tyto alba ) one poisoned rodent per day over a six day period. 57

The mean total residue in poisoned rats was 3.86 ± 1.36 mg/kg. At a mean weight of

467.50 ± 25.86 g, each owl consumed 5.89 ± 2.07 mg/kg of coumatetralyl or, approximately 1 mg/kg per day. As Fisher et al . (2003) describe, this level is substantially lower than the eight day LD 50 for hens of 50 mg/kg reported by

Worthing and Hance (1991).

Apart from a small efficacy field trial conducted by Fisher et al. (2003) which documented that coumatetralyl concentrations in R. sordidus were highly variable, limited data are available concerning actual field residue levels for either rodenticide in sugarcane agro-ecosystems. Therefore, it is unknown whether laboratory trial residue data approximate those found in the field and no study has rigorously estimated the secondary poisoning hazard due to the availability of poisoned rodents to avian predators in a field situation. This chapter investigates the potential risk of secondary poisoning to avian predators in Australian sugarcane growing areas by applying baits to crops using the industry IPM protocol, and then, determining the chemical residues in rodents trapped in adjacent open habitats.

Materials and Methods

The study was conducted during late April - early May 2002 in the Herbert River

District near Ingham, North Queensland. This region encompasses the range of habitats encountered throughout sugarcane growing districts including rainforest, open forests, grasslands and extensive sugarcane fields and therefore results should be representative of major sugarcane growing areas. The region also has a high diversity of raptors (Young and De Lai 1997; Chapter 2). Although the industry- approved baiting window occurs between October and March and is aimed at the

early suppression of non-breeding rodents that colonise crops, this study was undertaken to coincide with peak rodent abundance based on current industry monitoring records and historical trapping records (Wilson and Whisson 1993). This therefore provides a “worst case” scenario in terms of the highest number of poisoned rodents available for movement into open adjacent habitats.

Sites that historically supported high rodent densities were selected in each of four separate locations within the Herbert River District. Each site consisted of six canefields (approximately 2 ha each) separated by at least 100 m and all adjacent to a common grassland/open forest habitat.

Three adjacent canefields at each site were allocated randomly to rodenticide treatment and the three canefields within each rodenticide treatment were assigned randomly to a baiting strategy treatment. The design was not fully randomised with respect to rodenticide treatment to minimise potential interaction between rodenticides. If a fully randomised design was used, all animals trapped would have to be analysed for residues of both rodenticides. The study design is summarised in

Table 3.1.

Table 3.1: Study design showing treatments and replicates

Baiting strategy Rodenticide Rattoff® Racumin® Entire crop baited 4 sites 4 sites

Crop baited with rows 0-3 unbaited 4 sites 4 sites

Crop baited with rows 0-6 unbaited 4 sites 4 sites

Crops were randomly assigned to three treatments: entire crop baited, crops baited with rows 0-3 unbaited, and crops baited with rows 0-6 unbaited. Leaving 0-3 and 0-

6 rows unbaited resulted in an unbaited buffer adjacent to the grassland/open forest habitat of approximately 4 m and 8 m respectively. The rationale of incorporating this treatment into the experimental design was to determine if buffer zones would decrease edge effects and affect the likelihood of secondary poisoning occurring.

Baits were laid according to standard industry protocols. Single, 10 g Rattoff® sachets were placed on a 10x10 m grid, an application rate equal to 1 kg/ha. Three

25 g Racumin® wax blocks wired into bait stations were laid on a 30x30 m grid, an application rate of 0.9 kg/ha. Bait availability and persistence was monitored in each canefield every three days throughout the experiment and bait was replenished over the course of the experiment as per industry standard. At each canefield baited with

Rattoff®, five single sachets were pegged to the ground at each corner of the field and inspected for consumption. Similarly, 20 bait stations in each canefield baited with Racumin® were monitored for consumption. Rattoff® baits were not replenished during the experiment as per industry standard.

Prior to bait application, 20 snap traps were placed in each canefield for a period of three nights, resulting in 60 trap nights by each of the 24 canefields. Traps were spaced 5 m apart and baited with linseed oil soaked cardboard. Traps were placed in the half of the crop furthest from the adjacent habitat and the location of trap lines randomised each night to minimise the possible depletion of local foraging populations and avoid influencing potential crop/adjacent habitat movement.

Following bait application, at each site 20 snap traps were set per night in the adjacent open habitat approximately 1.5 m from the edge of each crop. This resulted in a minimum of 200 and a maximum of 280 trap nights in each of the 24 treatments

(Table 3.2). The interior of each crop was trapped once every three-nights for the duration of the experiment with twenty snap traps on each occasion. This resulted in

60 to100 trap nights in each canefield following bait application (Table 3.2). Traps were placed in the half of the crop furthest from the open habitat. Traps were spaced at 5 m intervals and baited with raw linseed oil soaked cardboard.

Each morning, trapped rodents were tagged with a site-specific identifier, wrapped individually in a plastic bag and placed in a cold esky. Immediately following total trap clearance, carcasses were dissected. Organs of interest (see Zinc phosphide analysis and Coumatetralyl analysis sections below) were placed in a sterile vial, labelled and placed in a freezer. The time between removal from the field and dissection did not exceed four hours on any occasion. Carcasses were disposed of using an accredited pathological waste system.

All experiments at a given site were stopped when trapping resulted in no in-crop captures over three successive nights. Adjacent open habitats were trapped with three times the effort of crops, as there is low rodent utilisation of these habitats relative to mature crop (Wilson and Whisson 1993).

Zinc phosphide analysis

As Tkadlec and Rychnovsky (1990) demonstrated that 98 % of zinc phosphide remained in the gastrointestinal tract (90 % in the stomach), therefore only the gastrointestinal tract was examined for zinc phosphide residues in this study. The entire gastrointestine of rodents trapped in zinc phosphide baited crops was removed and tissue samples were cut into fragments up to 1 cm in maximal dimension, weighed and placed in a reaction vessel (250 mL culture bottles (Schott) fitted with a glass side arm (Labglass Pty Ltd) containing an 8 mm diameter silicone septa). To this vessel, 50 mL of 10 % sulphuric acid was added, the bottle sealed and shaken for

3 hours at room temperature. After shaking, the phosphine concentration in the headspace of the reaction vessel was determined by gas chromatography.

Phosphine determination was carried out using a gas chromatograph (Aglient 6890) fitted with a 15 m x 0.53 mm GS-Q column (J&W Scientific) and using helium as the carrier gas. The injection port temperature was 110 °C and the column temperature 80 °C. A Nitrogen Phosphorus detector (NPD), optimized for phosphorus was used to determine phosphine concentrations. The detector temperature was 325 °C, hydrogen flow was 3.0 mL/min, airflow 60.0 mL/min, with

nitrogen used as the makeup gas. The headspace was sampled using a 10 µL gas syringe (Hamilton), with 5 µL injected onto the column.

Standards were prepared by adding know amounts of zinc phosphide to reaction vessels and adding 50 mL of 10% sulphuric acid. The standard curve response was modeled using regression analysis and the concentration of the phosphine in the sample flasks determined from this model.

Tests on known zinc phosphide concentrations in three spiked bovine muscle tissue samples at levels of 771, 826 and 1790 mg/kg gave recoveries of 83.1, 115, and 98% respectively. Mean run-to-run variation over three runs was 4.78 %, and the limit of determination (LOD) for this method was 0.01 mg/kg.

Coumatetralyl analysis

Coumatetralyl analysis was undertaken independently on the stomach and liver of trapped rodents. The decision to analyse both organs was made because, even though

Record and Marsh (1988) demonstrated that the liver was the most significant organ associated with the retention of anticoagulant rodenticides, if predators were to swallow prey whole, any rodenticide residual located in the stomach would contribute to the overall quantity of rodenticide ingested.

The method of analysis was based on the high performance liquid chromatography method of Hunter (1985). Liver and gut samples were finely sliced and placed in a

150 x 25 mm screw cap tube and 30 mL of acetone-chloroform solution (1:1 solution) added, the sample mixed and 0.5 g of anhydrous sodium acetate added as a 63

drying agent, followed by remixing. The vial was sonicated for 10 minutes and the supernatant transferred to a separate 150 x 25 mm screw cap tube. The residue was resuspended in 30 mL of acetone-chloroform solution, the sample mixed and then sonicated for 10 minutes. The supernatant was again transferred to the separate 150 x 25 mm screw cap tube. This process was repeated one more time with all three supernatants combined. The resultant extracts were combined and evaporated to dryness under a nitrogen atmosphere and reconstituted in 1 mL of methanol for analysis.

The sample was analysed using an Alliance 2690 high performance liquid chromatograph (Waters Corporation). Separation was achieved using a Xterra C18

MS 2.1 x 150 mm column with 3.5 um pore size (Waters Corporation) operating at a temperature of 35 oC with detection at 280 nm using a Waters 996 photodiode array detector. The mobile phase was 0.25% acetic acid in water, on-line mixed with

0.25% acetic acid in methanol, with a gradient starting at 32% acetic acid in water reducing to 25% at 10 minutes, and then to 5% at 15 minutes. The limit of determination using this method is 0.02 mg/kg.

Standards were prepared by diluting known amounts of coumatetralyl in methanol.

The standard curve response was modeled using regression analysis and the concentration of the coumatetralyl determined from this model.

Tests on spiked liver samples, gave recoveries of 85.1, 78.8, 80.9 and 86.5 % for four samples spiked at 1 mg/kg and 78.9, 72.8, 86.6, 89.3 and 90.6 % for five samples spiked at 10 mg/kg. Mean run-to-run variation over three runs was 2.78 %. 64

Assessment of secondary poisoning hazard

To assess secondary poisoning risk, two avian predators, the large female masked owl ( Tyto novaehollandiae ), weighing approximately 1260 g, and the small male nankeen kestrel ( Falco cenchroides ), weighing approximately 165 g, were chosen as model species because they are commonly sighted in the district (Chapter 2), are known to hunt small mammals (see Marchant and Higgins 1990 and references therein), and their body weights cover the range for avian predators common around sugarcane growing districts (Chapter 2). These predators should therefore differ vastly in their sensitivity to rodenticide doses.

Zinc phosphide LD 50 ’s reported by Johnston and Fagerstone (1994) were used in this study. At 20 mg/kg, the LD 50 is approximately 25 mg for the 1260 g masked owl and approximately 3 mg for the 165 g kestrel. The eight day LD 50 for coumatetralyl of 50 mg/kg described by Madden (2002) was also used. Therefore the LD 50 for the masked owl is approximately 63 mg per day for eight days, and approximately 8 mg per day for eight days for the kestrel.

Some confusion exists in the literature as to whether the LD 50 of coumatetralyl is based on body weight of the bird as described by Madden (2002) or is based on consumption of feed dosed with 50 mg of coumatetralyl as described by Hermann

(1963). If the dosed feed scenario is accepted, many assumptions are required, but this reduces the number of rodents needed to be consumed to ingest an LD 50 reported in the results by approximately a factor of 10. The confusion surrounding this issue is summarised in Appendix 1.

The above mentioned LD 50 information has been adopted in lieu of published data which estimates the survival probability of specific avian species or toxicants studied. Death is recorded in this model as occurring when an the LD 50 is consumed.

Outputs therefore give a range of mortalities from 50% to 100%. This has the effect of potentially overestimating death rate by up to a factor to two.

This research was conducted under Queensland University of Technology ethics approval (Ref No. QUT 2021A) and Queensland Parks and Wildlife Services scientific research permit No. F1/000373/00/SAA.

Results

Rodent Trapping

A total of 350 rodents were trapped during this study. Rattus sordidus was the main species captured (n = 333) along with incidental captures of Melomys sp. (n = 14) and Rattus rattus (n = 3). Results are analysed using pooled capture data.

Prior to the application of rodenticide, trap success (Table 3.2) was similar in all canefields randomly allocated to rodenticide treatment and baiting strategy (F rodenticide

= 1.63, df = 1, 18, p = 0.22; F baiting strategy = 0.675, df = 2, 18, p = 0.52; F rodenticide*baiting strategy = 0.702, df = 2, 18, p = 0.51). Trap success was also similar in the canefields over the period that bait was available (Table 3.2) regardless of the type of rodenticide applied or baiting strategy used (F rodenticide = 0.379, df = 1, 18, p = 0.55;

Fbaiting strategy = 1.379, df = 2, 18, p = 0.28; F rodenticide*baiting strategy = 0.199, df = 2, 18, p

= 0.82).

Table 3.2 : Trap success (rodent captures per 50 trap nights) before and during baiting with 2.5% zinc phosphide (Rattoff®) and 0.0375% coumatetralyl (Racumin®) with different baiting strategies in sugarcane crops. Numbers in parentheses are individual captures/trap nights. Prior to Bait During Bait Application bPercentage of Captures Application Positive for Rodenticide Residue a Rodenticide Baiting Site Crop Trap Trial Duration Crop Trap Adjacent Habitat Trap Crop Success Adjacent Success Strategy Success (days) Success Success a) Zinc Phosphide 0-3 1 1.7 (2/60) 14 4.5 (9/100) 0.2 (1/280) 44.4 (4/9) 0.0 (0/1) (Rattoff®) 0-3 2 5.0 (6/60) 11 5.0 (8/80) 0.2 (1/220) 50.0 (4/8) 0.0 (0/1) 0-3 3 4.2 (5/60) 13 4.4 (7/80) 0.4 (2/260) 0.0 (0/7) 100.0 (2/2) 0-3 4 4.2 (5/60) 10 0.0 (0/60) 0.0 (0/200) 0.0 (0/0) 0.0 (0/0) 0-6 1 3.4 (4/60) 14 2.5 (5/100) 1.4 (8/280) 40.0 (2/5) 12.5 (1/8) 0-6 2 1.7 (2/60) 11 2.5 (3/60) 0.2 (1/220) 33.3 (1/3) 0.0 (0/1) 0-6 3 4.2 (5/60) 13 11.9 (19/80) 0.4 (2/260) 10.5 (2/19) 0.0 (0/2) 0-6 4 0.9 (1/60) 10 1.9 (3/80) 0.0 (0/200) 33.3 (1/3) 0.0 (0/0) Full 1 5.9 (7/60) 14 1.9 (3/80) 1.1 (6/280) 0.0 (0/3) 16.6 (1/6) Full 2 4.2 (5/60) 11 1.9 (3/80) 0.5 (2/220) 100.0 (3/3) 0.0 (0/2) Full 3 6.7 (8/60) 13 5.5 (11/100) 2.9 (15/260) 27.3 (3/11) 20.0 (3/15) Full 4 0.9 (1/60) 10 2.5 (3/60) 0.3 (1/200) 0.0 (0/3) 100 (1/1) Total 3.5 (51/720) 144 3.9 (74/960) 0.7 (39/2880) 27.0 (20/74) 20.5 (8/39) b) Coumatetralyl (Racumin®) 0-3 1 3.4 (4/60) 14 0.0 (0/100) 0.0 (0/280) 0.0 (0/0) 0.0 (0/0) 0-3 2 3.4 (4/60) 11 5.0 (6/60) 0.0 (0/220) 0.0 (0/6) 0.0 (0/0) 0-3 3 14.2 (17/60) 13 10.0 (16/80) 0.6 (3/260) 12.5 (2/16) 0.0 (0/3) 0-3 4 6.7 (8/60) 10 1.3 (2/80) 1.8 (7/200) 0.0 (0/4) 28.6 (1/7) 0-6 1 1.7 (2/60) 14 1.5 (3/100) 0.5 (3/280) 33.3 (1/3) 66.7 (2/3) 0-6 2 0.9 (1/60) 11 6.9 (11/80) 0.0 (0/220) 18.2 (2/11) 0.0 (0/0) 0-6 3 5.9 (7/60) 13 10.6 (17/80) 0.6 (3/260) 23.5 (4/17) 66.7 (2/3) 0-6 4 10.0 (12/60) 10 10.0 (12/60) 0.0 (0/200) 22.2 (2/9) 0.0 (0/0) Full 1 3.4 (4/60) 14 0.0 (0/80) 0.2 (1/280) 0.0 (0/0) 0.0 (0/1) Full 2 2.5 (3/60) 11 5.0 (8/80) 0.0 (0/220) 25.0 (2/8) 0.0 (0/0) Full 3 4.2 (5/60) 13 5.0 (10/100) 1.0 (5/260) 20.0 (2/10) 60.0 (3/5) Full 4 5.9 (7/60) 10 0.0 (0/60) 1.3 (5/200) 0.0 (0/1) 0.0 (0/5) Total 5.1 (74/720) 144 4.4 (85/960) 0.5 (27/2880) 17.6 (15/85) 29.6(8/27) a0-3 = all crop baited except for rows 1-3 (approx. 4 m) 0-6 = all crop baited except for rows 1-6 (approx. 8 m) Full = all crop baited b Percentage of toxicant positive captures per 100 rodent captures 67

During the period of the trial, a similar number of rodents were captured in open habitats adjacent to canefields baited with Rattoff® and Racumin®, namely 39 (0.7 individuals per 50 traps) and 27 (0.5 individuals per 50 traps) respectively (Table

3.2). Trap success in open habitats was also similar regardless of the rodenticide treatment or baiting strategy applied to adjacent canefields (F rodenticide = 0.224, df = 1,

18, p = 0.64; F baiting strategy = 1.220, df = 2, 18, p = 0.32; F rodenticide*baiting strategy = 0.921, df = 2, 18, p = 0.42). Incrop catch rates were higher than in open adjacent habitats in accordance with Chapter 2.

The similarity between rodent captures both prior to and during baiting in all treatments and sites allows for a direct comparison of chemical residues in rodents between baiting strategies and rodenticides.

Rodenticide concentrations

Applied rodenticides were consumed during the trial. Of the 20 stations monitored in each canefield, an average of 12.6 ± 1.7 Rattoff® sachets and 14.1 ± 1.0 Racumin® bait stations had bait consumed (Table 3.3).

Table 3.3: Number a of Rattoff® baits and Racumin® bait stations which had some portion of bait consumed at the end of each study under different baiting strategies. Rodenticide Site Baiting Strategy Rattoff® Racumin® 1 0-3 9 13

2 0-3 18 10 3 0-3 17 16 4 0-3 3 17 1 0-6 10 12 2 0-6 17 18 3 0-6 17 13 4 0-6 13 15 1 Full 6 6 2 Full 18 16 3 Full 19 19 4 Full 4 14 aTwenty baits/bait stations monitored per canefield.

Residual rodenticide concentrations were detected in 20/74 and 15/85 rodents caught in Rattoff® and Racumin® baited crops respectively (Table 3.4). A total of 16 rodents tested positive for residual concentrations of zinc phosphide, the active ingredient in Rattoff® (n = 8) and coumatetralyl, the active ingredient in Racumin®

(n = 8) in open adjacent habitats (Table 3.4). There was no association between capture location (crop or open habitat) and baiting strategy for rodents testing positive for either rodenticide (G zinc phosphide = 2.59, df = 1, p = 0.11; G coumatetralyl =

0.29, df = 1, p = 0.59). There was also no difference between the total amount of rodenticide recovered from rodents trapped in the crop or open adjacent habitats for

Racumin® baited paddocks (t coumatetralyl = 0.75, df = 21, p = 0.46) (Table 3.4).

However, there was a difference between total residues recovered from rodents caught in crops and adjacent open habitat associated with Rattoff® application (t zinc

phosphide = 3.14, df = 26, p = 0.004). This difference was affected largely by two open habitat-caught rodents which had approximately 1.5 (0.57 mg) and 2.2 (0.69 mg) times the highest concentration detected in crop-caught rodents (0.31 mg) (Table

3.4).

The mean (± SE) quantity of zinc phosphide recovered per rodent in open adjacent habitats was 0.25 ± 0.09 mg. The maximum quantity recovered and the upper 95 %

CI calculated were 0.69 mg and 0.48 mg respectively.

Table 3.4: Mean quantity (milligram) of rodenticide residue recovered from tissue from rodents trapped in crops baited with 2.5% zinc phosphide (Rattoff®) and 0.0375% coumatetralyl (Racumin®) and open adjacent areas.

Rodenticide Baiting Capture n Mean (± SE) Strategy location

Zinc Phosphide (Rattoff®) 0-3 Crop 8 0.06 ± 0.04 0-6 Crop 6 0.02 ± 0.01 Full Crop 6 0.05 ± 0.02

0-3 Adjacent 2 < 0.01 ± 0.0005 0-6 Adjacent 1 0.57 Full Adjacent 5 0.28 ± 0.12

Coumatetralyl (Racumin®) 0-3 Crop 3 < 0. 02 ± 0.01 0-6 Crop 8 0.03 ± 0.01 Full Crop 4 0.21 ± 0.18

0-3 Adjacent 1 0.02 0-6 Adjacent 4 0.02 ± 0.007 Full Adjacent 3 0.03 ± 0.01

Gut and liver residues of anticoagulants such as coumatetralyl indicate respectively recent and historic consumption on the part of the animal. Of the 15 crop-caught rodents that tested positive for coumatetralyl, all had detectable liver concentrations

but only nine had detectable gut concentrations (Table 3.5). Of the eight open habitat-caught rodents that tested positive for coumatetralyl, five recorded both gut and liver concentrations, two had gut but no liver concentrations and one recorded liver but no gut concentrations (Table 3.5). There was no difference between either liver or gut concentrations between crop and adjacent habitat captures (t liver = 0.11, df = 19, p = 0.92; t gut = 0.93, df = 14, p = 0.37).

The mean (± SE) quantity of coumatetralyl recovered per rodent in open adjacent habitats was 0.02 ± 0.005 mg. The maximum quantity recovered and the upper 95 %

CI calculated were 0.05 mg and 0.04 mg respectively.

Table 3.5: Mean quantity (milligram) of Coumatetralyl residue recovered from Gut and Liver tissues from rodents trapped in-crop and in open adjacent areas.

Capture Location Organ Baiting n Mean ± SE Strategy

Crop Gut 0-3 1 0.02 Gut 0-6 5 0.03 ± 0.01 Gut Full 3 0.25 ± 0.24

Liver 0-3 2 0.01 Liver 0-6 9 0.01 ± 0.005 Liver Full 4 0.02 ± 0.008

Adjacent Habitat Gut 0-3 1 0.01 Gut 0-6 3 0.02 ± 0.006 Gut Full 3 0.02 ± 0.009

Liver 0-3 1 0.01 Liver 0-6 3 0.02 ± 0.004 Liver Full 2 0.02 ± 0.004

Assessing secondary poisoning hazard

Three events were deemed necessary for an avian predator to ingest an LD 50 of either rodenticide. First, given that avian predators restrict hunting to areas with low, open canopies in sugarcane growing districts (as per Chapter 2), the predator must find and hunt in an open habitat adjacent to a baited crop. Second, a forager must capture a poisoned rodent and third, one or both of these processes must be repeated until enough toxicant is ingested to reach an LD 50 .

Assuming that poisoned and unpoisoned rodents are equally catchable and that the potential for avian predators to capture poisoned and unpoisoned rodents is equal, the probability of an avian predator ingesting a poisoned rodent is:

P1 * P 2

Where: (P 1) = Area baited / District Area

(P 2) = Number of poisoned rodents caught in open habitats adjacent to poisoned crops / Total number of rodents caught in open habitats adjacent to poisoned crops.

The probability of secondary poisoning events occurring was very low in all four years for which data are available, and is dependent on the quantity of rodenticides sold (Table 3.6). For example in 2001, the year with the largest quantity of bait sold, the probability of ingesting a rodent poisoned with zinc phosphide was 0.01 (Table

3.6).

To consume an LD 50 , a forager must repeat this process until enough zinc phosphide is ingested. Under the IPM, baits are available for approximately 30 days. At a mean quantity of 0.25 mg of zinc phosphide recovered per rodent, the masked owl (LD 50 =

25.2 mg) would need to eat 100 poisoned rodents over a relatively short period of time and the kestrel (LD 50 = 3.3 mg) would need to eat approximately 13 poisoned rodents in a short time period.

Similarly, at a mean quantity of 0.02 mg of coumatetralyl, a masked owl (LD 50 = 63 mg per day over eight days) would need to consume 3150 poisoned rodents per day and a kestrel (LD 50 = 8.3 mg per day over eight days) would have to consume 415 poisoned rodents per day. If the dosed feed scenario (see Appendix 1) is accepted, a masked owl would need to consume 300 rodents while a kestrel would need to consume 40 rodents over an eight day period to achieve this level.

Table 3.6: Quantities of rodenticide sold, areas baited and the estimated amount of zinc phosphide and coumatetralyl residue available in open adjacent habitats in the Herbert River District after bait application. Year District Amount of Maximum Probability of ingesting a Estimated amount of residue in rodents c available in the Area a bait sold a area baited b poisoned rodent district (ha) (kg) (ha) P1 P2 Combined Mean d Worst case e Upper probability 95%CI f Zinc Phosphide (Rattoff®) 2001 58300 2800 2800 0.05 0.2 0.01 85 340 155 (3.4, 25.8) g (13.5, 103.0) (6.2, 45.0) 2002 56820 640 640 0.01 0.2 0.002 20 80 35 (0.8, 6.1) (3.2, 24.2) (1.4, 10.6) 2003 57640 1750 1750 0.03 0.2 0.006 55 220 100 (2.2, 16.7) (8.7, 66.7) (4.0, 30.3) 2004 59160 1480 1480 0.03 0.2 0.006 45 180 80 (1.8, 13.6) (7.1, 55.5) (3.2, 24.2)

Coumatetralyl (Racumin®) 2001 58300 80 100 0.002 0.3 0.0006 0.2 1.0 0.4 (0.003, 0.02) (0.02, 0.12) (0.006, 0.05) 2002 56820 380 460 0.009 0.3 0.003 1.0 5.0 2.0 (0.02, 0.12) (0.08, 0.61) (0.03, 0.2) 2003 57640 110 130 0.002 0.3 0.0006 0.3 1.4 0.6 (0.005, 0.04) (0.02, 0.17) (0.01, 0.07) 2004 59160 90 110 0.002 0.3 0.0006 0.2 1.2 0.5 (0.003, 0.02) (0.02, 0.15) (0.008, 0.06) a Figures supplied by the Herbert Cane Productivity Services Limited (HCPSL) b Information supplied by Bureau of Sugar Experiment Stations (BSES) c Figures determined from actual residues recovered in the current study and recalculated as estimates per kilometre of adjacent open habitat. d Mean estimates (mg / km of baited perimeter) of zinc phosphide = 0.14 and Coumatetralyl = 0.01 e Worst case estimates (mg / km of baited perimeter) of zinc phosphide = 0.57 and Coumatetralyl = 0.05 f Upper 95% confidence limit (mg / km of baited perimeter) of zinc phosphide = 0.26 and Coumateralyl = 0.02 g Figures in parentheses represent the number of LD 50 ’s available to a 1260 g masked owl ( Tyto novaehollandiae ) with an LD 50 of 25.2 mg of zinc phosphide and 63.0 mg of coumatetralyl and a 165 g kestrel ( Falco cenchroides ) with an LD 50 of 3.3 mg of zinc phosphide and 8.3 mg of coumatetralyl based on the estimated quantities of residue in rodents.

Discussion

This study is novel in that it demonstrates that under field conditions live poisoned rodents do become available to avian predators and therefore demonstrates that secondary poisoning associated with the application of rodenticide is possible. Further, the low level of rats caught outside the crop is in accordance with the results presented in Chapter 2. Interestingly, there is no significant difference in the proportion of rodents poisoned in crop and those that occur in adjacent habitats. Although Cox and Smith (1992) suggest that a significant proportion of rodents which ingest rodenticides in cage trials showed behavioural changes which could increase exposure to predators, results from this study do not reflect this result. If Cox and Smith (1992) assertion were correct, a higher proportion of poisoned rodents would be expected in open areas although rodent behaviour around snap traps may not reflect behaviour which may lead to greater vulnerability to predators.

Field residues of zinc phosphide recorded in this study are substantially lower than worst case residue levels reported in rodents in laboratory trials, suggesting that reliance on laboratory trials may overestimate potential risk. Parker and

Hannan-Jones (1999) reported laboratory residue levels ranging from 0.18 – 9.05 mg of available zinc phosphide in wild caught R. sordidus offered a no choice zinc phosphide diet. Although the lower figure fits within the range of residues recorded in the current study, the upper limit is approximately 13 times the highest field residue level recorded (0.69 mg). This represents approximately 0.4 and 3 LD 50 ’s for a masked owl and a kestrel compared to a maximum of 0.02 and

0.2 LD 50 ’s respectively recorded in the current study. Further, Sterner et al . (1998) and Sterner and Mauldin (1995) recovered maximum whole carcass zinc phosphide residues of 1.80 mg ( Microtus canicaudus fed a maximum of 5 SRO oat groats) and 4.95 mg ( Microtus spp . fed zinc phosphide ad libitum), representing approximately 2.5 and 7 times the highest field level recorded in the current study.

Similarly, field residues of coumatetralyl recorded in the current study are again substantially lower than those reported in laboratory trials. Fisher et al . (2003) reported a mean total carcass level of 1.42 mg and 0.41 mg respectively in Rattus norvegicus and R. tiomanicus fed 0.375 % coumatetralyl over three nights without alternative food under laboratory conditions. These figures represent approximately 28 and 8 times the highest residue level recorded in open adjacent habitats (0.05 mg) and are approximately 2 and 0.5 times the highest level recorded in the crop (0.74 mg) respectively.

Fisher et al . (2003) reported levels ranging from 0.02 – 6.51 mg/kg recovered from R. sordidus after field application of coumatetralyl. Converting these figures using the supplied mean weight of 148 g per rodent, this range equates to 0.003 –

0.96 mg per rodent and does not vary significantly from the range of 0.0001 –

0.74 mg recorded in the current study. Results presented here also agree with

Fisher et al . (2003), who reported that gut concentrations of coumatetralyl were variable due to the presence of undigested bait.

In summary, it appears that laboratory studies are likely to overestimate the amount of toxicant available to predators. This may be due to artefacts of the design of these experiments that have the potential to modify the feeding and foraging behaviour of restricted animals. Therefore it is important to undertake field trials to accurately estimate secondary poisoning risk.

This study suggests that the secondary poisoning hazard to avian predators resulting from the application of zinc phosphide and coumatetralyl based rodenticides is minimal if bait is used in accordance with the industry IPM strategy. The low risk is primarily due to the low level of rodenticide use in the district, which in turn is due to the success of the habitat manipulation components of the IPM. Since baiting has been included in the IPM, less than 5 % of the Herbert River District has been baited in each year. The risk is further reduced due to the inaccessibility of poisoned rodents in full canopy cover cane to avian predators (Chapter 2) and, in particular by:

a) The low level of rodenticide use over the district results in a low

probability of a foraging avian predator encountering an open habitat

adjacent to a baited crop;

b) The proportion of poisoned animals moving into adjacent open

habitats is low and therefore the probability of capturing a

poisoned rodent within an open habitat is low; and

c) The low levels of rodenticide residue obtained from poisoned

rodents.

These factors, in conjunction with the low toxicity that both baits present to avian predators suggest that the potential for secondary poisoning events to occur in sugarcane growing areas is remote.

The mechanisms involved in potential secondary poisoning processes are complex and any statement of risk must take account of complicating processes. For example, some areas of complication are a) the effect on predators of sub-lethal doses of toxins, b) the behaviour of the predator in relation to prey consumption c) the behaviour of poisoned prey and d) potential limitations associated with the timing of the current study to coincide with peak rodent densities.

Addressing the later point, this study was timed to coincide with peak rodent abundance to ensure that an adequate number of rodents were captured because previous studies (e.g. Wilson and Whisson 1994) have demonstrated that rodent densities are low during the industry baiting window of October to March.

Undertaking this study outside the industry baiting window may affect results due to a) changes in rodent diet through the growing cycle which means that the uptake of bait could be different dependent on dietary requirements and b) as movement and dispersal of rodents between adjacent habitats and canefields changes over the course of the growing season. Both of these factors could potentially impact on the availability of poisoned rodents to avian predators.

Sub-lethal doses of rodenticides have been reported to alter behaviour and thus the ingestion of non-lethal quantities of bait may indirectly harm predators. For example, Bell and Dimmick (1975) reported behavioural changes in great horned

owls ( Bubo virginianus ) fed poisoned prairie voles ( Microtus ochrogaster ) over a three day period. While no predator deaths occurred during their study, the apathy exhibited by these owls was suggested to potentially prove fatal (Bell and

Dimmick 1975). Given this information and the fact that the industry baiting window (October to March) coincides with the breeding period of avian predators in North Queensland, there is some concern that ingestion of sub lethal doses of rodenticide (see Brakes and Smith 2005 for a review) may disrupt breeding behaviour and, if rodenticides are available during the period that chicks are reared, could negatively impact the next generation. Although the probability of ingesting poisoned rodents is low, further studies would be required to understand this problem.

Another confounding issue relates to feeding behaviour involving poisoned prey.

Bell and Dimmick (1975) reported that feeding behaviour of the gray fox

(Urocyon cinereoargenteus ) was unpredictable in that, on some days entire vole carcasses poisoned with zinc phosphide were consumed, whilst during other feeding sessions only the heads were consumed. Johnston and Fagerstone (1994) also suggested that most animals will refuse to eat the gastro-intestinal tract of zinc phosphide poisoned animals. In contrast, O’Connor et al . (2003) and Fisher et al . (2003) have reported that ferrets ( Mustela furo ), weka ( Grallirallus australis ) and barn owls ( Tyto alba ) ate the entire carcasses of coumatetralyl poisoned rodents, which led to the death of two ferrets but no observable signs of toxicosis in the other experimental animals. This suggests that coumatetralyl poisoned animals may not be as easily distinguished by predators as zinc phosphide poisoned animals and, if available during the period when chicks are

reared, may cause instances of secondary poisoning or have negative sub-lethal impacts on young chicks.

Before a definitive statement on the long-term effects of secondary poisoning in

Australian canefields can be made, further study is required to determine a) if both coumatetralyl and zinc phosphide poisoned rodents are refused by predators and b) the potential impacts on immature (low body weight) predators. The way in which poisoned prey behave also needs examination particularly as it relates to the catchability of poisoned rodents.

The advantage of the field-based technique used in this study is that it develops a broader understanding of the secondary poisoning process by a) providing quantification of the amount of poison available to predators and b) the probability of encountering poisoned areas and rodents. Both of these processes are underpinned by an understanding of how predators and their prey use the system which enables the sugarcane industry to manage this risk through adherence to its bait application protocol. Further, this technique is readily adaptable to other broad-scale agricultural systems (e.g. in the grain growing industry) that share similar cropping patterns and potentially similar predator/prey interactions. Results suggest that if baits are used in accordance with industry standards within the industry IPM framework, there will be minimal secondary poisoning risk posed to avian predators from the use of either rodenticide. The potential for secondary poisoning hazard is further examined via the use of a stochastic computer model in the following chapter.

Chapter 4

Factors influencing secondary poisoning risk of avian predators in agricultural systems: a case study in sugarcane.

Introduction

Assessing secondary poisoning risk in agricultural systems is a complex process because any statement of risk must account for the organism/organism and organism/resource interactions that influence it. To adequately account for these interactions and determine their significance in the secondary poisoning process, an ecologically-based assessment of how predators and prey utilise the various components of the system is required. Rarely however is this type of study performed.

Such an assessment was conducted in the Australian sugar industry to determine the risk to avian predators caused by the use of in-crop rodenticides. Chapter 2 demonstrated a differential use of components of the system with rodents over- utilising crops with high levels of canopy cover and poorly utilising open canopy areas. In contrast, diurnal raptors focussed hunting efforts in open areas (e.g. farm roads) adjacent to canefields. This study demonstrated that accessibility of prey is a key element in determining secondary poisoning risk in this system. As the industry baiting requirements allow baiting in closed canopy crops only, the likelihood of raptors encountering poisoned prey is minimal unless poisoned rodents leave the crop and become available to predators in open adjacent areas.

These findings were incorporated into the design of a large-scale field trial

(Chapter 3) which baited crops following industry standards. Results of trapping 81

conducted in open areas adjacent to baited crops demonstrated that while poisoned rodents do become available to avian predators, toxicant levels in these rodents was relatively low. Based on this information, a simple mathematical model was developed, based on the likelihood of a predator encountering a poisoned crop within the district and then encountering a poisoned rodent outside this crop.

Results suggested that the probability of secondary poisoning events occurring were extremely low. Based on an ecological assessment of secondary poisoning risk, the application of rodenticides at current low levels in the Australian sugar industry was unlikely to cause major secondary poisoning incidents.

A shortfall of the model advanced in Chapter 3 is that it generalises hazard without considering biologically relevant factors that further influence it. For example, in Chapter 2, I reported a high diversity of raptors utilising sugar producing areas with weights varying between 0.16 kg – 1.2 kg. Given that secondary poisoning is dose-dependent, a low bodyweight predator will be at greater risk of toxicosis than a larger predator. Similarly, the foraging range, number of prey eaten per day and the quantity of toxicant ingested are important components of this interaction. These factors, and the interaction between them need to be examined in order to provide a more refined assessment of risk.

In response to this need, this chapter presents the results generated from a stochastic spatial computer model specifically developed for this chapter and designed to incorporate biological factors into the ecological framework determined in Chapter 3. This chapter specifically aims to investigate the effect that increasing the area of an agricultural system under bait and further

manipulating the proportion of rodents poisoned available to avian predators has on the mortality rates of a suite of avian predators at varying toxicity levels.

Further, given limited specific information about the sensitivity of modelled species to rodenticides, a general investigation into the impact of decreasing LD 50 is undertaken.

Materials and Methods

Model overview

A stochastic spatial model was specifically developed for this chapter and is included as Appendix 2 of this thesis. The model was based on the ecological factors which impact risk as described in Chapter 2 and Chapter 3 and the physical attributes of the Herbert River District, a 64 000 ha sugar producing area in North Queensland. It was designed as a means of investigating which, if any, biological factors most significantly affect secondary poisoning risk. The model is coded in Microsoft Visual Basic and runs under Microsoft Excel. Essentially, the model enables users to input data related to a range parameters (including predator body weight, the number of prey positive for toxicant which are available to a predator and, the quantity of toxicant within a prey item) and, after running the input simulation provides output related to predator mortality. The model’s inputs are described below and a model description follows.

Model inputs District characteristics

The physical attributes of the model simulate the Herbert River District in North

Queensland, a large cane producing district with approximately 64 000 ha of land available for sugarcane production. As paddock size varies greatly in the Herbert

River District, in the model an arbitrarily assigned paddock size of 2 ha is used in unison with the size of paddocks studied in Chapter 3.

In the model, rodenticides are distributed non-randomly to reflect the patchy nature of rodent damage in the district. Wilson and Whisson (1993) demonstrated that the Herbert River District could be categorised as three sub-districts based on rodent damage potential categories of High, Medium and Low. These sub- districts cover 37% 44 % and 19% of the whole district respectively. In the model, these damage-area relationships are approximated. As the model lacks the facility to divide the district into three sectors (for ease of programming), four sectors were used with bait apportioned in the following way: Sector 1 – 37%, Sector 2 –

22%, Sector 3 – 22% and Sector 4 – 19% (percentage of district baited between 5 and 50%) and Sector 1 – 33%, Sector 2 – 23%, Sector 3 – 24% and Sector 4 –

20% (percentage of district baited = 75%). At 100 % of district baited, all sectors were equally apportioned bait. Therefore, the distribution of baits is patchy at low levels of application and as application rates increase distribution become randomly applied.

Rodenticides investigated and the quantities of these available to predators

Currently, two rodenticides are available for use in the Australian sugar industry, the acute rodenticide Rattoff® (2.5 % zinc phosphide) and the chronic, first generation anticoagulant Racumin® (0.037 % coumatetralyl). As the only distinguishing feature between these two rodenticides in the model is their LD 50 ’s

(20 mg/kg and 50 mg/kg respectively), for brevity, this chapter investigates a range of secondary poisoning scenarios associated with the use of Rattoff® as this rodenticide is the most widely used in the district, the most poisoned rodents caught in open adjacent areas were poisoned with this rodenticide and it has the lower LD 50 . This rodenticide therefore likely presents the worse–case secondary poisoning scenario of the either rodenticide. In order to determine parity between model results from this chapter and those of the simple model advanced in

Chapter 3, both Rattoff® and Racumin® were modeled using the data provided in

Chapter 3. Results were similar and are provided in Table 4.2.

To determine the effect that the quantity of rodenticide available in prey has on secondary poisoning hazard, five zinc phosphide quantities were chosen. These were 0.25 mg, 0.47 mg, 0.69 mg, 1.3 mg and 1.8 mg. Of these, 0.25 mg and 0.69 mg were the mean and worst-case scenario levels derived in field trials in sugarcane (Chapter 3), 1.8 mg was the worst case recorded in a laboratory trial by

Sterner et al. (1998) and 0.47 mg and 1.3 mg were chosen as mid points between these experimentally derived values.

A standard deviation of 0.1mg of toxicant was applied to the above quantities.

This figure was derived from the maximum whole carcass residue level recorded in Sterner et al. (1995) of 1.8 ± 0.12 mg. Field results from Chapter 3 were extremely variable due to the low number of rodents sampled in this part of the study and therefore deemed unrealistic and therefore not suitable for use.

Avian predators modelled

To determine the effect of predator bodyweight in the secondary poisoning process, three raptors common in the Herbert River District and with diverse bodyweights were selected. Where possible, each species was assigned a mean bodyweight (Table 4.1) based on published data (Marchant and Higgins 1990).

Given that a low bodyweight predator could be at high risk from secondary poisoning (see Chapter 3), a low bodyweight predator (denoted as juvenile) with a bodyweight of 0.05 kg was incorporated into the study.

A standard deviation of 0.01 kg was applied to the above predator bodyweights.

This figure was chosen as it approximates the range of bodyweights derived from

Marchant and Higgins (1990).

Due to a paucity of data, limited scientifically verifiable information is available regarding the quantity of prey eaten per day by the selected raptors. In this chapter, a prey intake which approximates 20 % body weight is used based on the following information. Tully et al. (2000) suggest a general rule of thumb, stating that small raptors eat 20 % of body weight; medium raptors consume 10 – 15 % of body weight while large raptors consume 6 – 8 % of bodyweight daily. Marti

(1970) compared food intake of four caged owl species and determined that intake ranged between 4.7 % and 15.9 % of body weight daily. Therefore, based on a daily intake of 20 % of body weight, the selected raptors would eat at worst case

(in terms of risk potential) between 1 and 3 prey items a day (Table 4.1).

Table 4.1: Avian predator bodyweights and conjectured number of prey consumed per day used to investigate secondary poisoning risk in the Australian sugarcane industry.

Weight Number of prey Raptor species (kg) consumed per day

Juvenile 0.05 1

Nankeen kestrel ( Falco cenchroides ) 0.16 1

Barn owl ( Tyto alba ) 0.34 2

Masked owl ( Ninox novaehollandiae) 0.90 3

The number of days spent foraging was held constant at 30 days reflecting industry advice that rodenticides are active for this period. In lieu of any information regarding raptor foraging ranges in sugar producing areas, foraging range was kept constant for all raptors at 50 plots (100 ha). Standardising foraging range will not bias model output as preliminary investigation demonstrated that if all other model inputs are kept constant; varying home range size does not impact the outcome (that is, percent mortality) of the model. For example, all else being equal, varying a barn owl’s home range from 50 plots (100 ha) to 100 plots (200 ha) resulted in a 2 % increase in mortality (63% and 65% respectively).

Impact of varying LD 50 levels

To determine the impact that varying LD 50 has on mortality rates of avian predators, two levels, 10 mg/ kg and 60 mg/ kg were investigated. The following conditions were kept constant:

1. 50 % of the district was randomly baited;

2. 50 % of rodents were positive for rodenticide; and

3. All other parameters where kept constant as per avian predators modelled

section above.

The above conditions were investigated at two mean prey toxin levels, 0.25mg and 0.69 mg, in order to determine how high prey toxicant loads affect predator sensitivity to changes in LD 50 .

Model description

1. The sample area (maximum 264 x 264 units) for the simulation is based on

two user inputs [ crop area (ha)] and a unit factor [ plot area per cell ] (see

Figure 4.1). In this case, [ crop area ] = 64,000 ha (area of Herbert River

District) and [ plot area per cell ] = 2 ha (average size of a sugarcane

paddock).

2. The location of baited paddocks is assigned at random throughout the

sample area, and is based on a user input of the number of plots (in this

case, 2 ha sugarcane crops) poisoned [ plots poisoned ].

Figure 4.1: Screen shot of User Input screen in PoisonV7 model.

3. The weight of a predator is determined by random selection of a value

from a normal distribution of predator weight based on two user inputs

[mean of distribution ] and [ standard deviation of distribution ].

4. The predator is randomly assigned to a plot (sugarcane crop) within the

sample area (Figure 4.2).

5. The foraging area for the predator is generated around the randomly

assigned plot (4), and is point based on a user input [ foraging range (plot

areas) ]. This foraging area could therefore include poisoned (red squares)

and unpoisoned plots (green squares) (Figure 4.2)

Figure 4.2: Screen Shot of Sample Area of PoisonV7 model.

6. The predator is randomly allocated a location along the perimeter of the

plots within the foraging range based on the number of plot areas

adjoining the perimeter.

7. The status of the plot area adjacent to this location (baited/not baited) is

noted.

8. The bird selects a rodent from this location. If the adjacent plot area is not

baited, toxicant level ingested is recorded as zero in the output.

Accumulated toxin ingested is also recorded.

9. If the adjacent plot area is baited, the presence of toxin (+/-) in the selected

rodent is determined at random from two user inputs [ no. of prey per unit

perimeter positive for toxin ] and [ no. of prey per unit perimeter negative

for toxin ]. 90

10. If the selected rodent is positive for toxin, the quantity of toxin ingested

by the bird is determined by random selection of a value from a normal

distribution of the amount of toxin in a prey item based on two user inputs

[mean of distribution ] and [ standard deviation of distribution ], and the

quantity of toxin ingested is recorded. Accumulated toxin ingested is also

recorded.

11. The rodent selection process (points 6 to 9) is repeated within the same

foraging area based on a user input of the number of prey consumed per

day [ no. of prey per day ].

12. The foraging area selection and rodent selection processes (points 4 to 9)

are repeated based on a user input [ no. of foraging days ]. This is based on

the longevity of the poison within the system.

13. The bird selection, foraging area selection and rodent selection processes

(points 3 to 9) are repeated based on a user input [ no. of predators ].

14. For each bird, every time a rodent is selected, the status of the bird

(alive/dead) is determined by comparing the accumulated toxin ingested

value with the amount of toxin required to kill the particular bird. This

latter value is obtained from the weight of the bird and a user input [ LD 50 ].

The model has an option whereby the user can divide the total sample space into

1, 2, 4, 9, or 16 equal sectors and allocate different numbers of baited paddocks to each sector. This allows simulation of “hotspots” of baiting activity where the baited paddocks are not distributed randomly throughout the total sample space.

Outputs therefore give a range of mortalities from 50% to 100%. This has the effect of potentially overestimating death rate by up to a factor to two.

All model scenarios used in this chapter were simulated using a total of 5000 iterations (that is, 5000 predators). Confidence limits were calculated following the

F-distribution method discussed in Zar (1999).

Results

The model output is consistent with the simple model proposed in Chapter 3

(Table 3.6) suggesting that at low levels of toxicant use, the probability of secondary poisoning is low.

Regardless of predator bodyweight or the quantity of toxicant available, there was a positive relationship between predator mortality and increasing either the percentage of the district poisoned (Figure 4.3 a - d) or the number of poisoned prey available (Figure 4.4 a - d). Mortality levels increased sharply once percent of the district baited exceeded 20% (Figure 4.3), except for adult predators at low toxicant levels where mortality remained low (Figure 4.3 c and d). When the percentage of rodents poisoned exceeded 20%, mortality rates were generally high for all except larger predators (Figure 4.4 c and d).

The level at which highest mortality was reached was strongly influenced by the quantity of toxicant available to predators. Low body weight predators suffered greater mortality at lower toxicant quantities than higher bodyweight predators (Figure 4.3 a – d and 4.4 a - d). For example, juvenile mortality levels (Figure 4.3 a and 4.4 a) were uniformly high across the range of toxicant levels investigated while masked owls (Figure 4.3 d and 4.4 d) suffered high mortality levels only after toxicant levels exceeded a mean of 0.69 mg.

There was a negative relationship between the average number of days to predator death and increases in both the percentage of the district baited and the number of poisoned rodents available to predators across all species (Figure 4.5 a - d and 4.6 a - d). Increasing the mean quantity of toxicant available to predators intensified this trend with low bodyweight predators exhibiting a greater sensitivity (Figure 4.5 a and 4.6 a) at lower toxicant levels than higher bodyweight predators (Figure 4.5 d and 4.6 d).

The LD 50 of rodenticides applied to crops has a significant impact on avian predator mortality. Generally, reducing the LD 50 of the toxicant applied in the system had the effect of increasing avian predator mortality while increasing the toxicant’s LD 50 has the opposite effect (Figure 4.7 a and b). However, this general trend was not evident in juvenile predators. These low body weight predators were relatively insensitive to changes in LD 50 (Figure 4.7 a and b). Generally, lowering the quantity of poison available to predators (that is, from 0.69 mg to 0.25 mg) resulted in lower levels of predator mortality (Figure 4.7) except for juveniles that again were relatively insensitive to these changes. Juvenile mortality levels only reduced significantly when a combination of high LD 50 (60 mg) and low available poison levels (0.25 mg) were modelled (Figure 4.7 a).

Juvenile

0.25 mg 100 0.47 mg 0.69 mg 1.30 mg 1.80 mg 80

60 Mortality % Mortality 40

0 0 20 40 60 80 100 Percent of district poisoned

a) Juvenile weighing 0.05 kg consuming one prey per day

Kestrel

100 0.25 mg 0.47 mg 0.69 mg 80 1.30 mg 1.80 mg

60 Mortality % Mortality 40

0 0 20 40 60 80 100 Percent of district poisoned b) Kestrel weighing 0.16 kg consuming one prey per day

Barn Owl

100 0.25 mg 0.47 mg 0.69 mg 1.30 mg 80 1.80 mg

60 Mortality % Mortality 40

0 0 20 40 60 80 100 Percent of district poisoned c) Barn Owl weighing 0.34 kg consuming two prey per day

Masked Owl

100 0.25 mg 0.47 mg 0.69 mg 80 1.3 mg 1.8 mg

60 Mortality % Mortality 40

0 0 20 40 60 80 100 Percent of district poisoned d) Masked Owl weighing 0.90 kg consuming three prey per day

Figure 4.3 (a – d): The effect of increasing crop area under rodenticide on mortality of four predator species. The number of poisoned rodents was held constant at 20%. Data represent quantity of toxicant (mg) as per legend .

Juvenile

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60 Mortality % Mortality 40 0.25 mg 0.47 mg 0.69 mg 1.30 mg 20 1.80 mg

0 0 20 40 60 80 100 Percent of rodents consumed that are poisoned a) Juvenile weighing 0.05 kg consuming one prey per day Kestrel

0.25 mg 100 0.47 mg 0.69 mg 1.30 mg 80 1.80 mg

60 Mortality % Mortality 40

0 0 20 40 60 80 100 Percent of rodents consumed that are poisoned

b) Kestrel weighing 0.16 kg consuming one prey per day

Barn owl

0.25 mg 0.47 mg 100 0.69 mg 1.30 mg 1.80 mg

60 Mortality % Mortality 40

0 0 20 40 60 80 100 Percent of rodents consumed that are poisoned

c) Barn Owl weighing 0.34 kg consuming two prey per day

Masked Owl

0.25 mg 100 0.47 mg 0.69 mg 1.30 mg 1.80 mg 80

60 Mortality % Mortality 40

0 0 20 40 60 80 100 Percent of rodents consumed that are poisoned

d) Masked Owl weighing 0.90 kg consuming three prey per day

Figure 4.4 (a – d): The effect of increasing the number of poisoned rodents available to four predator species. The crop area under poison was held constant at 50%. Data represent quantity of toxicant (mg) modelled as per legend.

Juvenile

25 0.25 mg 0.47 mg 0.69 mg 1.30 mg 20 1.80 mg

10 Aver. daysto death

0 0 20 40 60 80 100 Percent of district poisoned

a) Juvenile weighing 0.05 kg consuming one prey per day

Kestrel

20 Aver. daysto death 10 0.25 mg 0.47 mg 0.69 mg 1.30 mg 1.80 mg 0 0 20 40 60 80 100 Percent of district poisoned b) Kestrel weighing 0.16 kg consuming one prey per day

Barn Owl

0.25 mg 10 Aver. daysto death 0.47 mg 0.69 mg 1.30 mg 1.80 mg

0 0 20 40 60 80 100 Percent of district poisoned c) Barn Owl weighing 0.34 kg consuming two prey per day

Masked Owl

0.25 mg Aver. daysto death 10 0.47 mg 0.69 mg 1.30 mg 1.80 mg

0 0 20 40 60 80 100 Percent of district poisoned d) Masked Owl weighing 0.90 kg consuming three prey per day

Figure 4.5 (a – d): Effect of increasing the area of district under bait on the average numbered days to predator death. Data represent quantity of toxicant (mg) modelled as per legend. Note that birds reaching 30 days remain alive.

Juvenile

0.25 mg 30 0.47 mg 0.69 mg 1.30 mg 1.80 mg

Aver. days to death Aver. 10

0 0 20 40 60 80 100 Percent of rodents consumed that are poisoned a) Juvenile weighing 0.05 kg consuming one prey per day

Kestrel 0.25 mg 0.47 mg 30 0.69 mg 1.30 mg 1.80 mg

Aver. daysto death 10

0 0 20 40 60 80 100 Percent of rodents consumed that are poisoned b) Kestrel weighing 0.16 kg consuming one prey per day

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Barn Owl

20 Aver. daysto death 10 0.25 mg 0.47 mg 0.69 mg 1.30 mg 1.80 mg 0 0 20 40 60 80 100 Percent of rodents consumed that are poisoned

c) Barn Owl weighing 0.34 kg consuming two prey per day

Masked Owl

0.25 mg Aver. daysto death 10 0.47 mg 0.69 mg 1.30 mg 1.80 mg

0 0 20 40 60 80 100 Percent of rodents consumed that are poisoned

d) Masked Owl weighing 0.90 kg consuming three prey per day

Figure 4.6 (a - d): The effect on increasing the number of poisoned rats on the average numbered days to predator death. Data represent quantity of toxicant (mg) modelled as per legend. Note that birds reaching 30 days remain alive.

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0.25± 0.1 mg / kg toxicant in prey items

80 10 mg 60 mg

Mortality%

0 Juvenile Kestrel Barn Owl Masked Owl

a) The effect of varying LD 50 on avian predator mortality at low levels of prey item toxicant

0.69± 0.1 mg / kg toxicant in prey items

100 10 mg 60 mg

Mortality% 40

0 Juvenile Kestrel Barn Owl Masked Owl

b) The effect of varying LD 50 on avian predator mortality at a high level of prey item toxicant

Figure 4.7 (a and b): The effect of varying LD 50 (10 mg/kg and 60 mg/kg) on a suite of avian predators. Crop area under poison and the number of poisoned rodents was held constant at 50%.

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Discussion

Results reaffirm the premise of the simple mathematical model presented in

Chapter 3 and demonstrate that the likelihood of secondary poisoning events occurring increases as the area of the district under bait and the number of poisoned rodents available to predators increase. Further, increasing the quantity of rodenticide available in prey potentiates this general trend and its effects are most pronounced in low bodyweight predators. These results demonstrate that secondary poisoning is a complex process and any strategy aimed at quantifying risk must account for both ecological and biological factors relevant to the risk process.

Although the assessment of secondary poisoning risk is complex, the options available for managing risk are comparatively simple. Of the factors that influence hazard, only the area of the district under bait and the timing of bait application can be manipulated to minimise the potential for adverse events to occur. Under the scenarios investigated in this chapter, once the area of the district baited exceeded 20%, avian predator mortality generally increased sharply. If this threshold is a true indication of field dynamics, maintaining the area of the district baited below this threshold should minimise secondary poisoning events occurring.

The timing of bait application to crops within the district may also be crucial in minimising risk. Results indicate that low bodyweight predators are most at risk from secondary poisoning. If bait application coincides with the period when juveniles enter the population, there is a heightened potential that these low 103

bodyweight predators will be adversely affected. Obviously, minimising the use of rodenticides during this period should reduce hazard, however the potential positive conservation outcomes associated with this risk reduction must be balanced with the need to protect crops via the use of rodenticides. This situation occurs in the Australian sugar industry with the industry baiting window (October

- March) coinciding with the breeding season of many raptors in North

Queensland. This baiting period is timed to coincide with the annual colonisation of crops by rodents and aims to suppress seed populations and force peak breeding activity closer to harvest. This then decreases damage potential. Since zinc phosphide and coumatetralyl have been available for use in the Australian sugar industry, less than 5 % of the district has been baited. At this level, the model predicts that secondary poisoning risk is remote however any future increase in this level may result in heightening secondary poisoning risk. The objective of minimising secondary poisoning risk must then be traded-off with the need to protect crops and maximise yield.

In order to minimise secondary poisoning hazards, the Australian sugarcane industry has chosen both zinc phosphide and coumatetralyl for their low toxicity to avian predators. Results suggest that mortality in predator populations is sensitive to changes in toxicant strength with low bodyweight (juvenile) predators proving to be particularly sensitive to decreases in LD 50 . At an industry level, this should be considered when and if new rodenticides are considered for incrop use.

The potential for secondary poisoning events to occur is likely to be overestimated in the model’s output and therefore the results present a worst-case scenario of

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secondary poisoning risk for two reasons. First, the model assumes a 100 % conversion rate of prey ingested rodenticide to avian predators. Due to a paucity of field data, it is difficult to determine by how much, if at all, this conversion rate leads to an overestimation of risk. A survey of predator feeding strategy is important in determining this. If a predator consumes a prey whole, it will ingest all rodenticide available in that prey item. However, if a predator dissects its prey during consumption it may be at a lower risk of ingesting large quantities of rodenticide particularly if this strategy confers a greater sensitivity to the presence of bait in prey items. For example, Johnston and Fagerstone (1994) reported that many animals will refuse to consume the gastro-intestinal tract of zinc phosphide poisoned prey. Also, Newton et al. (1994) and Gray et al . (1994) have demonstrated that in laboratory studies, owls fed rodenticide-poisoned rodents eliminated approximately 25% of the ingested rodenticide via pellets. Therefore a survey of predator feeding strategies should highlight those that are most at risk from secondary poisoning and refine estimates of rodenticide ingestion by predators. Further, if predators eat significantly more rodents than assumed in this study, this would significantly impact these results. Reliable estimates of the number of prey eaten per day are therefore required.

Second, the average number of days to predator death is very high in all but the lowest bodyweight predators and at the highest toxicant levels investigated. This suggests that the model overestimates risk given that studies (Koehler et al. 1995;

Brown et al. 2002; Twigg et al. 2002) have demonstrated that once zinc phosphide is applied in crops, the majority of rodent deaths occur in the first few days of application. While industry recommendations suggest that rodenticides are

105

available to incrop rodents for a period of thirty days, it is unclear whether poisoned rodents will be consistently available to predators over this entire period.

Further, zinc phosphide is unlikely to have cumulative effects on raptors which have consumed poisoned rodent prey leading to further overestimation. A study designed to determine the time period over which poisoned rodents are available to predators and whether the proportions of poisoned rodents available changes over time would clarify this issue.

The model developed for this study is novel and is the first attempt at quantifying the role of ecological and biological components of secondary poisoning risks in complex agricultural systems. The inputs in the model are universal and therefore this model should have application in other agricultural systems that share similar cropping patterns and predator/prey interactions. Results demonstrate that secondary poisoning risk associated with the currently available rodenticides in the Australian sugar industry is remote most likely due to the current low level of rodenticides use and the low level of toxicity associated with these chemicals.

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Chapter 5

General Discussion

The objective of this thesis was to investigate avian predator secondary poisoning risk in the Australian Sugar Industry associated with the use of rodenticides. It is apparent that, even in this simple agricultural system, that predator prey processes are complex and therefore an understanding of the ecological interactions between predators and their prey needs to be evaluated prior to any assessment of risk.

The main findings of this thesis were:

1. Interactions between avian predators and rodent prey must occur exclusively

in crops with open canopies or associated open habitats for secondary

poisoning to occur. Availability of prey is then a key component of

secondary poisoning risk;

2. Low numbers of poisoned rodents became available to avian predators post

rodenticide application. Secondary poisoning risk should therefore be

minimised if rodenticide application follows industry protocols; and

3. Modelling demonstrates that the likelihood of secondary poisoning events

occurring is low when standard rates of application are used with accepted

industry baiting protocols.

Prior to this thesis there was limited knowledge of the interaction between avian predators and rodent prey in sugar agro-ecosystems and therefore, limited information on which to base risk assessment. The information available consisted of a) an anecdotal study (Young and De Lai 1997) which reported that there was a 107

decline in owl populations coincident with the application of the rodenticides

Klerat® and b) laboratory derived LD 50 ’s that suggested that the rodenticides currently available pose minimal risk to avian predators.

Young and De Lai’s (1997) paper suggested that owl populations were affected by the use of the now de-registered rodenticide Klerat® (Brodifacoum). By their own admission, their study was not based on population estimates or conducted using scientific methodology, but was derived from anecdotal knowledge of population densities over a twenty-year period. While their study drew attention to the issue of secondary poisoning in sugarcane agro-ecosystems, the nature of their study means that it is difficult to verify the extent of any population variation, and they provided no evidence to link any change in population density with the use of this rodenticide. In order to attribute risk, an interaction between baited rodents and predators must be established and further, any statement about changes in predator density caused by rodenticides needs to be demonstrated, not merely inferred.

Both rodenticides currently available for use in the sugar industry are suggested to present low risk to avian predators due to their high laboratory derived LD 50 .

While laboratory derived LD 50 are essential in setting toxic thresholds, laboratory trials cannot encompass the complexity of the organism/resource or organism/organism interaction. Sole reliance on laboratory derived data is then simplistic. Laboratory derived data while indicating a toxic threshold, cannot reflect the complexity of the interactions that lead to secondary poisoning and therefore cannot provide a satisfactory answer to what is fundamentally a field- based ecological question.

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The chapters which make up the basis of this thesis address this information gap by:

a. determining the ecological relationship between both rodent prey and avian

predators and hence determine where this population interaction takes place;

b. determining the availability of both poisoned rodents and the quantity of

toxicant available for ingestion to avian predators; and

c. developing a stochastic model which enables future risk to be assessed under

various baiting scenarios.

The basis of the assessment framework is the understanding of the ecological interaction between rodent prey and avian predators. Throughout this study, both populations used the available canopy levels in vastly different ways. Rattus sordidus was always associated with high levels of canopy cover which afford high level protection from avian predators. This habitat use is consistent with

Wilson and Whisson (1993) who demonstrated that rodents invade closing canopy crops and reach highest numbers in mature cane. In contrast, avian predators were commonly associated with open canopy crops/open adjacent areas and this use reflects the high accessibility these areas offered. Importantly, while these areas are poorly utilised by rodents, results demonstrated that they are used albeit at a comparatively low level. Therefore, the interaction between predators and their prey are highly constrained in this system. While the highest densities of rodents occur in inaccessible, closed canopy crops, the interaction occurs only in open adjacent areas which comprise, for the majority of the growing season, only a small proportion of the total district as a whole.

109

Given this highly constrained interaction and that the industry baiting protocol prescribes that rodenticides are only applied in closed canopy crops, logically, secondary poisoning can only occur if poisoned rodents leave the crop and become available to predators. Results suggest that only a fraction of rodents found outside the crop are poisoned and further, that these rodents contain relatively low levels of toxicant. Therefore, for an avian predator to consume a lethal dose of toxicant secondarily, it must a) encounter a poisoned crop and b) capture a poisoned rodent and repeat one or both of these processes until a lethal dose in ingested. Results suggest that this is highly unlikely in all but extreme cases.

The outcome of this assessment framework is the development of a stochastic model that provides a means of assessing future risk based on the ecological interactions described previously and information such as the amount of bait sold in a district and the area under crop in a district which are readily available in sugar producing districts. Simulation results run under various scenarios suggest that with standard baiting protocols and existing levels of bait application, the risk of secondary poisoning is negligible.

In summary, the framework presented in this thesis provides an assessment of secondary poisoning risk based on an understanding of the ecological interactions between rodent prey and avian predators, demonstrated that the risk associated with secondary poisoning is negligible and further provides a means of future assessment of risk under various rodenticide use scenarios.

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While the work from this thesis provides a sound foundation for examining secondary poisoning, a number of issues arise. One important consideration is that the outcomes of this study are based on the interaction between raptors and their rodent prey occurring in a subset of available habitats. Although several hundred hours of observations were made in this study, there were no observations made of interaction between raptors and rodents. The assumption that physical interaction between raptors and rodents is more likely to occur in locations where raptors spend more time is reasonable, but the conclusions of this work would be strengthened by observations of prey consumption by raptors.

Another issue relates to the rodent species in these systems. Given its broader geographical range and hence its greater potential impact to the sugar industry, this thesis has focussed primarily on potential risk associated with rodenticide baiting for the canefield rat ( R. sordidus ). Another rodent species, the grassland melomys (Melomys burtoni ) is implicated in the damage process but in a much reduced geographical range, being generally confined to the Wet Tropics of

Northern Queensland. Recent research has been conducted to investigate the biology of this species in far north Queensland sugarcane crops (Dyer 2007). This work demonstrated that unlike R. sordidus which colonises sugarcane crops early in the growing season, M. burtoni colonise crops late in the growing season. This suggests that the Rodent IPM currently used by that targets R. sordidus is not directly transferable to areas which are subject to crop damage by M. burtoni . The outcomes of this research and future studies on controlling M. burtoni damage and any management strategies which use rodenticides to minimise rodent damage

111

will likely require an evaluation of potential secondary poisoning hazard separate to the one presented here.

Further issues relate to model inputs. Some of the information that forms the basis of this model could be assessed in more detail as they are fundamental to model outcomes. Components of the feeding behaviour of the raptors, and rodent behaviour after bait ingestion are examples.

There is little published data available to determine the number of prey that avian predators consume under field conditions over the course of a day or indeed how this data may change with season. For example, major increases in prey utilisation over the breeding season may significantly impact the likelihood that predators consume poisoned prey and feed these to their young. An examination of the quantity of prey consumed would provide valuable information for estimating the potential for secondary poisoning events occurring. The following areas would also benefit from further research:

a. The time period that poisoned rodents are available to avian predators is also

important. Industry sources suggest that rodenticide is available for a period

of 30 days. This is most likely an overestimate given other studies have

documented that the majority of rodent deaths occur in the first few days of

application. A study aimed at documenting the potential exposure time of

avian predators to poisoned rodent prey would be beneficial in refining

hazard assessment, and;

b. Another factor is the quantity of poison ingested by avian predators. The

model described in Chapter 4 assumes a 100% conversion rate between the

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toxicant occurring in rodent prey and that ingested by the predator. This

assumes that predators ingest the whole prey item and this may not be the

case. This then likely results in an overestimate of the dose which is retained

by the predator. For example, Newton et al. (1994) found during laboratory

feeding trials, barn owls fed on flucomafen-dosed mice regurgitate pellets

which contained a mean quantity of rodenticide equal to 27% of the ingested

dose. Similarly, Gray et al . (1994) reported elimination of 25% of the

ingested dose of both brodifacoum and difenacoum rodenticides. A survey of

actual rodenticide assimilation would be helpful in further refining risk.

Although these are minor issues which would enable a greater refinement in assessment, two issues have been identified which must be rapidly addressed.

First, the confusion surrounding the LD 50 associated with Coumatetralyl and the method used to determine it needs clarification. This information is fundamental and without it, it is impossible to determine the risk the use of this rodenticide poses to avian predators. Clarification of this is paramount.

Second, industry sources suggest that protocols relating to bait application in open canopy crops have been relaxed with baiting permitted in these crops when visual signs of damage occur during the early crop growth stages. Due to the strong association demonstrated in this thesis between avian predators and open canopy areas, this decision may dramatically increase the potential interaction between predators and poisoned rodents. While an assessment of the risk associated with this decision is required, the logic of bait application as a means of cost-effective

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crop protection at this early stage also needs to be assessed, particularly given the extremely high use of trash-blanketing in the industry (Wilson and Whisson

1993).

For example, Whisson (1996) demonstrated that although rodents established low level populations in trash-blanketed crops after harvest, population levels remained low through the early growth period (December to March) and stomach contents of rodents trapped in trash-blanketed crops predominately comprised non-cane vegetation. Further, breeding intensity was low in trash-blanketed areas, evident in the low percentage of female pregnancies and population levels increased only when dispersing juveniles entered these crops from adjacent weedy areas late in the growing season (May).

Given the low level of use, the predominance of non-cane vegetation in stomachs of trapped animals, the low fecundity in trash-blanketed areas and without any information relating perceived damage levels in early growth stages to economic losses at harvest, it appears control at this early stage is unwarranted. Therefore, the use of rodenticides in open canopy needs to be investigated both in terms of risk associated for secondary poisoning and cost-effective and efficient rodent control technique.

The framework developed to assess the risk of secondary poisoning in this thesis provides a strong alternative to more commonly used techniques. For example,

Brown (1994) suggested that searching for carcasses was the most commonly employed technique used to survey impacts on non-target animals for rapid-acting

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rodenticides. While widely accepted, this technique provides a snap-shot of risk over a short period of time, relies on finding affected animals over vast cropping areas, does not provide any information regarding the ecological interaction between populations and, does not provide any information which will enable secondary poisoning risk to be assessed under differing rodenticide application scenarios or incorporate any significant information regarding the levels of rodenticides available to predators, or indeed any information regarding ecologically important details about home range. The framework designed in this thesis and the model generated provides all this information and therefore should be considered as a strong alternative to currently used techniques.

Although this thesis has focussed on the Australian sugar industry, the framework presented here has broader application to agricultural systems which share similar cropping processes to sugarcane. The approach used to assess hazard in this thesis would be particularly suited to broad-acre crops such as wheat where rodenticides are used to control mouse populations, and raptors are common predators in these systems. Minimal field trials would be required to adapt the methodology used in this thesis to other cropping systems, requiring knowledge of district attributes such as total area under crop and area baited – information which is readily available in most organised agricultural systems, a survey of avian foraging patterns and an assessment of the availability of poisoned prey to avian predators.

The methods used to accumulate these data are simple and relatively cost- effective.

For example, Brown and Lundie-Jenkins (1999) reported actual cases of secondary poisoning of raptor species associated with the application of 115

strychnine used to control a mouse plague on the Darling Downs in central

Queensland, Australia. Applying the simple mathematical model P 1 X P 2 described in Chapter 3, P 1 can be calculated as the area under bait (250 000 ha) / total cropping area (600 000 ha) giving a P 1 value of 0.4. In the absence of an estimate of P2 for this system, assuming a similar value as described in sugarcane

(P2 = 0.2) the combined probability of ingesting a poisoned rodent is low at 0.08.

Even assuming a significantly higher poisoned rodent availability rate of P2 = 0.5 this returns a low combined probability of (P 1 X P 2) 0.2. These results suggest that secondary poisoning for this species is low. In approximately 180 person hours of carcass search time, Brown and Lundie-Jenkins (1999) found a total of nine dead raptors, four testing positive for strychnine.

The outcomes of this thesis demonstrate that poisoned rodents become available for predation and studies like Brown and Lundie-Jenkins (1999) demonstrate that non-target deaths associated with rodenticide use do occur. One critical question which neither approach has addressed experimentally but is paramount to assessing impact is at what point secondary poisoning becomes a problem in a non-target population. Obviously, the effect of number of deaths in a population is dependent on its size with small and localised populations likely to suffer greater impacts from the loss of individuals than larger populations. It is critical therefore that any broad statement of secondary poisoning impact must take into account the size of the population which will likely be impacted. A survey of population levels of the various raptors which hunt around sugarcane crops should be undertaken to determine their resilience to population losses.

While this thesis has focussed on potential secondary poisoning events involving raptor and rodent interactions, other small mammal predators exist in and around sugarcane crops which could potentially be at risk. These species include a suite

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of snakes (both elapid and colubrid) and in the more northerly canegrowing regions, Australia’s largest carnivore, the spotted-tailed quoll ( Dasyurus maculates gracillis) .This species is listed as Endangered in the Commonwealth Environment Protection and Biodiversity Conservation Act (1999) and by the Queensland Government ( Nature Conservation Act 1994 ). To date, no studies have been undertaken to determine the impact of rodenticide use on snake populations which are known to utilise canefields and no studies have been conducted to determine if this species is at risk from secondary poisoning. Given the conservation status of spotted-tailed quolls, an assessment of secondary poisoning risk is warranted.

This thesis is the first to use an ecological approach to investigate secondary poisoning events in sugarcane producing areas and demonstrates the utility of this approach. This thesis has demonstrated that poisoned rodents can become available in open areas where raptors hunt and therefore demonstrates that secondary poisoning interactions can occur in sugarcane producing districts albeit at low levels given the current limited use of rodenticides by the sugarcane industry. Further, the methodology used is simple to conduct and relatively cheap and provides a strong and adaptive framework in which modelling can be undertaken to investigate various secondary poisoning scenarios and prioritise future research directions.

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Appendix 1

Some discrepancy exists over the interpretation of the 8 day coumatetralyl LD 50 of

>50 mg/kg for hens. For example, Madden (2002) quotes an 8 day LD 50 of >50 mg/kg of body weight, referencing a Bayer publication (Hermann 1963) that reports an 8 day dosed feed rate of >50 mg (of coumatetralyl) / kg (of feed).

Worthing and Hance (1991) report an 8 day LD 50 of >50 mg/kg without specifying whether this is a dosed feed rate or if it is related to body weight. The consequence of this confusion is very significant in terms of the secondary poisoning hazard. For example, in the current study, it changes the number of rodents required to be consumed to ingest an LD 50 by a factor of 10. For example, at 50 mg/kg of body weight, a masked owl (LD 50 63 mg) would need to consume approximately 3150 rats at a mean dose of 0.02 mg coumatetralyl. Assuming a hen weighs 1000 g and eats 100 g of food/day, the dose recorded for hens would be 5 mg/day. Scaling this to a 1260 g masked owl, its LD 50 would be approximately 6 mg and at a mean dose of 0.02 mg of coumatetralyl it would need to consume approximately 300 rodents.

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Appendix 2

Please Note: This model is a Microsoft Excel file. Microsoft Excel must be installed prior to running / opening the file. As the model relies heavily on

Macros, Macros must be enabled for the file to run.

Installation advice

To run the model:

1. Insert CD into computer CD drive

2. Open File on CD named Poison V7.xls

3. Ensure Macros are enabled

4. Enjoy in moderation

Chapter 4 provides a model description for reference.

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