SYMPOSIUM 7: MANAGEMENT—URBAN RODENTS AND RODENTICIDE RESISTANCE This file forms part of ACIAR Monograph 96, Rats, mice and people: rodent biology and management. The other parts of Monograph 96 can be downloaded from <www.aciar.gov.au>. © Australian Centre for International Agricultural Research 2003 Grant R. Singleton, Lyn A. Hinds, Charles J. Krebs and Dave M. Spratt, 2003. Rats, mice and people: rodent biology and management. ACIAR Monograph No. 96, 564p. ISBN 1 86320 357 5 [electronic version] ISSN 1447-090X [electronic version] Technical editing and production by Clarus Design, Canberra 431 Ecological perspectives on the management of commensal rodents David P. Cowan, Roger J. Quy* and Mark S. Lambert Central Science Laboratory, Sand Hutton, York YO41 1LZ, UNITED KINGDOM *Corresponding author, email: [email protected] Abstract. The need to control Norway rats in the United Kingdom has led to heavy reliance on rodenticides, particu- larly because alternative methods do not reduce rat numbers as quickly or as efficiently. However, such reliance has led to concerns that repeated use of rodenticides poses unacceptable risks to other animals as well as encouraging resis- tance in the target species. In agricultural areas, frequent use of poison baits is unavoidable when control is concen- trated on individual, resource-rich patches, such as farm buildings, because even if total elimination is achieved, reinvasion is inevitable. Population density is highest around buildings during winter when food supplies are most abundant and locations for baits most restricted. Control can be improved by, for example, changing the location of food sources within buildings, so that the environment is less predictable to rats. Rats with the smallest home ranges are most likely to succumb to rodenticide treatment, but those with the largest ranges who nevertheless consume poi- son bait may pose the greatest risk to predators. Populations reduced to very few animals may take two years or more to recover to their previous levels if there are no nearby reservoir colonies. Not surprisingly, recovery can be severely impeded by limiting the food supply, but in practice the resources of food and harbourage can rarely be controlled to a significant degree on a working farm. Consequently, reducing cover to expose rats to increased predation risks is prac- ticable, but the most successful control strategy necessitates breaking links between populations in resource-rich patches. This may be achieved by treating several patches simultaneously, as well as removing rats in transit between them. Although the initial control may be carried out with rodenticides, the need for subsequent treatments using poi- son baits should be greatly reduced. Introduction while ideal, is difficult to prove, let alone achieve. Hence, populations invariably recover, largely because the under- The need to control commensal rodents is rarely controver- lying reasons for the problem population developing in the sial, as the public has always associated rats, in particular, first place are seldom addressed. As the recovery with lethal, disease-causing organisms, such as bubonic progresses, at some point re-treatment becomes inevitable, plague. While plague has long been absent from some but experience has shown that the same level of control is countries, including the United Kingdom (UK), the poten- not guaranteed on subsequent occasions. The dynamics of tial health threat from commensal rodents is now focused each new population, including the behaviour and physi- on other zoonotic diseases, such as leptospirosis (Weil’s ology of individuals, are subject to change, not only disease) and salmonellosis. While pressure to control because of the selection pressure imposed by intensively problem populations continues, the means of doing so are applied control measures, but also because resources vary in coming under closer scrutiny, particularly regarding issues distribution and abundance. In this paper, we use the of humaneness and environmental impact, in addition to Norway rat (Rattus norvegicus) in rural habitats in the UK cost-effectiveness. Nevertheless, control tactics need to as a model to illustrate how these ecological processes reduce rapidly the number of potentially disease-carrying influence the effectiveness and safety of commensal rodent rodents and non-lethal management methods are generally management strategies. unable to achieve this. For example, repellents at best keep The Norway rat first arrived in the British Isles nearly rodents at a distance and at worst push the problem else- 300 years ago, then spread rapidly and largely displaced where, while fertility control might only reduce problems the ship rat (Rattus rattus) that had been present since over too long a period. Moreover, for many, the only accept- Roman times. Today, the Norway rat in rural areas is seen able level of control that mitigates disease risks is total predominantly as a storage pest that exploits supplies of elimination of the rodent population, an objective that, harvested cereals, root crops and livestock feeds to be 433 Rats, Mice and People: Rodent Biology and Management found in farm buildings. It also lives along field margins Hampshire farms was cereal growing (67%), livestock but rarely digs burrows in the fields. Changes in the agri- rearing on those in North Yorkshire (61%), and 42% live- cultural landscape that came with increasing mechanisa- stock/42% arable on the Sussex farms. Farming activity on tion resulted in loss of harbourage for rats as field one site in Hampshire, four in Sussex and eight in North boundaries were ploughed up to make larger fields. Yorkshire was classified as mixed. The location and type Consequently, the significant damage that the rats report- of stored food accessible to rats on each farm was edly caused to standing crops is now negligible. Despite recorded. Rats in Hampshire were typically warfarin-resis- being a relatively recent arrival and its association with tant with some resistant to difenacoum and bromadiolone human activities, the species can survive independently in also, while rats in Sussex were mostly anticoagulant- the UK, but large (damaging) populations seem to occur susceptible (Cowan et al. 1995); resistance to anticoagu- only on working farmsteads with abundant resources. lants has also been found in North Yorkshire rats. Popula- While much of their food may be found indoors, rats tion size was estimated immediately before a rodenticide prefer to nest in burrows dug in undisturbed ground treatment using a calibrated tracking plate technique (Quy between buildings or along adjacent hedgerows and et al. 1993). Briefly, tracking plates were evenly spread ditches. Control measures seem to be particularly across each site at a nominal density of 400/ha. The area of successful if they intercept rats before the animals reach each plate covered with rat footprints was given a score their food source and the most extreme form of intercep- and summing the scores gave an index of total rat activity. tion is to dispense poison bait directly into the burrow A mean of 3–4 consecutive daily total scores was then (Quy et al. 1996). However, active burrows are not always converted via a calibration curve to give an estimate of rat easy to find and rats are also highly mobile (Taylor and population size. Quy 1978), such that some burrows may be 1 km or more The ranging behaviour of rats was recorded between away from food sources. 1994 and 1996 on two farms in the county of Warwickshire Controlling Norway rats presents similar challenges to as part of a study investigating the role these rodents play as achieving successful control of other commensal rodents. vectors of cryptosporidiosis (Quy et al. 1999). Adult rats of The most practicable technique on farmsteads is to use >300 g living in and around cattle sheds and those living rodenticide bait, but the animals live in a complex, three- along field margins on an arable farm 5 km away were fitted dimensional habitat where attractive food is, for weeks at a with radio-transmitters that also contained mortality time, virtually unlimited and where places to put the bait sensors. Each tagged rat was tracked intensively for a week may be restricted. Under such circumstances, bait may be approximately every 4 weeks until the animal died, was totally ignored while damage and contamination continue lost, or the tag came off. Fixes were obtained every 6 hours unabated. However, the food supplies can also ‘disappear’ and a home range was calculated by the minimum convex suddenly and there may be little time to intensify baiting polygon method for each rat with >10 fixes. Every effort before the rats disperse. Harbourage is always available, was made to recover animals that had apparently died in although it often varies in quantity, quality and distance from order to determine the cause of death. Anticoagulant treat- food sources. Control measures are usually instigated on ments were carried out periodically on the livestock farm farms when the population reaches some arbitrary nuisance by the farm staff, but no control was carried out along the or damage threshold, but it is only when such measures fail field margins on the other farm. that the number of rats which can be supported by farmstead habitats is shown to greatly exceed the numbers previously The rate of recovery of 20 rat populations in the considered to be too high. As rat population density county of Surrey in south-eastern England was monitored increases, it sometimes becomes impossible to lay the at 2-monthly intervals for approximately one year required amounts of bait and efficacy is reduced or treat- between 1976 and 1979. Each population was reduced by ments prolonged. In this paper, we will consider the ecolog- an acute rodenticide and, where necessary, an anticoagu- ical factors that affect the efficacy of rodenticide use, in lant to eliminate as far as possible every rat.