Population Phenology of the Tropical Fruit Fly, Bactrocera Tryoni
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Population phenology of the tropical fruit fly, Bactrocera tryoni (Froggatt) (Diptera: Tephritidae), in Queensland, Australia W. Sakuntala Nayanatara Muthuthantri, B.Sc. (Agriculture) (Hons) Submitted in fulfilment of the requirements for a Master of Applied Science School of Natural Resource Sciences Queensland University of Technology Brisbane, Australia 2008 Key words Bactrocera tryoni, breeding, Dymex, insect, integrated pest management, population phenology, climate, larval host, over-wintering i Abstract Bactrocera tryoni, the Queensland fruit fly, is established along the entire Australian east coast. It is a major pest of horticulture and arguably the worst horticultural insect pest in Australia. Adult flies lay eggs into fruit and resultant larvae feed on the flesh of the fruit. The population biology of B. tryoni has been well studied in temperate regions, where it has been established that climatic factors, particularly temperature and rainfall, limit population growth. In contrast, in subtropical and tropical regions, the population dynamics of the fly have been little studied. This thesis investigates the fly’s phenology and abundance changes across subtropical and tropical Queensland and asks what factors govern the population cycles of B. tryoni in this state. Winter breeding and abundance of the fly, a component of the seasonal cycle which in south-east Queensland is fundamentally different from that observed in temperate Australia, is also investigated. A historical, extensive multi-year and multi-site trapping data set with from across Queensland was analysed to look at the effects of temperature, rainfall and relative humidity on B. tryoni trap catch. Trap data was further compared with the predicted phenology data generated by a DYMEX® based B. tryoni population phenology model. The phenology model used was based on a previously published model, but was also modified to more explicitly look at the effects of host plant availability and the presence or absence of non-reproductive over-wintering flies. Over-wintering field cage studies and a winter-spring field trapping study, both carried out in Brisbane, supplied additional data on B. tryoni’s population abundance and capacity to breed during winter in the subtropics. Results show significant variation of monthly fly abundance for nine sites across Queensland. Abundance changed across sites in non-predictable ways. Annual population phenology within a site was, for some sites, highly consistent from year to year, but inconsistent for other sites. All sites in the subtropics showed some form of population depression during the cooler months, but breeding was continuous, albeit ii reduced at nearly all sites. Some tropical sites, where the climate is regarded as highly favourable for B. tryoni, still showed dramatic peaks and troughs in annual population abundance. There were relatively few significant correlations observed between weather factors and fly populations for any site. Output from the DYMEX population model suggested that fruit availability is a major driver of population dynamics in the tropical north of the state, while weather is more important in the subtropical south. The population dynamics of B. tryoni at sites along the central Queensland coast, where it is assumed that a mix of both weather and host fruit availability drive local populations, were poorly captured by the population model. Field cage results showed that B. tryoni successfully bred during winter in Brisbane, with pupal emergence starting in mid-winter (1st week of August), peaking in early spring (2nd week of September). Trap catch at orchards in Brisbane increased with increasing temperature and fruit availability, but diminished with decreasing temperature and fruit availability. The results suggest that B. tryoni has an optimal climate for population growth in the tropics, but fruit availability for offspring production limits population growth. In the subtropics however, both climate and fruit availability determine the population size. Winter temperatures are marginal for B. tryoni population growth in the subtropics. iii Table of Contents Keywords i Abstract ii Table of contents iv List of figures viii List of tables xi Declarations xiii Acknowledgements xiv Chapter 1: General Introduction 1 1.1 Introduction 1 1.2 Fruit flies as agricultural pests 1 1.3 Bactrocera tryoni (Queensland fruit fly) 2 1.4. Temperature effects on B. tryoni physiology and ecology 4 1.4.1 Acclimatization of B. tryoni with respect to temperature 4 1.4.2 Developmental acclimation process of B. tryoni 5 1.4.3 Post-teneral acclimation process of B. tryoni 5 1.4.4 Temperature and B. tryoni survival 6 1.4.5 Survival rate of B. tryoni in warming environments 7 1.4.6 Survival rate of B. tryoni in cooling environment 7 1.4.7 Temperature effect on B. tryoni longevity 9 1.4.8 Ecological aspects in relation to physiological adaptation of B. tryoni to temperature 9 1.5. Bactrocera tryoni reproduction 11 1.5.1 Temperature effect on B. tryoni gamete development 11 1.5.2 Temperature effect on mating of B. tryoni 13 1.5.3 Indirect effects of temperature on B. tryoni reproduction 13 1.5.4 Ecological relevance of B. tryoni reproduction with temperature 14 iv 1.6. Effect of moisture on population abundance of B. tryoni 15 1.6.1 Effect of moisture on physiology of adult B. tryoni 15 1.6.2 Effect of moisture availability on B. tryoni breeding 16 1.6.3 Effect of moisture on B. tryoni immature stages 17 1.6.4 Ecological impacts of moisture on B. tryoni 17 1.7. Host fruit and B. tryoni populations 19 1.8. Dispersion and B. tryoni abundance 20 Chapter 2: Bactrocera tryoni Phenology in Queensland 24 2.1 Introduction 24 2.2 Methodology 25 2.2.1 Study area 26 2.2.2 Data collection 26 2.2.3 Inter-site comparisons of mean monthly B. tryoni abundance 28 2.2.4 Intra-site comparisons of monthly B. tryoni abundance 29 2.3 Results 30 2.3.1 Inter-site comparisons of mean monthly B. tryoni abundance 30 2.3.2 Seasonal abundance across sites 31 2.3.3 Population fluctuation and weather 36 2.3.4 Seasonal phenology across site 37 2.3.5 Within site year to year weather fluctuation 39 2.3.6 Within site year-to-year population fluctuation 40 2.3.7 Relationship between within-site population fluctuations and weather 40 2.4 Discussion 44 2.4.1 Year to year population phenology of B. tryoni 44 2.4.2 Impact of temperature 45 2.4.3 Role of rainfall and humidity 46 2.4.4 Host availability 47 v Chapter 3: DYMEX Modelling of B. tryoni in Queensland 50 3.1 Introduction 50 3.2 Methodology 52 3.2.1 Fruit incorporated as a constant factor (Model 1) 52 3.2.2 Host considered as a variable (Model 2) 53 3.2.3 Simulation runs 59 3.2.3.1 Data files 59 3.2.3.2 Initialisation of model 62 3.2.4 Observed data compared with model prediction 621 3.3 Results 63 3.3.1 Fruit effect set as a constant (Model 1 results) 63 3.3.2 Fruit effect set as a variable (Model 2 results) 66 3.3.3 Correlation between predicted and observed results 69 3.4 Discussion 71 3.4.1 Population growth and abiotic variables (Model 1) 72 3.4.2 Population growth and model two 74 Chapter 4: Over-wintering Biology of B. tryoni in South-east Queensland 77 4.1 Introduction 77 4.2 Methodology 79 4.2.1 Field cage experiment 79 4.2.2 Winter to spring abundance of B. tryoni in an orchard environment 84 4.2.3 Dymex modelling with special relevance to winter reproduction 84 4.3 Results 86 4.3.1 B. tryoni survival and reproduction in field cages 86 vi 4.3.2 Over wintering of B. tryoni in natural orchard habitat 90 4.3.3 Population fluctuation of B. tryoni predicted from Model 1A and Model 2A 90 4.4 Discussion 95 4.4.1 Over-wintering physiology of B. tryoni breeding 95 4.4.2 B. tryoni population abundance in winter in subtropical habitats 96 4.4.3 Model modification with over-wintering physiology of B. tryoni 97 Chapter 5: Discussion 99 5.1 Summary of results: Factors affecting the population phenology of B. tryoni in subtropical and tropical Queensland 99 5.1.1 Climatic effect 100 5.1.2 Host effect 101 5.1.3 Winter effect 102 5.2 Further research 103 5.2.1 Relationship between host and B. tryoni behaviour 103 5.2.2 Improving the population dynamics model for B. tryoni 104 References 105 vii List of Figures Figure 1.1: Female Bactrocera tryoni (Froggatt) [Photo by P. Zabrowski] 3 Figure 2.1: Mean (+SE) monthly abundance of Bactrocera tryoni at nine sites in Queensland. Columns surmounted by the same letter are not significantly different at p = 0.05 30 Figure 2.2: (a) Population phenology of male (closed circle) and female (open circle) Bactrocera tryoni across nine sites in Queensland, Australia, with (b) corresponding long term temperature (closed triangle) and rainfall (vertical bar) averages for the same sites. 1= Cairns, 2 = Atherton, 3 = Ayr, 4 = Maryborough, 5 = Rockhampton, 6 = Toowoomba, 7 = Gatton, 8 = Sunnybank, 9 = Stanthorpe. The number of years data contributing to any one site is given in Table 2.1. 35 Figure 2.3: Annual phenology of combined teneral male and female (closed triangle), non-gravid female (open circle) and gravid female (closed circle) Bactrocera tryoni populations in nine locations in Queensland. Within a graph, each plot presents the monthly proportional number of flies only of that age class, not the proportion of that age class as a proportion of all age classes combined.