Environmental and physiological correlates of response by three fruit fly (Diptera: Tephritidae) species to commercial lures

Abstract

Traps are important tools for managing economically damaging pest fruit flies (Diptera: Tephritidae). Trap captures help fruit and vegetable producers to monitor fruit fly populations and shape their pest management action thresholds. However, biotic, and abiotic factors influence how fruit flies respond to attractive stimuli used in traps. Physiological changes within a fly can change its resource orientated behaviours. In a similar manner, environmental changes can limit or encourage insect activity that governs this behavioural response. Consequently, baited traps only catch subsets of the population under certain weather conditions. Understanding how fruit fly lure responses change is important for improving interpretation of trap captures with direct benefits for fruit fly monitoring and management programmes. In this dissertation, I aimed to test how fruit fly physiology, the environment, and lure weathering impact lure response. I assessed the response of three economically important fruit fly species to several commercial fruit fly lures. To do this I released known numbers of flies in field cages, which enabled unambiguous measurement of the relative response of species to lures under the experimental conditions. The response of Ceratitis capitata (Wiedemann) was tested for BioLure, E.G.O PheroLure and trimedlure. The response of C. cosyra (Walker) was tested for BioLure and E.G.O PheroLure, while the response of Bactrocera dorsalis (Hendel) was measured for BioLure and methyl eugenol. For each species and lure combination, response by a group of flies (n=25) was determined in a semi-field setting in relation to sex, diet (protein deprived and protein rich), and age (two-, 10-, and 20-days post emergence). The effect of nutrient reserves (protein, lipid, and carbohydrate content), and body weight on lure response was also quantified using colorimetric biochemical assays adapted from van Hendel’s method. The environmental effects of temperature (°C), relative humidity (%RH), and light intensity (lux) on lure response were also measured. Additionally, the effects of temperature and relative humidity on the weathering of the lures BioLure, E.G.O PheroLure, methyl eugenol and Capilure (trimedlure mixed with extenders) was evaluated in a field setting. The. The weight of each lure was measured at daily intervals for 30 days to determine weight loss of each lure product in relation to daily mean temperature and relative humidity. Additionally, I tested the effect that semiochemical pre-release feeding, diet (protein rich and iii protein deprived), and age (four-, 10-, and 20-days post emergence) had on the response of B. dorsalis males (n=20) to methyl eugenol baited traps in a field cage setting. Semiochemicals were fed to flies 48 hours prior to field cage testing, with methyl eugenol, eugenol, or no semiochemicals (control) being tested. Semiochemicals were mixed into the relevant diet treatment food at a concentration of 1.5%. I further assessed the effects that body weight and total body nutrient composition (protein, lipid, and carbohydrate content) had on the response of B. dorsalis to methyl eugenol. Body nutrient composition data was obtained with the revisited van Hendel’s method which uses colorimetric biochemical assays. The effect that temperature (°C), relative humidity (%RH), and light intensity (lux) had on the response of B. dorsalis to methyl eugenol lures was also measured. Overall, the effect of fly physiology and the environment on response is specific to the species and lure tested. Despite BioLure being considered a female biased lure there was no consistent bias towards female captures. Only in C. capitata were protein deprived females more likely to respond to BioLure. Irrespective of species, access to dietary protein decreased the response to BioLure baited traps. This suggests that abundant natural sources of protein may decrease the accuracy of fruit fly monitoring that relies on BioLure as the main attractant. Age affected the response of flies to male lures. Immature C. capitata and C. cosyra responded more strongly to E.G.O PheroLure and trimedlure. However, immature B. dorsalis were the least responsive to the male lure methyl eugenol. I found that the effects of temperature on response are largely lure and species specific. The response of B. dorsalis increased at warmer temperatures irrespective of the lure tested. Similarly, the response of flies to male lures increased with temperature. However, I observed that the response of C. capitata and C. cosyra to BioLure is not affected by temperature. Warmer temperatures resulted in greater lure weight loss for all tested lures, but this occurred to varying degrees. Combining the results of lure volatilisation and fly response, I identified that the increased response of C. capitata males to trimedlure at warmer temperatures is mostly driven by increased fly activity. Whereas increased response of C. capitata and B. dorsalis to E.G.O PheroLure and methyl eugenol, respectively, at warmer temperatures coincides with greater release rates of the lures. These results show that monitoring conducted when temperatures are cooler is likely to underestimate fruit fly populations. Feeding on either methyl eugenol or eugenol reduces the response of B. dorsalis males to methyl eugenol baited traps by similar amounts. Access to dietary protein had little effect iv onthe reduced response by males to methyl eugenol when fed semiochemicals. This suggests that feeding semiochemicals to B. dorsalis males may facilitate the simultaneous application of the sterile insect technique and the male annihilation technique. Furthermore, this shows that protein supplementation will not counteract the methyl eugenol response suppression induced by pre-release semiochemical feeding. These results also show that eugenol is a promising alternative to methyl eugenol to achieve the required reduction in response to methyl eugenol. Overall, this dissertation highlights that fly physiology and the environment significantly impact the response of fruit flies to commercially used lures. Specifically, access to natural sources of protein may decrease the accuracy of trap surveys that rely on BioLure. The minimal response of flies occurring at temperatures below 12 °C indicates that trapping surveys deployed below this will largely misrepresent population sizes and are likely to miss incipient fruit fly incursions. It is thus crucial to integrate fruit fly physiology and environmental variables into fruit fly monitoring and survey programmes to increase their accuracy. My results further indicate that eugenol is an effective and safe semiochemical that can be used in joint MAT-SIT programmes. I also observed that protein supplementation and semiochemical pre-release feeding can occur simultaneously in MAT-SIT programmes without increasing the response of B. dorsalis to methyl eugenol. Overall, this dissertation highlights the complex nature of tephritid lure response and the value that quantifying and integrating these effects into monitoring and control programmes can have on fruit fly management.