Postharvest quarantine treatment of fresh fruits is often required to overcome trade restrictions against fruit flies and other pests. The use of heat applied as hot air provides a suitable quarantine treatment for many products, particularly tropical fruits. This project investigated the response to heat of three critical Australian fruit fly pests, Bactrocera tryoni, B. jarvisi and B. cucumis. Bactrocera papayae, an introduced species present at the time, was included in some tests.
Initial studies on the development of these species under defined laboratory conditions showed that there was a constant and reproducible difference in the duration of embryonic development for each species. Furthermore, histological studies showed that embryonic development for each species proceeded at a proportional rate over this period. This information allowed experiments to be performed using embryos at comparable stages of physiological development which differed from chronological age.
Heat tolerance varied with fruit fly species: B. jarvisi being significantly more heat tolerant than other Australian tephritids tested, while B. cucumis was least tolerant. This result will be an important consideration in the development of quarantine heat treatments against Australian fruit flies. Fruit fly life stages showed variations in heat tolerance. Results of this research have identified the most tolerant life stages to heat treatment, including the most tolerant age within the egg stage in Bactrocera tryoni, B. jarvisi and B. cucumis. This information will allow quarantine treatments to be tested against the most tolerant age/stage.
Comparative studies of heat response between laboratory-reared and wild fruit flies showed that long periods of laboratory culture did not cause any significant increase in susceptibility to heat in lab-reared flies. In some cases, laboratory-reared insects were significantly more heat tolerant than their wild counterparts. The use of laboratory-reared insects to develop quarantine heat treatments will provide an effective treatment in a commercial situation so long as quality control in laboratory cultures is maintained.
Comparison of responses to heat of life-stages treated in fruit and in vitro showed that in vitro treatment was a good predictor of species responses in fruit. That is, the most heat tolerant species in fruit could be identified through in vitro water bath studies. However, in vitro treatment was not a good predictor of stage responses in fruit. This study showed that it may not be appropriate to assume that stage responses determined by hot water immersion (in vitro) studies will reflect the responses occurring in the practical (commercial) situation of treatment of eggs and larvae in fruit.
Incidental investigation of the technique of using solutions of sugar or salt to separate larvae from diet by flotation showed that there can be an interaction with heat treatment. Larvae that were collected by flotation in solutions of sugar or salt were more susceptible to heat treatment than those collected from water. Further testing should be undertaken before this technique is used to separate insects that will be challenged through the application of heat or other stressors.