Seasonal biology of drosophila suzukii

Drosophila suzukii is an agricultural pest originating from South-East Asia that recently invaded Europe and the Americas and threatens the worldwide fruit industry. The findings of Aurore Panel have direct implications for current pest management strategies. 

Panel: 'This highly polyphagous insect can produce 5–15 generations per year, and it infests a wide range of fruit crops as well as many wild host plants. The females use their serrated ovipositor to insert eggs in the flesh of ripening fruits. By feeding on the pulp, the larvae damage the fruits that then become unmarketable. In addition, D. suzukii is characterized by a high phenotypic plasticity which allows it to tolerate thermal fluctuations associated with seasonal changes. When the development of juvenile stages takes place in autumn, the adult flies emerge with a winter morphotype (WM) characterized by a strong abdominal pigmentation and a large body size. In contrast, when the development takes place in summer, the flies are smaller and emerge with a lighter summer morphotype (SM). These morphological differences are accompanied by complex physiological and behavioural differences that allow D. suzukii to optimize its reproduction and/or survival according to its environment.

Most current control strategies are based on the application of synthetic broad-spectrum insecticides to eliminate D. suzukii adults. This chemical protection is often supplemented by cultural measures including sanitation, mass-trapping of the pest and reduction of harvest intervals. The repeated application of pesticides not only disrupts existing Integrated Pest Management (IPM) programs, it also endangers the health of consumers and growers and causes the development of resistance of the pest to some active classes of pesticide ingredients. Moreover, chemical control is limited to the cultivated environment and does not encompass the wild habitats which serve as refugia for the pest. The practical implementation of biocontrol strategies is still at a very early stage. Therefore, there is a strong need to rapidly increase our understanding of the factors that influence the population growth, reproductive success, and survival of D. suzukii. With this knowledge, an IPM strategy can be developed to protect fruit crops from D. suzukii and to biologically control D. suzukii populations. The findings of my PhD project are presented in this dissertation and may contribute to designing such an IPM programme. The specific aim of my experimental work was to investigate the key life history traits that govern the establishment and population dynamics of D. suzukii in north-western Europe. I particularly focused on (1) D. suzukii population structure; (2) survival strategies and population dynamics in winter and early spring; and (3) life history strategies and phenotypic differences between the two seasonal morphotypes. 

My combined results revealed that local winter survival and maintenance of D. suzukii populations were widespread in invaded areas from north-western Europe. They also showed that the key starting point for annual seasonal population buildup were D. suzukii females that overwintered at sites in which D. suzukii had reproduced before winter. In fact, it was predominantly the overwintered D. suzukii females that infested the earliest ripening cherries, not their offspring from early spring hosts. Thus, I showed that these females led to the development of the first generation of D. suzukii summer morphs and the population peak recorded in late spring. Interestingly, I observed that prevailing environmental conditions affected the developmental programme of the insect pest, which led to different life-history strategies depending on the time of year. SM flies were characterized by a high reproductive output and a low mortality, especially during the reproductively active period of life. They could capitalize fully on arising breeding opportunities and sustained these for several months, but they were vulnerable to cold stress, especially SM males. WM flies, in contrast, showed less fluctuations in age-specific risks of mortality, and had a substantially lower total reproductive output. They could cope well with extended periods of cold stress and a lack of mates after cold exposure, but at a cost to their reproductive potential. Finally, I found that temperature tolerance of D. suzukii was primarily influenced by adult temperature regime in both sexes with a minor effect of juvenile thermal conditions for cold tolerance only. My findings also showed strong plasticity effects in cuticular hydrocarbon (CHC) profiles in both sexes in response to environmental temperatures experienced at the adult stage. In contrast, morphological traits such as wing size and tibia length were mostly determined by juvenile thermal conditions. Interestingly, in males, abdominal pigmentation was primarily influenced by the thermal conditions experienced during juvenile development whereas in females it was modulated by both developmental and adult temperatures. These results go against the commonly accepted view that the SM and WM of D. suzukii are separate entities since I showed that the different traits typical of each morph vary independently in their response to temperature. For some traits, the critical period for determining the trait value is during the juvenile, for some during the adult stage, and a few are affected by both.

These findings have direct implications for current pest management strategies such as the Sterile Insect Technique (SIT), the release of natural enemies, the application of pesticides or bait sprays, etc. since the success of all these density reduction methods is determined by the timing of the intervention, which is directly influenced by the biology and ecology of the pest.'

Dissertation: http://hdl.handle.net/11370/de9b57b4-77b2-4e1a-91d5-3684d3b908be