Seasonal biology of <i>Drosophila suzukii</i>:Genetic and phenotypic variation in the Netherlands

Drosophila suzukii is an agricultural pest originating from South-East Asia that recently invaded Europe and the Americas and threatens the worldwide fruit industry. 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