- Adaptive Ovipisition Decisions in Parasitoids
- Arms Races and Local Adaptation
- Biological Control
- The Structuring of Communities and the Coexistence of Species
- Life History Trade-offs in Insect Parasitoids
- Sexual Selection and Sympatric Speciation
- Climate Change and the Evolution of Insect Life Histories and Reproductive Behaviour
- Nature Management
Adaptive Ovipisition Decisions in Parasitoids
Much of my research concerns adaptive oviposition decisions in parasitoids: After my thesis I continued to develop and test theory for adaptive superparasitism, to study patch time allocation, search modes, competitive modes and habitat choice in parasitoids.
Arms Races and Local Adaptation
Using a large number of populations of Asobara tabida and Drosophila melanogaster collected all over Europe, we studied the evolution of cellular immune responses by D. melanogaster to parasitoids and the evolution of virulence in the parasitoid. We showed that A. tabida is more virulent on D. melanogaster in southern Europe and that D.melanogaster is more resistant to A. tabida in southern Europe. This is evidence for antagonistic co-evolution and local adaptation between host and parasitoid.
From 1985, I was involved in research on the biological control of cassava mealybug in collaboration with the International Institute of Tropical Agriculture at Ibadan, Nigeria. Our research concerned an analysis of why Epidinocarsis lopezi was a successful natural enemy and why the coccinellid Diomus spec failed. Studies on sex allocation by E. lopezi showed why the mass rearing first produced 70% males, and provided the solution to obtain 65% females in stead.
This collaboration was continued with a project on the mango mealybug, Rastrococcus invadens. Applied biological control research on insect pests in Europe have included a study on biological control of citrus mealybug in glasshouses, control of iris mealybugs in bulb storage sites, and the biological control of Asparagus beetle with the parasitoid Tetrastichus asparagi, notably the development of mass rearing of the parasitoid.
The Structuring of Communities and the Coexistence of Species
With a grant from WOTRO for research in the tropics I started community ecological work on Drosophila in 1987 at Barrow Colorado Island, Panama in collaboration with the Smithsonian Institute.This work was continued until 2004 with 2 additional WOTRO-grants. The work showed that spatial heterogeneity and independent aggregation of species reduces the overlap between species to less than 2% and allows the coexistence of more than 30 species of Drosophila without a significant interspecific competition. Mortality caused by parasitoids does not play a role in Drosophila species coexistence, because temporal variation in fruit abundance together with longer generation times and lower fecundities make that parasitoid do not regulate Drosophila populations.
With a WOTRO grant I started in 1990 research on community ecology of rock-dwelling cichlid fish in Lake Victoria, Tanzania. The project provided evidence that niche segregation plays an important role in co-existence.
Together with the University of Lyon and Tours and the INRA laboratory at Antibes and several researchers at Leiden , I did research on the coexistence of sexual and a-sexual populations of Venturia canescens parasitoids, showing that differences in habitat choice between sexual and a-sexual populations allows their coexistence.
Life History Trade-offs in Insect Parasitoids
A comparison of populations of A. tabida from northern and southern Europe showed that southern populations allocate more energy to egg production and northern populations more to longevity. This difference can be explained by differences in the abundance and spatial distribution of the hosts. A similar study along a climatic cline of Asobara persimilis in Australia showed no differences in allocation, but a higher metabolic rate and nocturnal activity in populations from a hot and dry environment, with a lower metabolic rate in populations from hot, but humid climates.
Sexual Selection and Sympatric Speciation
To explain the radiation of haplochromine cichlids in Lake Victoria, we started research on the hypothesis that sexual selection for male coloration can result in reproductive isolation between closely related colour morphs, which can then evolve divergent ecological adaptations. We could prove that (1) females do select partners on the basis of colour, (2) females prefer more brightly coloured males over less brightly coloured and hence (3) that selection can be divergent (4) That brightly coloured males are more resistant to parasites and, hence (5) that females select males for good genes. We could also show why species split into a red and a blue species. In addition we showed that (6) sex ratio selection against a gene for superfemaleness results in the evolution of preference and that (7) reproductive isolation can evolve within 50 generations. This research has been granted by several WOTRO-project grants and an ALW- programme-grant.
Climate Change and the Evolution of Insect Life Histories and Reproductive Behaviour
Since my appointment in Rennes, my research focuses of how climate change will affect the evolution of behaviour and life history in parasitoids: Like all ectotherms, metabolic rate of parasitoids is determined by the ambient temperature. Predicted climatic changes in Western Europe involve an increase in average temperatures, both in winter and summer. In addition a higher frequency of the occurrence of heat waves, draughts, storms and flooding has been predicted. In contrast to other organisms parasitoids lack the ability to store excess energy in the form of lipid reserves. This lack of lipogenesis makes them totally dependent on the lipids that they have acquired from the host during the larval state. A consequence of an increase in metabolic rate at higher temperatures will be that parasitoids will discount the lipid reserves faster. This will allow them to be more active, but at the expense of a shorter adult lifespan, a lower fecundity and/or a smaller adult body size. Hence, climatic change will change life history constraints and select for a shift in the trade-offs that determine the optimal reproductive decisions of parasitoids. Optima and trade-offs will shift. We have developed theory to predict how trade-offs will shift, and to explain why hot and dry climates select for higher metabolic rates and are doing empirical research to test the theory.
Together with Dr. Pim Arntzen, Curator at the Netherlands Centre for Biodiversity, Naturalis, I started to study biodiversity of amphibians in relation to landscape in northern France.
From July to December 2010, I was member of the International Committee on the Management of Large Herbivores in the Oostvaardersplassen (ICMO). This committee advised the Dutch government on management and animal welfare of large mammals in nature reserves. I also advised the City Council of Amsterdam on the management of fallow deer in the dunes used by the Amsterdam Water Supply Company.