Louise E.M. Vet
Department of Entomology, Wageningen Agricultural University, P.O. Box 8031, 6700 EH Wageningen, The Netherlands.
Since the early 80's evolutionary ecologists inspired the incorporation of a multitrophic and more evolutionary approach into the field of chemical ecology of insect-plant relationships. Traditionally the approach was to unravel mechanisms of stimulusresponse patterns for specific systems. Most insects were envisaged to respond to stimuli in an invariable fashion and a major emphasis was (and still is) on the elucidation of the origin and exact chemical identity of the chemicals involved. With regards to insect behaviour, most chemical ecologists appreciated the presumed consistency of behavioural outcome in their experimental animals. This in contrast to behavioural ecologists that value the fact that individuals differ, being aware that selection acts on this variation. Using optimality principles, behavioural ecologists are interested in how individuals make adaptive decisions. As many of these decisions are made on the basis of chemical information, collaboration of chemical and behavioural ecologists is fruitful and very promising. Hence, chemical ecologists are increasingly asking questions on the evolutionary significance (the 'why') of stimulus-response patterns in insects, and this evolutionary approach is complementing, not contrasting a causal-analytical one (the 'how'). Consequently, we see a greater focus on studying the evolutionary significance and mechanism of variability, both at the stimulus and at the response side. ln the field of parasitoid chemical ecology we see increasing attempts to go beyond the details of a specific system and to look for generalities that may lead to testable hypotheses to guide future research.
ln the present contributlin the setting is the dynamic triangle of plant, herbivore and natural enemy. My focus will be on the foraging behaviour of the natural enemy, which is guided by chemical information from the different trophic levels. l will address variability in stimulus and response and hypothesize on general patterns in the way semiochemicals are used by natural enemies. What is the relative importance of plant and herbivore cues and why? l will argue that continuous selection for inconspicuousness on herbivores to escape parasitization and predation has shaped the searching strategies of their natural enemies. How do searching natural enemies deal with variation in major foraging cues? The importance of differences in reliability of stimuli on the evolution of behavioural traits will be adressed. When and to which type of stimuli (host/plant derived) can we expect fixed responses and when can we expect selection for behavioural plasticity? Are searching strategies correlated to the animals diet breadth, its life history, the herbivore stage attacked? I will plead that a comparative approach, in combination with empirical behavioural studies is useful in answering such questions.
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