3 Foraging strategies of parasitoids in complex chemical environments Nicole Wäschke 1 , Torsten Meiners 1 and Michael Rostás 2 1 Department of Applied Zoology/Animal Ecology, Freie Universität Berlin, Germany 2 Bio-Protection Research Centre, Lincoln University, Canterbury, New Zealand Chemical Ecology of Insect Parasitoids, First Edition. Eric Wajnberg and Stefano Colazza. © 2013 John Wiley & Sons, Ltd. Published 2013 by John Wiley & Sons, Ltd. Abstract Parasitoids of herbivorous insects need to locate their hosts in complex chemi- cal environments. Such complexity arises from a plethora of volatile and non- volatile chemical compounds produced by the hosts, by their host plants, and by the surrounding vegetation. To complicate the issue, biotic and abiotic factors that affect hosts and habitat can lead to further variability of chemicals in both space and time. In this chapter, we investigate the chemical complexity encoun- tered by parasitoids while searching for suitable hosts, and their foraging strat- egies to cope with this complexity. First we review parasitoid responses to vegetation and vegetation odour diversity at a large scale. At a small scale, we then focus on the chemical complexity of the host plants’ variability and its influence on parasitoid foraging. At the level of parasitoid host recognition and acceptance, the variability of chemicals and complex interactions between host plants and host cues are explored. Finally, we review how parasitoids use their behavioural, sensory and neurophysiological adaptations to successfully locate their hosts in heterogeneous environments, and how their foraging strategies are shaped by different life history traits. 3.1 Introduction Insect parasitoids need to find suitable hosts for reproduction, otherwise their genes will not be passed on to future generations. This crucial process of locating a host can be divided