Introduction

The process by which parasitoids select their hosts can be viewed in two different, but complementary, ways. The first seeks to understand the proximate cause of events by which parasitoids select hosts (the ‘how’ questions). This involves behavioral and physiological studies that show how the parasitoid interacts with its environment to locate its hosts. The second way in which to view host selection is to understand the ultimate reasons why a parasitoid selects a host (the ‘why’ questions). This involves studies that show how different strategies influence the parasitoid's reproductive success and, therefore, the Darwinian fitness of the searching parasitoid. Thus, the researcher aims to understand why the parasitoid chooses a particular host type among those that are available.

Our aim in this chapter is to emphasize the ‘why’ of host selection by parasitoids and how this understanding can aid in identifying suitable natural enemies for biological control. It can also aid in creating conditions within a crop that maximize their success. We will also emphasize that a successful understanding of the ultimate causation of host selection requires a thorough knowledge of both ‘how’ parasitoids select their hosts and the natural history of the system under study. To illustrate this point, we focus on two parasitoid systems of practical importance, the Trichogramma–lepidopteran system and the Aphytis–red scale system.