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In this chapter, we consider practical aspects of the foraging behaviour of insect natural enemies in its widest sense. Initially, most insect natural enemies must locate the habitat where potential victims may be found. Within that habitat, the victims themselves must be discovered. Once a patch of potential targets is identified, the predator or parasitoid must choose its victim. Furthermore, in judging host quality, a parasitoid must decide whether to feed from the host, to oviposit, or to do both. If she does decide to oviposit, then there are questions of sex allocation and offspring number that need to be addressed (Figure 1.1). All of these activities fall under the aegis of ‘foraging behaviour’.

This chapter is concerned with approaches and techniques used in studying those aspects of parasitoid and predator life-cycles that are relevant to the topics covered by other chapters in this book. To illustrate what we mean, consider the female reproductive system of parasitoids, discussed in some detail in section 2.3. As pointed out by Donaldson and Walter (1988), at least some knowledge of its function, in particular of the dynamics of egg production, is crucial to a proper understanding of foraging behaviour in parasitoids. The state of the ovaries may determine: (a) the duration of any preoviposition period following eclosion; (b) the rate of oviposition, (c) the frequency and duration of non-ovipositional activities, e.g. host-feeding; and (d) the insect’s response to external stimuli, e.g. odours, hosts (Collins and Dixon, 1986) (subsection 1.5.1). Note that egg load (defined in subsection 1.2.2) is now often incorporated into foraging models, as it is becoming increasingly clear that key foraging decisions depend upon the insect’s reproductive state (Jervis and Kidd, 1986; Mangel, 1989a; Chan and Godfray, 1993; Heimpel and Rosenheim, 1995). It also follows from the above that a female parasitoid’s searching efficiency depends upon the functioning of its reproductive system and this may in turn influence parasitoid and host population processes (Chapter 7).

Genetical research on insect natural enemies is rare and basically limited to parasitoid wasps and some coccinellid beetles. To our knowledge, no genetic studies have been done on other wellknown groups such as parasitoid flies, robberflies, scorpion flies or mantids. Even among the well-studied parasitic Hymenoptera, genetics has largely been neglected. For example, sex allocation strategies (see 1.9) have been intensively studied in this group, but hardly anything is known about the underlying genetic mechanism of sex determination. In many organisms there is geographic variation in the expression of biological traits, and in parasitic Hymenoptera such variation has been reported for sex ratio, host preference, oviposition strategies, superparasitism, diapause induction, virulence, developmental time, and other traits (see subsection 6.5), yet despite this broad suggestive evidence for the existence of genetic variation for many traits, relatively little is known about the underlying genetics.

In sexually reproducing animals, reproduction entails insemination (the transfer of the male’s sperm to the female) and fertilisation (the fusion of a sperm and an egg to create a diploid zygote). In many insect species, including predators and parasitoids, insemination and fertilisation are temporally separated, from minutes or hours to years (as in many social insects). The term “mating behaviour” refers to the behavioural events surrounding insemination, which ensure successful sperm transfer by the male and uptake by the female as well as, in many species, post-copulatory male behaviours that have evolved in response to sperm competition (Parker, 1970a). Alexander et al. (1997) divided mating behaviour into: pair formation, courtship, copulation, insemination and the events immediately following insemination, including temporary pair-maintenance, and we discuss mating behaviour mainly according to these divisions. Our discussion also includes components of mate competition and fertilisation. A typical sequence of mating behaviour, from the search for mates up to insemination and mate-guarding is shown in Figure 4.1. Figure 4.2 represents the behaviour of the parasitoid wasp Cotesia rubecula (Braconidae) as a specific example.

The term ‘mating system’ is used to describe how males and females obtain mates in a population (Emlen and Oring, 1977; Thornhill and Alcock, 1982; Davies, 1991; Brown et al., 1997). A particular mating system may be characterised by the events surrounding pair formation, courtship, copulation and the postcopulatory events (Brown et al., 1997). Individual males and females engage in reproductive behaviours that maximise their own fitness, frequently to the detriment of their mates. Evolutionary biologists have come to regard events surrounding mating as a set of intrasexual and inter-sexual ‘battles’ which reflect the sometimes common, sometimes differing, reproductive interests of males and females (e.g. Davies, 1991; Brown, et al. 1997; Choe and Crespi, 1997; Alonzo and Warner, 2000).

In nature, any particular habitat contains animal and plant species which exist together in both time and space. Many of these species will interact with each other, for example when one species feeds on another or when two species compete for the same food or other resource. A group of species having a high degree of spatial and temporal concordance, and in which member species mutually interact to a greater or lesser extent, constitute a community (Askew and Shaw, 1986). The size and complexity of a community will depend upon how broadly that community is defined. For example, we could consider as a community the organisms which interact with each other within a particular area of woodland, the herbivore species which compete for a particular food plant or the complex of natural enemies associated with a particular prey or host species. Here, we are especially interested in communities of natural enemies, which are often surprisingly species-rich (Carroll and Risch, 1990; Hoffmeister and Vidal, 1994; Settle et al., 1996; Sunderland et al., 1997; Memmott et al., 2000).

The reasons for studying the population dynamics of insect natural enemies are basically twofold. Firstly, predators and parasitoids are an important component of terrestrial communities (LaSalle and Gauld, 1994), so therefore are of central interest to the ecologist who attempts to unravel the complexity of factors driving the dynamics of species interactions. Secondly, the knowledge gained from studies of predator and parasitoid populations may be of immense practical value in insect pest management (Hassell, 1978, 2000b; DeBach and Rosen, 1991; Van Driesche and Bellows, 1996).

This chapter considers ways in which phytophagy by parasitoids and predators may be studied, particularly with respect to conservation biological control programmes that involve the manipulation of insect natural enemies by means of supplemental food provision. Many insect natural enemy species, at some stage during their life cycle, exploit plant materials in addition to invertebrate materials, and the nutrients obtained from the former affect growth, development, survival and reproduction (subsections 2.7.3 and 2.8.3).