Food sources

Several species of parasitoid wasps and hoverflies require nectar and/or pollen as food to provide energy and protein for egg maturation (Schneider, 1948; Jervis and Kidd, 1986). On the basis of this, it is reasonable to assume that an increased availability of suitable flowers in or near crops can increase natural enemy effectiveness (Powell, 1986). Hence, a number of studies have tried to identify suitable flowering plants to include in field boundaries to attract parasitoids and predators (Zandstra and Motooka, 1978; Lovei et al., 1993; Hickman et al., 1995; Cowgill et al., 1993). A number have been able to establish that the presence of suitable flowering plants has increased the incidence of natural enemies (Molthan and Rupert, 1988; von Klinger, 1987). Few, however, demonstrate subsequent reduction in pest populations. Numbers of the bean aphid Aphis fabae in sugar beet plots drilled with tansy leaf (Phacelia tamacetifolia) between rows or at plot edges were reduced by syrphids compared with populations in control plots (Sengonca and Frings, 1989).

Manipulation of habitats can improve the availability of alternative hosts or prey for important natural enemies. Natural enemies that are not host or prey-specific will search for alternative food or hosts in the absence of the pest prey or host species. If these alternative sources of prey are not found in the vicinity of the cropping system then there will be a tendency for the natural enemies to disperse away from the crop. This may mean that when the pest population in the crop starts to increase, the natural enemies that could potentially reduce the rate of growth are not within the crop vicinity and a delayed return, perhaps later in the season, may be too late for them to significantly affect the pest population. The presence of a secondary prey source has been shown to be important in a number of cases. Anthrocorid bugs (Onus majusculus) which are used for the control of western flower thrips (Frankliniella occidentalis) have been shown to benefit from the presence of alternative prey species in situations where thrips are scarce (Brodsgaard and Enkegaard, 1997). In rice systems, the egg parasitoids Anagrus spp. and Oligosita spp., which are important control agents for rice planthoppers, use some delphacids as alternative hosts. The delphacids survive in non-rice habitats such as field bunds, irrigation canals and roadsides (Yu et al., 1996). Also in rice systems, Settle et al. (1996) hypothesized that the abundance of alterna tive detritivore prey for generalist predators give these natural enemies a 'head start' on later developing pest populations leading to a stability in rice ecosystems by decoupling predator populations from a strict dependence on herbivore populations. The early appearance of the parasitoid Anagrus epos of the grape feeding leafhoppers Erythromeura spp. was explained by companion planted prune trees being a refuge for an alternative host for the parasite, the prune leafhopper Edwardsiana prunicola (Pickett et al., 1990). There are probably many such examples of such relationships; the difficulty is identifying them and then being able to manipulate the habitat in such a way as to make effective their use.

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