13.1. Prey/host location
The foraging behaviors of insects, like all other behaviors, comprise a stereotyped sequence of components. These lead a predatory or host-seeking insect towards the resource, and on contact, enable the insect to recognize and use it. Various stimuli along the route elicit an appropriate ensuing response, involving either action or inhibition. The foraging strategies of predators, parasitoids, and parasites involve trade-offs between profits or benefits (the quality and quantity of resource obtained) and cost (in the form of time expenditure, exposure to suboptimal or adverse environments, and the risks of being eaten). Recognition of the time component is important, as all time spent in activities other than reproduction can be viewed, in an evolutionary sense, as time wasted.
In an optimal foraging strategy, the difference between benefits and costs is maximized, either through increasing nutrient gain from prey capture, or reducing effort expended to catch prey, or both. Choices available are:
- where and how to search;
- how much time to expend in fruitless search in one area before moving;
- how much (if any) energy to expend in capture of suboptimal food, once located.
A primary requirement is that the insect be in the appropriate habitat for the resource sought. For many insects this may seem trivial, especially if development takes place in the area which contained the resources used by the parental generation. However, circumstances such as seasonality, climatic vagaries, ephemerality, or major resource depletion, may necessitate local dispersal or perhaps major movement (migration) in order to reach an appropriate location.
Even in a suitable habitat, resources rarely are evenly distributed but occur in more or less discrete microhabitat clumps, termed patches. Insects show a gradient of responses to these patches. At one extreme, the insect waits in a suitable patch for prey or host organisms to appear. The insect may be camouflaged or apparent, and a trap may be constructed. At the other extreme, the prey or host is actively sought within a patch. As seen in Fig. 13.1, the waiting strategy is economically effective, but time-consuming; the active strategy is energy intensive, but time-efficient; and trapping lies intermediate between these two. Patch selection is vital to successful foraging.
(After Malcolm 1990)