These studies have indicated the importance of carabid predation in particular situations and against particular prey types. These beetle are generally polyphagous and the predator/prey interactions studied include such diverse prey as cutworms (Best & Beegle, 1977 and Brust et al. 1985, 1986), codling moth larvae (Riddick & Mills, 1995), aphids (Dixon & McKinlay, 1992; Holopainen & Helenius, 1992 and Ekbom, 1994), armyworms (Clark et al. 1994), colorado beetle (Cappaert et al. 1991; Heimpel & Houghgoldstein, 1992), cricket eggs (Griffith & Poulson, 1993), hemipteran eggs (Filippitsukamoto et al. 1995), slugs (Symondson & Liddell, 1995) and snails (Digweed, 1993). The papers quoted are a small sample of an extensive literature, most of which concentrates on the role of these beetle as epigeal predators. If one seeks information on their role below the soil surface it is much more difficult to find. Chaabane et al.(1993) has shown that carabids will feed on earthworm flesh in the laboratory and the gut content analysis (Holopainen & Helenius, 1992) has shown that collembola are ingested although these may not have been true soil dwelling species. The searching and foraging behaviour of carabids has been studied on the soil surface by using pitfall traps in the corn fields (Best et al.1994), soyabean fields (House & All, 1981), cereals (Edwards et al. 1988) and potatoes (Armstrong, 1995).
In the laboratory and in the field, Griffith & Brown (1992) examined foraging effort by assessing the number of holes dug by carabids to reach a buried cricket egg. Using antigens, Symondson & Liddell (1995) showed that carabids could forage into the soil for slugs. Filippitsukamoto et al. (1995) reported that carabids were the first known predators of hemipteran eggs laid in the soil. Humphreys & Mowat (1994) reported that carabids were the predators of cabbage root fly.
A body of theory views predation from the perspective of an economic process consisting of benefits derived from capturing and consuming prey versus the cost associated with obtaining energy (energy expended to pursue, capture and subdue a prey). The most successful predator maximises its energy intake per unit time or minimises its time spent acquiring a given amount of energy (Malcolm, 1992) The latter strategy minimises the risk of predation to the searcher.
Prey items (species, age class and quality) are assumed to exist in patches and it is also assumed that these patches differ in prey density or quality. The forager exploiting and choosing patches in a manner that maximises energy intake or maximises its prey encounter rate.
The theory assumes that these rules, evinced as behaviour, are subject to natural selection and predators which possess the most effective set of rules (e.g. those that lead to maximising energy intake) are the most fit, ie, they contribute most to the reproducing gene pool of the next generation.
Predators are believed to be important agents of selection and to elicit an impresive variety of adaptations in prey. The criteria outlined above suggest that selection in favour of antipredatory traits can occur only when some members of a prey population survive to reproduce after being detected, that is, when predators are less than 100 per cent efficient at one or more stages of their interactions with prey.
Prey animals have available to them numerous ways of avoiding predation. Not all these ways are equally effective, nor are they appropriate for all phase of an encounter with a given predator.
For a given phase of predation, the definiton of efficiency of antipredatory defence is the number of encounters that result in prey survival during the phase in question divided by total number of encouters in that phase. The overall antipredatory effectiveness is the subjugation phase (Ives & Dobson, 1987).
In many situations, it is difficult to measure the relative abudance and efficiency of predators. Predator impact depends upon such unknowable quantities as the predators rate feeding, time of feeding, selectivity and rate of encounter with the prey. Some of these relationships could be reconstructed if predators victims can be recognised.
An alternative approach is to study predation by offering baits Chaabane et al. (1993) used earthworm flesh as baits for carabids in the laboratory. Carcamo (1994) employed fly pupae as a bait for carabids to observe pupal disappearance in some crops. Lys (1995) has used artificial prey to calculate predation rates of carabid and staphylinids in winter wheat. Tolonen (1995) used Drosophila melanogaster pupae as bait to measure predation rates.
If predators leave measurable or countable traces of their activity on unsuccessfully attacked victims, the proportion of the prey population that was exposed to non-lethal attack during the subjugation phase can be calculated.
