Behavioural studies and prey enrichment studies

Research programmes aimed at evaluating the effectiveness of natural enemies should ideally start with behavioural studies to immediately identify important predators/parasitoids and their consumption/parasitism rates (Luck, 1990). However, this is rarely done and if carried out at all is left until the programme is well established.

Behavioural studies can be carried out in semi-natural conditions in the laboratory and can provide important information about predator behaviour (Marks, 1977b; Gardner and Dixon, 1985) but it is impossible to entirely recreate field conditions in the laboratory and this may lead to important mechanisms being overlooked. Field studies provide the opportunity to identify some of the underlying behavioural and ecological mechanisms involved in the process of predation and parasitism; as an approach, however, it does offer some formidable experimental problems.

Field studies of insect behaviour are highly labour intensive and data only accrue slowly because of the difficulties involved in observing small invertebrates at low densities and often also at night. High densities of the predator have been used in field arenas to overcome the problems of trying to observe the beetle Agonum dorsale at night (Griffiths, 1982; Griffiths et al., 1985). Red torch light was also used to aid observations after laboratory experiments had established that it did not affect the behaviour of the beetle. The problems associated with observations of small highly mobile Hymenoptera are even more horrendous, although Waage (1983) at least showed that it was possible using artificially infested plants and two observers with binoculars. Waage (1983) collected and dissected the larvae of Plutella xylostella observed during the experiment to determine how many had been parasitized. Similar collections and dissections of artificially infested insects have been used to determine levels of parasitism (Way and Banks, 1968) and are generally referred to as prey enrichment studies. They usually work best with nonmobile stages such as eggs but care must be taken to arrange them in realistic positions on host plants (Luck et al., 1988).

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