PATTERNS, MECHANISMS AND SPATIAL SCALE OF AGGREGATION IN GENERALIST AND SPECIALIST PREDATORY MITES (ACARI, PHYTOSEIIDAE)

Prey species often distribute themselves patchily in their habitats. I n response to this spatial variation in prey density, some predator sp ecies aggregate in patches of higher prey density. This paper reviews a series of laboratory experiments to demonstrate the patterns of resp onses by phytoseiid predators (Phytoseiulus persimilis, Typhlodromus o ccidentalis and Amblyseius andersoni) to spatial Variation in the dens ity of their spider mite prey (Tetranychus urticae) and reveal the beh avioural mechanisms underlying the observed patterns. In addition, pat terns of aggregation were examined at a variety of spatial scales on p lants in greenhouses. The patterns, mechanisms and spatial. scale of a ggregation in three predatory species are discussed in relation to the ir Varying degrees of polyphagy. The results show that a specialist pr edator species (1) aggregates more strongly than generalist predators, (2) does so not because it finds prey patches of high density more ea sily but because it remains in these patches longer than generalist pr edators and (3) tends to aggregate more often at lower levels of spati al scale than generalist predators. It is suggested that these conclus ions, based mainly on laboratory studies of a small sample of species, should be tested in the future on a wider selection of specialist and generalist species at different scales in the held.