Absence of odour-mediated avoidance of heterospecific competitors by the predatory mite Phytoseiulus persimilis

Arthropods use odours associated with the presence of their food, enemies a nd competitors when searching for patches. Responses to these odours theref ore determine the spatial distribution of animals, and are decisive for the occurrence and strength of interactions among species. Therefore, a logica l first step in studying food web interactions is the analysis of behaviour of individuals that are searching for patches of food. We followed this ap proach when studying interactions in an artificial food web occurring on gr eenhouse cucumber in the Netherlands. In an earlier paper we found that one of the predators of the food web, the predatory mite Phytoseiulus persimil is Athias-Henriot, used to control spider mites, discriminates between odou rs from plants with spider mites, Tetranychus urticae Koch, and plants with spider mites plus conspecific predators. The odours used for discriminatio n are produced by adult prey in response to the presence of predators, and probably serve as an alarm pheromone to warn related spider mites. Other pr edator species may also trigger production of this alarm pheromone, which P . persimilis could use in turn to avoid plants with heterospecific predator s. We therefore studied the response of the latter to odours from plants wi th spider mites and 3 other predator species, i.e. the generalist predatory bug Orius laevigatus (Fieber), the polyphagous thrips Frankliniella occide ntalis and the spider-mite predator Neoseiulus californicus (McGregor). Bot h olfactometer and greenhouse release experiments yielded no evidence that P. persimilis avoids plants with any of the 3 heterospecific predators. Thi s suggests that these predators do not elicit production of alarm pheromone s in spider mites, and we argue that this is caused by a lack of coevolutio nary history. The consequences of the lack of avoidance of heterospecific p redators for interactions in food webs and biological control are discussed .