Dietary specialization and infochemical use in carnivorous arthropods: testing a concept

For the location of hosts and prey, insect carnivores (i.e., parasitoids or predators) often use infochemical cues that may originate from the host/prey itself but also from the food of the host/prey, a food plant, or another feeding substrate. These cues can be either specific for certain host/prey complexes or generally present in various complexes, and the reaction of the carnivores to these cues is either innate or learned. According to the concept on dietary specialization and infochemical use in natural enemies, the origin and specificity of the infochemical cues used and the innateness of the behavioural response are dependent on the degree of dietary specialization of the carnivore and its host/prey species. This concept has been widely adopted and has been frequently cited since its publication. Only few studies, however, have been explicitly designed to test predictions of the concept. Thus, more than 10 years after publication and despite of its broad acceptance, the general validity of the concept is still unclear. Using data from about 140 research papers on 95 species of parasitoids and predators, the present literature study comparatively scrutinises predictions from the concept. In accordance with the concept, learning to react to infochemicals and the use of general host and host plant cues was more often found in generalists than in specialists. In addition, more specialists were using specific infochemicals than generalists. In contrast to the concept, however, there was no significant difference between specialists and generalists in the proportion of carnivore species that use infochemicals during foraging and also extreme generalists are using infochemical cues for foraging. Likewise, an innate reaction to infochemicals was found in both specialists and generalists. Several reasons why infochemical use, including an innate reaction to infochemicals, is adaptive in generalist carnivores are discussed. We conclude that the concept has been a useful paradigm in advancing the chemical ecology of arthropod carnivores, but needs to be modified: the use of infochemicals is expected in all arthropod carnivores, regardless of dietary specialization.