Individual and combined effects of the generalist Forficula auricularia and the specialist Episyrphus balteatus on Dysaphis plantaginea - are two predators better than one?

The interactions between specialist and generalist natural enemies could affect the success of biological pest control, especially in augmentation programmes. The aim of this laboratory study was to quantitate the individual and combined effects of the generalist predator Forficula auricularia L. (Dermaptera: Forficulidae) and the specialist Episyrphus balteatus De Geer (Diptera: Syrphidae) on the population growth of the rosy apple aphid (RAA), Dysaphis plantaginea Passerini (Hemiptera: Aphididae). The experiment was carried out using apple seedlings with five levels of initial RAA infestation. In most cases, both predator species (alone and in combination) were able to reduce RAA densities or to keep them at their initial level. This suppression effect was greater when RAA densities were low and resulted in near‐complete mortality of the RAA colony after 4–6 days. Without predators, RAA population size almost doubled within 10 days, whereas with predators, RAA population size was reduced by about 20 (alone) and 50% (in combination). Although RAA numbers in the two‐predators treatment were ca. 81% of RAA numbers in the one‐predator treatment for pooled numbers across dates and densities, the difference between the two treatments was not significant. Some predator mortality occurred with first and second instars of the syrphid larvae and only at the two lowest initial RAA infestation levels. This mortality may be explained by inadequate food supply, intraguild predation (IGP) (in 13.3% of the possible cases), or both. However, the incidence of IGP under natural conditions could be even less frequent, as both predators were never observed on the same leaf.