Functional response of a generalist insect predator to one of its prey species in the field

1. The shield beetle Cassida rubiginosa Muller (Coleoptera, Chrysomelidae) is one of the most prominent herbivores on creeping thistle, Cirsium arvense (L.) Scop. and is considered as a biological control agent against this weed. In order to investigate if augmented densities of this species could be sustained, predation on C rubiginosa larvae was studied in the field with the aid of continuous video surveillance of individual prey during three seasons at two different study sites. 2. The paper wasp Polistes dominulus Christ (Hymenoptera, Vespidae) was responsible for 99.4% predation on C rubiginosa larvae at both study sites. Prey densities were always reduced to a few individuals at the sites after a few days only. Thus, P dominulus may annihilate the effect of augmentative releases of the beetle as a biocontrol agent. 3. P dominulus showed no aggregation in patches of high host density (numerical response). The observed density dependence was attributed to the functional response of the wasp. 4. P dominulus showed a Type III functional response. In mark-and-recapture experiments we found no indication that this type of functional response was the result of single wasps specializing on this particular prey type. 5. Type III functional responses in generalists can result by switching to prey species which become temporarily abundant. Since in a natural setting a mixture of prey species coexists in different abundance and which are differentially accepted by generalist predators, it is imperative to study their functional response in the field.