Impact of cocoon predation and parasitism on endemic populations of the common pine sawfly, Diprion pini (L.) (Hymenoptera, Diprionidae) in different forest types

摘要

1. Predation and parasitism on litter-buried cocoons of the common pine sawfly Diprion pini (L.) were compared in different forest types with endemic sawfly populations by field exposure of laboratory-reared cocoons during three consecutive years (1993-1995). 2. The impact of cocoon predation was dependent on season and forest type. The highest predation (up to 95 percent) was found during autumn in forest stands with a dense understory vegetation. 3. Cocoon parasitism varied between year, season and forest type. The highest parasitoid attack was observed in pure pine forests with more or less barren soils, but did not exceed 24 percent of exposed cocoons. 4. Cocoons were exposed in small patches. Predators tended to exploit all cocoons of a patch, whereas parasitoids only attacked a few cocoons of a patch. Predation was similar on cocoons placed in the litter and those buried more deeply in the soil, whereas parasitism of soil-buried cocoons was rare. 5. These results indicate that predators canhave a remarkable potential for limiting endemic sawfly densities, if habitat conditions in a forest maintain their population and support their foraging behaviour. A notable effect of parasitoids on sawfly cocoons deposited in the litter is obviously restricted to typical pure and barren pine forests, but may play there a similar role as predation.