The role of egg predation in the population dynamics of Gastrophysa polygoni (Coleoptera) in cereal fields.

Abstract

From 1977 to 1979 counts were made of the numbers ( m^-2) of G. polygoni (L.) found in cereal fields in southern England and age-specific life tables prepared for each generation. In six of the eight generations, mortality was highest during the egg stage and the key factor operated on this age class. Second-instar larval mortality was the next most important factor. Between generations there was a significant positive correlation between egg mortality, expressed as a k-value, and the estimated total population of the stage, but formal proof of density dependence was not obtained. Egg mortality varied from 21 to 74%, but no parasites or pathogens were found attacking the eggs. The percentage of infertile eggs was never high and cannibalism (by first-instar larvae) was not important. In the laboratory, eggs were taken from plants by a range of predators such as Tachyporus spp. (Staphylinidae), Agonum dorsale (Pont.) and Demetrias atricapillus (L.) (Carabidae). Egg mortality was reduced significantly when polyphagous predators were excluded from areas of the crop and predation, in particular by Carabidae and Staphylinidae, was considered by far the most important egg mortality factor.