The Ecology of Gastrophysa polygoni (L.) (Coleoptera: Chrysomelidae) in Cereals.

Abstract

The aims of this project were to investigate the biology of Gastrophysa polygoni in cereals. Assessments were to be made of the natural mortality factors affecting the different developmental stages and of mortality due to agricultural practices. Also to study the inter-relationships between the beetle and its host plant and to assess how these relationships (with the knowledge of the beetle's biology and its habitat) determine its spatial distribution and abundance between cereal fields.

During routine field sampling, information was gathered on the phenology of the beetle over eight generations in three years on three field sites. There were large losses in the numbers of the developmental stages within a generation, losses tended to be higher in the first than in the second generation. Aspects of the biology of oviposition, fecundity, longevity and duration of development were investigated.

In the laboratory G.polygoni larvae survive most well, developmental time is shortest and fecundity was highest on Polygonum aviculare and Polygonum convolvulus. In the field, P. aviculare was the preferred host plant. Other species of the Polygonaceae were not acceptable host plants. Deteriorations in host plant condition due to senescence and herbicide applications were reflected in poor rates of larval survival.

A series of relative mortalities were calculated for the developmental stages of G.polygoni. Mortalities during the second-larval instar were the highest and during the third-larval instar the lowest. Mortality during the egg stage was found to be the key factor in determining population trend. Mortalities during the egg stage were not density dependent. Predator exclusion experiments in the field showed that predation by ground-living, polyphagous, entomophagous insects was mainly responsible for the large losses of eggs and larvae. The effects of waterlogging, infertility, larval cannibalism and the effects of parasites and pathogens were considered relatively unimportant.

G.polygoni was susceptible to a range of agricultural practices in cereal fields. The spraying of herbicides and fungicides, harvesting and straw burning were harmful to populations.

Proportionally more egg batches were found when the density of the host plant was relatively high and a threshold weed density was found below which oviposition did not take place. The crop rotation pattern and straw burning also affected the spatial distribution and abundance of G.polygoni. Reasons for the apparent decline of G.polygoni over the last 40 years in the context of changing patterns in agricultural practice and predictions for the possible future success of this species are discussed.