Interactions Between the Chrysomelid Beetle Gastrophysa viridula, the Weed Rumex obtusifolius and the Herbicide Asulam

Abstract

The effects of separate and simultaneous grazing by Gastrophysa viridula Degeer and herbicide treatment (asulam) on Rumex obtusifolius L. and the influence of asulam on G. viridula were investigated. Asulam had little influence on the survival or rate of development of G. viridula when used as a contact agent on eggs, first-instar larvae and adults. The ingestion of asulam-contaminated leaves reduced the fecundity of female beetles by 64% and the rate of development of the whole generation by 4 days. The presence of asulam in R. obtusifolius leaves had no significant influence on the oviposition and feeding site selection of gravid beetles. In the field, there were fewer second generation G. viridula on asulam-treated plants than on untreated plants; the total, shoot and root dry weights of asulam-treated plants were significantly lower than those of untreated plants. Grazing significantly reduced the dry weight of asulam-free plants but did not further reduce the dry weight of asulam-treated plants. Plant survival was unaffected by grazing or low herbicide levels. Approximately 60% of plants survived treatment with high levels of asulam. The stage of beetle development when the host plants were sprayed was important for the continued survival of G. viridula. It is suggested that the density of beetles and the time of their release in relation to the spraying date of dock with asulam may be manipulated to improve the efficiency of any dock control programme.