Vegetation management in organic cabbages and pitfall catches of carabid beetles

Abstract

Intercropping has potential for reducing dependence on insecticides in vegetable crops. One reason for its effectiveness may be that it increases the numbers or activity of beneficial predators. Many carabid beetles are polyphagous predators which have been credited with playing an important part in suppressing some crop pests, and the aim of this work was to determine whether intercropping cabbages with white clover affected the pitfall catches of these beetles. Carabid catches from plots of cabbages intercropped with white clover were compared with catches from monocropped cabbage plots which had natural weed cover in 1993, and which were kept weed free by hoeing in 1994. Multivariate analysis separated the intercropped from the monocropped treatments in both years, on the basis of the occurrence and frequency of carabid species. In 1993, most of the species of carabid collected were found more frequently in plots with natural weed cover than in intercropped plots. In 1994, most species were collected in higher numbers in the intercropped plots while the developing clover sward gave an open vegetation cover, but once the clover cover in the intercropped plots was fully developed, some species seemed to be favoured by the weed free monocropped treatment, and others by intercropping. Only two species of carabid ( Pterostichus melanarius and Loricera pilicornis) were found in higher numbers in the intercropped cabbages in both years. It was concluded that carabid species differ in their responses to plant cover but that open, structurally diverse, vegetation is most likely to bring an increase in the activity of carabids in general.