Abstract
The damage caused by insect pests on cabbage plants is often reduced when planted together with cover crops; the reduction is accompanied by an increase in the population of ground-dwelling predatory natural enemies including carabid beetles. However, whether the increase of carabid beetles is caused by the existence of the cover crop itself or by the occurrence of lepidopteran pests on cabbage plants and/or cover crops is not yet clear. The effect of the removal of lepidopteran larvae was examined by spraying selective insecticides on the populations of ground-dwelling predatory natural enemies and using a pitfall-trapping method in cabbage fields with cover crops planted together. The carabid Chlaenius micans was the most abundant and the carabid Dolichus halensis was the next abundant natural enemy in spring cabbage fields. Both species were significantly more abundant in unsprayed plots than in sprayed plots. This may imply that the removal of lepidopteran larvae leads to the reduction of its predator through food deficiencies. The earwig Labidura riparia was the most abundant natural enemy in autumn cabbage fields. However, there was no difference in the population of any natural enemy species between the unsprayed and sprayed plots. This may be due to low population densities of lepidopteran larvae in the unsprayed plots, although they were higher than those in the sprayed plots.
