The effect of increased crop diversity on colonisation by pest insects of brassica crops

摘要

Many researchers have shown that the numbers of pest insects found on crop plants are reduced considerably when the crop is allowed to become weedy, when the crop is intercropped with another plant species, or when the crop is undersown with a livingmulch. Laboratory and field-cage experiments were done to determine how undersowing brassica plants (Brassica oleracea spp., cabbage, cauliflower, Brussels sprout and Brassica chinensis) with subterannean clover (Trifolium subterraneum) affected host plant selection by eight pest insect species of brassica crops. Experiments were done also to determine the effect on egg-laying by the cabbage root fly (Delia radicum) on brassica plants surrounded with plant species other than clover. A total of 24 'companion' plant species were tested, including 1) bedding plants, to provide a different range of plant architectures and leaf colours; 2) weeds; 3) aromatic plants such as rosemary and thyme and 4) companion plant species such as marigold. After studying how pest insects behave in undersown brassica crops, we suggest it is simply the number of green objects surrounding the host plants that is the major factor that prevents the pest insects from finding their host-plants. Hence, increasing plant diversity within brassica crops should help considerably to reduce pest insect numbers.