Effects of undersown clover on host-plant selection by Thrips tabaci adults in leek

The suppressive effects of undersown clover on Thrips tabaci Lindeman infes tation in leek is known but not the stages in thrips population dynamics th at are affected by intercropping and the mechanisms involved. Colonization or settling of adult onion thrips (T. tabaci) in monocropped leek (Allium p orrum L.) and in leek intercropped with strawberry clover (Trifolium fragif erum L.) was studied in two potted plant experiments. Potted leek plants wi th and without undersown clover were placed for a short period (2 and 5 day s) in monocropped or intercropped field plots when high thrips infestation was expected. Thrips adults were counted on all leaves and in the shaft dur ing this observation period. Thrips populations were monitored weekly in th e field plots throughout the entire growing season. In both potted plant experiments, there were consistently fewer thrips adul ts on intercropped leek plants than on monocropped plants. The plant growth parameters - number of leaves and stem diameter - were similar in all trea tments and thus did not explain differences in thrips adults. Natural enemi es were absent on both leek and clover, and thus cannot explain the differe nces in adult thrips numbers. Furthermore, no adults of T. tabaci were foun d on the clover in the potted plant experiment and only very few in the fie ld experiment. Thus it was concluded that strawberry clover was not an effe ctive trap crop for thrips. In the second potted plant experiment, clover was removed just before the l eek plants were introduced to the field, thus eliminating direct physical, visual and olfactory interference by clover. After the undersown clover was removed, the leek plants harboured only one-third of the number of thrips adults, as compared to the monocropped plants. This study supports the noti on that there are subtle links between intercropping and plant quality, and indicates that host-plant quality is an underlying cause of the reduction in adult thrips numbers in leek/clover intercropping.