A behavioural study to help clarify how undersowing with clover affects host‐plant selection by pest insects of brassica crops

Abstract

Laboratory and field‐cage tests were done to determine how undersowing brassica plants (Brassica oleraceae L. and B. rapa L.) (Cruciferae) with subterranean clover (Trifolium subterraneum L.) (Papilionaceae) affected host‐plant selection by eight pest insect species of brassica crops. The pest species tested were Pieris rapae (Lepidoptera: Pieridae) (the small white butterfly), Pieris brassicae (Lepidoptera: Pieridae) (the large white butterfly), Delia radicum (Diptera: Anthomyiidae) (the cabbage root fly), Phaedon cochleariae (Coleoptera: Chrysomelidae) (the mustard beetle), Plutella xylostella (Lepidoptera: Yponomeutidae) (the diamond‐back moth), Evergestis forficalis (Lepidoptera: Pyralidae) (the garden‐pebble moth), Mamestra brassicae (Lepidoptera: Noctuidae) (the cabbage moth) and Brevicoryne brassicae (Hemiptera: Aphididae) (the cabbage aphid). In all tests, except two in which the brassica plants were about three times as high as the clover background, 39%–100% fewer of the pest insect stage monitored were found on host plants presented in clover than on those presented in bare soil. Contrary to claims supporting the ‘enemies hypothesis’, differences in colonization alone appeared sufficient to account for the lower numbers of insects found when host plants are undersown with clover. To be effective in reducing plant colonization, the clover must cover 50%, and preferably more, of the vertical profile of the crop plants. As clover used as an undersown crop often has to be cut to make it less competitive with the main brassica crop, temporal aspects of the condition of the clover during critical periods of pest activity need to be recorded carefully before concluding that undersowing does not produce the effect desired against certain pest species under field conditions. The effective clover barrier is like any other treatment, if it is not present at the appropriate time it cannot be expected to reduce pest insect numbers.

Host‐plant selection by insects – a theory based on ‘appropriate/inappropriate landings’ by pest insects of cruciferous plants

“…To prevent this happening, the test plants in the experiments of Finch & Kienegger (1997) were left in their pots throughout the test periods. As the effects of the clover were still evident and often within minutes of starting an experiment (Finch & Kienegger, 1997), a generic mechanism based on the non-host plants (here clover) causing physiological changes in the host plants (Theunissen, 1994) cannot be supported.…”

“…As the effects of the clover were still evident and often within minutes of starting an experiment (Finch & Kienegger, 1997), a generic mechanism based on the non-host plants (here clover) causing physiological changes in the host plants (Theunissen, 1994) cannot be supported.…”

“…However, a mechanism relying solely on physical obstruction is not supported by the recent findings of Finch & Kienegger (1997). These authors used clover plants, which they desiccated so that the clover plants retained the same architecture as the living plants and differed only by being brown rather than green.…”

“…The numbers of eggs laid when the cabbage root fly (Delia radicum), the diamond-back moth (Plutella xylostella), and the large white butterfly (Pieris brassicae) were presented with host plants surrounded by desiccated (brown) clover did not differ from the numbers laid on host plants presented in bare soil. Hence, the physical presence of the clover was not sufficient on its own to reduce the numbers of eggs laid, a reduction occurred only when the surrounding clover was green (Finch & Kienegger, 1997).…”

Effects of agricultural diversification on the abundance, distribution, and pest control potential of spiders: a review

“…It is also established that diversification might influence agroecosystems through various modes of action. Diversification can impede pests' host plant finding behaviour (Root, 1973;Wratten & van Emden, 1995;Finch & Kienegger, 1997;Finch & Collier, 2000), and it can affect crop yields directly ( Theunissen & den Ouden, 1980;Tukahirwa & Coaker, 1982). Diversification might also impinge on pests indirectly, through boosting their natural enemies, such as spiders.…”