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Keywords:

  • intercropping;
  • trap crops;
  • attractants;
  • repellents;
  • interception barriers;
  • resource concentration hypothesis;
  • appropriate/inappropriate landing theory;
  • push-pull strategy;
  • host-plant finding

Abstract

Companion plants grown as ‘trap crops’ or ‘intercrops’ can be used to reduce insect infestations in field crops. The ways in which such reductions are achieved are being described currently using either a chemical approach, based on the ‘push-pull strategy’, or a biological approach, based on the ‘appropriate/inappropriate landing theory’. The chemical approach suggests that insect numbers are reduced by chemicals from the intercrop ‘repelling’ insects from the main crop, and by chemicals from the trap-crop ‘attracting’ insects away from the main crop. This approach is based on the assumptions that (1) plants release detectable amounts of volatile chemicals, and (2) insects ‘respond’ while still some distance away from the emitting plant. We discuss whether the above assumptions can be justified using the ‘appropriate/inappropriate landing theory’. Our tenet is that specialist insects respond only to the volatile chemicals released by their host plants and that these are released in such small quantities that, even with a heightened response to such chemicals, specialist insects can only detect them when a few metres from the emitting plant. We can find no robust evidence in the literature that plant chemicals ‘attract’ insects from more than 5 m and believe that ‘trap crops’ function simply as ‘interception barriers’. We can also find no evidence that insects are ‘repelled’ from landing on non-host plants. Instead, we believe that ‘intercrops’ disrupt host-plant finding by providing insects with a choice of host (appropriate) and non-host (inappropriate) plant leaves on which to land, as our research has shown that, for intercropping to be effective, insects must land on the non-host plants. Work is needed to determine whether non-host plants are repellent (chemical approach) or ‘non-stimulating’ (biological approach) to insects.