The influence of host and non‐host companion plants on the behaviour of pest insects in field crops

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.

New Synthesis: Plant Volatiles as Functional Cues in Intercropping Systems

“…The data suggest that stem borer adults perceive and avoid VOCs that are part of the host plant's VOC bouquet but function as inducible repellants because they are associated with reduced food quality. While this supports the repellant VOC hypothesis, it remains unknown if these compounds prevent pests from landing (repellency, as used by Finch and Collier) or only from ovipositing, as measured in the push-pull study (Hasannali et al, 2008).Second, Finch and Collier (2012) suggest that intercrops hypothetically could function as physical barriers and thus "hide" the target crop from herbivores. However, in the push-pull system, the most commonly used repellent crop, D. uncinatum, does not overgrow maize, and by the time maize plants are suitable hosts for stem-borers, they have emerged prominently above the Desmodium intercrop canopy.…”

“…VOCs) act as long-range cues that result in a non-random movement/distribution of pest insects, with reduced densities on the main crop. Finch and Collier (2012) in a recent critique of the mechanism underlying the push-pull system, contrast the functional VOC hypothesis with appropriate/inappropriate landing theory, which assumes random herbivore movement and oviposition/consumption responses based on host plant stimuli that are experienced by insects after landing. In this view, all short-and long range physical and chemical plant characteristics are potentially important for host choice by herbivores, rendering the two hypothetical frameworks non-exclusive.…”

“…However, the actual mechanism by which intercropping works remains elusive, and the proposed plant-chemistry based hypothesis was recently challenged (Finch and Collier, 2012). A common view proposes that intercrops come in two functional groups, trap plants and repellent plants, which have chemical characteristics making them attractive or repellent to a certain insect pest, thereby reducing pest pressure on the main crop.…”

“…Second, Finch and Collier (2012) suggest that intercrops hypothetically could function as physical barriers and thus "hide" the target crop from herbivores. However, in the push-pull system, the most commonly used repellent crop, D. uncinatum, does not overgrow maize, and by the time maize plants are suitable hosts for stem-borers, they have emerged prominently above the Desmodium intercrop canopy.…”

“…First, Finch and Collier (2012) argue that given the vast diversity of plant VOCs, it would be maladaptive for herbivores to perceive an undefined number of non-host chemicals as repellant compounds. Although this certainly is a viable argument, it also is important to note that it may be sufficient for host-searching insects to perceive and learn volatiles associated with their hosts' quality.…”