Plant competition in pest-suppressive intercropping systems complicates evaluation of herbivore responses

In the light of current theories on the effects of intercropping on pest reduction, population responses of the diamondback moth (Plutella xylostella), the cabbage aphid (Brevicoryne brassicae) and the life history traits of the large white butterfly (Pieris brassicae) were studied in a Brussels sprout (Brassica oleracea gemmifera)/malting barley (Hordeum vulgare) additive row intercrop and a Brussels sprout monoculture. More P. xylostella adults were caught in the monoculture than in the intercrop. Numbers of P. xylostella larvae and pupae per sprout plant were lower in intercropped plots than in monocultures. However, more larvae and pupae were found per m2 leaf area in the inter- than in the monocrop. Both the densities per plant and per m2 leaf area of B. brassicae populations were lower in the inter- than in the monocrop. After the barley withered and competition with Brussels sprout abated, aphid densities became higher in the inter- than in the monocrop. These findings may be explained by interspecific plant competition resulting in stressed sprout plants with a smaller size and delayed phenology relative to monocropped plants. Effects of differences in plant nutritional quality on herbivore performance were studied by offering leaves of inter- and monocropped sprout plants to larval P. brassicae. Performance and food utilisation were significantly better on leaves from the intercrop (lower dry weight consumption, higher growth rates) than from the monocrop. Defoliation rate was also higher on leaves of intercropped plants than on monocropped ones. The studies indicate that plant stress and consequent changes in developmental rate and nutritional quality of plants are playing a role in herbivore population responses to intercropping. It is argued that such confounding effects of plant competition in intercropping designs can hamper the evaluation of herbivore responses in pest-suppressive agro-ecosystems.