Habitat manipulation in lucerne (Medicago sativa l.): Strip harvesting to enhance biological control of insect pests

Habitat manipulation by strip harvesting as a means of enhancing biological control of insect pests was tested in a commercial field of lucerne. Vacuum sampling was used on five dates to measure the population densities of the predators Dicranolaius bellulus, Coccinella transversalis and spiders, as well as the lepidopteran pests Helicoverpa spp. and Merophyas divulsana. Predator population densities were consistently and significantly greater in strips with tall regrowth than in more recently harvested strips on all dates for C. transversalis and for all but one date in the case of D. bellulus and spiders. Densities of pest larvae did not differ between these treatments during periods of low pest abundance. In late January to mid-February, however, larvae were more numerous in unharvested strips. For both Helicoverpa spp. and M. divulsana, the high larval densities 11 days after a harvest were not reflected in samples taken 7 days later, in which densities had declined significantly. Catches of M. divulsana pupae on the latter date were also low in both treatments, suggesting that the fate of lepidopteran larvae, so numerous in the unharvested treatment on the previous sample date, was attributable to predation rather than to pupation. This conclusion is supported by the observation that the rate of predation of Helicoverpa spp. egg 'baits' was significantly greater in unharvested strips with dense predator populations than in harvested strips where predators were less dense. Similarly Helicoverpa spp. egg parasitism by Trichogramma spp. was significantly higher in unharvested strips than in harvested strips. Predator populations were higher in the strip harvest experiment than in a contiguous area of the same crop in which conventional harvesting was used. In the 'conventional' area, densities of Helicoverpa spp. larvae were higher but no such effect was apparent for M. divulsana. The present study supports previous findings, showing that natural enemies exploit unharvested strips as refuges, and that enhancing the within-field community of natural enemies by strip harvesting contributes towards pest management.