INTERCROPPING AS A MANAGEMENT STRATEGY AGAINST CARROT RUST FLY (DIPTERA, PSILIDAE) - A TEST OF ENEMIES AND RESOURCE CONCENTRATION HYPOTHESES

Intercropping with lucerne, Medicago littoralis Rohne ex. Loisel, as a management strategy for carrot rust fly, Psila rosae (F.), was studie d to understand why intercropping results in lower damage. Densities o f polyphagous predators were experimentally enhanced and decreased usi ng ingress and egress plots. The manipulation method was successful fo r carabids and for spiders in the family Lycosidae and the order Opili ones. The method was less effective for staphylinids and linyphiid spi ders. These results are discussed in relation to carrot rust fly damag e and earlier experiments with abundance of polyphagous predators in c arrots, Daucus carota L., in monoculture and intercrop. In field exper iments, D-vac catches were used to measure the number of flies in 2 tr eatments arranged in 3 different ways. More flies were caught in the m onoculture treatment when plots were placed next to each other or plac ed apart from each other and surrounded with ley (i.e., nonhost) veget ation, but no difference was found between treatments when plots were surrounded by bare soil. In greenhouse experiments, egg deposition was lower in intercropping. These results are discussed in relation to da mage levels, which were always lower in intercropping. We also discuss our results with regard to the possible outcome of scaling up our sma ll plot experiments to full scale cropping practices.