Pest-predator spatial relationships in winter rape: implications for integrated crop management

The brassica pod midge (Dasineura brassicae) is an important and widespread pest of winter and spring oilseed rape throughout Europe. Pods infested by D brassicae larvae split prematurely, releasing seeds, and the larvae drop to the soil into which they burrow to pupate. At this stage in its lifecyc le D brassicae is potentially vulnerable to predation by carabid beetles fo raging on the soil surface. This is the first study in the UK to focus on c arabid beetles as predators of D brassicae in the oilseed rape crop. The sp atio-temporal distributions of larvae of D brassicae dropping to the soil f rom the crop canopy and of adult carabid beetles active on the soil surface were analysed in two consecutive years. Insect samples were collected from spatially referenced sampling points across each crop. Counts of insects w ere mapped and analysed, and the degree of spatial association between pred ator and prey determined using Spatial Analysis by Distance Indices (SADIE) . Carabid species abundant and active during peak drop of first generation D brassicae larvae included Agonum dorsale, Amara similata, Harpalus rufipe s and Nebria brevicollis. The larvae of D brassicae had a marked edge distr ibution within the crop. SADIE analysis revealed significant spatial associ ation between larvae of D brassicae and adult H rufipes (P<0.05) in 1998, b ut not with adults of A dorsale, A similata or N brevicollis. In 1999, ther e was strong spatial association only between larvae of D brassicae and adu lt A dorsale (P < 0.01). Aggregation of N brevicollis adults occurred in so me areas of greatest D brassicae larval counts in 1999, but overall spatial association was not significant. The distributions are discussed in terms of their relevance to integrated crop management (ICM) strategies and spati al targeting of insecticides. (C) 2000 Society of Chemical Industry.