We assessed the effect of plant patch shape and surrounding vegetation on the density, emigration, and immigration of predatory coccinellids, and on the density of their aphid prey Brevicoryne brassicae (L.). Between spring 1997 and fall 1999, we set up square and I-shaped patches of Brassica oleracea Plenck surrounded by Medicago sativa L. or Allium porrum L. Medicago sativa is frequently used by coccinellids, whereas A. porrum is not. We used a factorial (2 × 2) randomized block design, and evaluated the density of coccinellids and aphids every 10 d. We also evaluated emigration and immigration of adult coccinellids through mark-recapture experiments. We quantified the population increase of aphids, and the final live mass of plants. All insects were more abundant in patches surrounded by A. porrum than in those surrounded by M. sativa, and coccinellids were occasionally more dense in square patches than in I-shaped ones. Coccinellids emigrated less from square patches, either surrounded by A. porrum or M. sativa, and immigrated more to patches surrounded by A. porrum. Aphids showed a higher population increase, and plants of B. oleracea ended up being heavier in patches surrounded by A. porrum, particularly in I-shaped patches. Surrounding vegetation and plant patch shape seem to have a direct effect on the density of coccinellids by modifying their immigration and emigration patterns, but also seem to have an indirect effect by changing plant growth and its effect on herbivore recruitment.
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