Effect of plant patch shape and surrounding vegetation on the dynamics of predatory coccinellids and their prey Brevicoryne brassicae (Hemiptera : Aphididae)

We assessed the effect of plant patch shape and surrounding vegetation on t he density, emigration, and immigration of predator) coccinellids. and on t he density of their aphid prey Brevicoryne brassicae (L.). Between spring 1 997 and fall 1999, we set up square and I-shaped patches of Brassica olerac ea Plenck surrounded by Medicago sativa L. or Allium porrum L. Medicago sat iva is frequently used by coccinellids, whereas A. porrum is not. We used a factorial (2 x 2) randomized block design, and evaluated the density of co ccinellids and aphids every 10 d. We also evaluated emigration and immigrat ion 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 nl. sativa, and coccinellids were occasionally more dense in square patches than in I-shaped ones. Coccinellids emigrated less from squ are patches, either surrounded by A. porrum or M. sativa, and immigrated mo re to patches surrounded by A. porrum. Aphids showed a higher population in crease, and plants of B. oleracea ended up being heavier in patches surroun ded by ii. porrum, particularly in I-shaped patches. Surrounding vegetation and plant patch shape seem to have a direct effect oil the density of cocc inellids by modifying their immigration and emigration patterns, but also s eem to have an indirect effect by changing plant growth and its effect on h erbivore recruitment.