Factors Influencing the Population Dynamics ofBrevicoryne brassicaein Undersown Brussels Sprouts

Abstract

Intercropping combines a commercial crop with a cover crop to reduce pest species abundance and make chemical control less necessary. Although this system has a long tradition, the underlying mechanisms remain partly obscure. In this study, Brussels sprouts (Brassica oleracea gemmifera L. cv. Kundry F₁) were undersown with ryegrass (Lolium perenne L. cv. Surprise). Half of the plants were grown in open containers to reduce rhizosphere competition in intercropped plots. The population dynamics of the cabbage aphid (Brevicoryne brassicae L.) were monitored on all plants. Plants inoculated with aphid colonies were transplanted to the field and used to assess the impact of natural enemies in relation to vegetation diversity. Intercropping Brussels sprouts significantly reduced densities of cabbage aphids. Winged females mainly colonized plants growing in control plots but also intercropped plants at the border of intercropped plots. Containerization of plants had neither an effect on the population levels nor an effect on the colonization of winged females. Plant quality did not play a crucial role during initial colonization and subsequent population increase. The hoverfly, Episyrphus balteatus DeG., or the parasitoid, Diaeretiella rapae M'Intosh, showed different numerical responses to increased densities of aphids in the two treatments. It is concluded that a complex interaction of several factors, i.e. dispersion, microhabitat condition, or natural enemies, accounts for reduced pest populations over time, at least in the field system tested.