Direct measurement of host and parasitoid recruitment for assessment of total losses due to parasitism in a continuously breeding species, the cabbage aphid, Brevicoryne brassicae (L.) (Hemiptera: Aphididae)

Abstract

AbstractIn studies in Massachusetts, the population density of Brevicoryne brassicae (L.) was measured over the lifespans of two groups of kale leaves from maturation to senescence, taking into consideration aphid spatial pattern (in colonies or as isolated insects) and aphid size as large (adults), medium-sized (nymphal instars 2–4) or small (first-instar nymphs) individuals. Total aphid densities per leaf in both leaf groups showed similar patterns of initial increase followed by a decrease as leaves aged over the 3–4-week period of leaf survival. Aphid densities reached peak values of 8·14 and 8·64 aphids per leaf for the two leaf groups studied, and 65–67% of all aphids observed occurred in colonies. Host and parasitoid recruitment to the aphid and the parasitoid immature populations were measured using a modification of the technique of Van Driesche & Bellows (1988). Total host recruitments per leaf were 43·7 and 64·6 aphids for the first and second leaf groups. Parasitoid recruitment was 6·8–8·1 for the first leaf group and 8·2–15·8 for the second. Recruitment values indicated 15·6–18.6% parasitism for the aphid cohort on the first leaf group and 12·7–24·4% for that on the second one.