Abstract:

Studies on tritrophic interactions involving three cultivars of common cabbage, Brassica
oleracea var capitata, with varying levels of susceptibility to aphids, cv Minicole, (green
leaved) partially resistant with antibiosis mechanisms, Ruby Ball, (red-leaved) with
antixenosis factors, and Derby Day (green-leaved), susceptible, were conducted with two
aphid species, Myzns persicae (Sulzer), a generalist, and Brevicoryne hrassicae (L.), a
crucifer specialist and two hymenopteran parasitoids, Aphidius colemani (Viereck) and
Diaeretiella rapae (McIntosh).
Investigations on the olfactory response of A. colemani in a four-way olfactometer showed
that odours influenced A. colemani response to varying degrees. The odours of the
cabbage cultivar on which the parasitoid had been reared was preferred over the odours of
the other cultivars. Parasitoids gave a significant response to the volatiles of the cultivar on
which they developed when tested against water. Overall, parasitoid responses increased when plant to plant comparisons were made as compared with plant to water controls, and again, the magnitude of parasitoid response to odours depended on cultivar with Derby Day and Minicole preferred over Ruby Ball. However, when provided with a choice between odours of infested plants, parasitoids did not show a significant preference for the cultivar on which they were reared. Laboratory studies were conducted to determine the effects of plant cultivar and aphid species on the performance of/(. colemani. Significantly greater aphid populations were found on Derby Day regardless of the presence or absence of parasitoids, compared with Minicole or Ruby Ball. Minicole had the greatest proportion of aphids parasitised and Derby Day the least. A significantly lower percentage of emerged parasitoids was recorded on Minicole for both aphid species compared with Derby Day and Ruby Ball. The total development time of female parasitoids reared on M. persicae did not differ significantly between Minicole and Derby Day but was significantly faster on Ruby Ball. Sex ratios, size and longevity of both male and female parasitoids on either host were not significantly influenced by cultivar.