Abstract:

Studies on tritrophic interactions involving three cultivars of common cabbage, Brassica
oleracea var capitata, with varying levels of susceptibility to aphids, cv Minicole, (greenleaved)
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.
Field studies showed that cultivar and plant growth stage rather than fertiliser influenced
aphid infestation and parasitism. Ini998, Ruby Ball was least colonised by aphids in early
stage of plant growth, while Minicole had the greatest aphid infestation. Similarly in early
season of 1999, Ruby Ball was least colonised but aphid densities were greatest on Derby
Day followed by Minicole. In both years, from mid to late stages of plant growth, the lowest
aphid infestation and the highest level of parasitism was observed on Minicole. Brevicoryne
hrassicae was the dominant species and had greater rate of parasitism compared with M.
persicae during the early part of the season. Diaeretiella rapae was the major parasitoid and
was found earlier than Aphidius sp. There was evidence of a beneficial interaction between
the degree of plant resistance and biological control at the early and mid stages of plant
growth. The implications of the results are discussed in relation to the integrated
management of these aphids.