Chemical ecology and olfactory behaviour of an aphid parasitoid and a lacewing predator

Female parasitoid (Aphidius colemani) olfactory preference was consistently biased in olfactometer experiments towards the Brassica cultivar on which it was reared when offered other Brassica cultivars as alternatives using either whole plants or detached leaves. Using gas chromatography, differences could be detected in the volatile composition of whole plants and detached leaves of five Brassica cultivars and the volatiles responsible for the fine distinctions the wasps are capable of making between plant cultivars are suggested. By subjecting female wasps to olfactory conditioning protocols, studies were carried out to understand the learning process for individual green leaf volatiles (6-carbon molecules), known to elicit behavioural responses in many insect species. Wasps were exposed to primary alcohols with differing carbon chain lengths in conjunction with aphids in an attempt to condition the wasps to the alcohols. When tested for learning, wasps changed their responses towards alcohols with molecules consisting of between 5 and 6 carbon atoms. The effect of cold temperature on olfactory preference was also investigated. After treating females at 0 °C for 0.5 h or longer, preference for the odour of the Brassica cultivar on which they were reared was lost for one hour after the cold treatment had finished, after which, the preference returned. However, when Brassica or unencountered plant odours were presented with clean air as the alternative choice, females could discern plant odour even immediately after the cold treatment. This suggests that the olfactory and locomotive systems were not altered by cold, whereas responses arising from learning-produced memories appear to have been inhibited temporarily and revealing underlying innate responses. The results indicate that temperature treatments could offer the possibility of dissecting innate and learnt behaviours in these parasitic wasps. The importance of controlling humidity arising from odour choices in olfactometers is also observed. In addition to assessing the parasitoid’s behaviour, various aspects of the role that neomatatabiol, a chemical compound closely related to aphid sex pheromone was investigated in relation to its role in the chemical ecology of the green lacewing Peyerimhoffina gracilis