Determinants of Variation in the Response of the Aphid Parasitoid Aphidius ervi to Aphid Sex Pheromones
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Variation in parasitoid responses to semiochemicals may be influenced by genetics, phenotypic plasticity and the individual's physiological state. Previous experimental work has indicated a high degree of variability among parasitoid individuals in their response to aphid sex pheromones but no work has been done to investigate the factors behind such variation. Some aspects of this variation were investigated in the aphid parasitoid Aphidius ervi (Haliday) (Hymenoptera: Braconidae) in the laboratory. Conducting behavioural experiments, the searching behaviour of females was investigated in the presence and the absence of the aphid sex pheromone (4GS,7S,7a/?)-nepetalactone, observing the same individual female parasitoids during two consecutive foraging attempts on different plants. The first set of observational experiments demonstrated the role of the pheromone as an arrestant in the searching behaviour of A. ervi and its additive effect when it was presented with other foraging cues such as aphid-induced plant volatiles. The second set of behavioural experiments showed significant differences in A. ervi responses to the pheromone depending on their physiological state. Virgin, well-fed and high egg-load females were more active in the presence of the aphid sex pheromone than mated, hungry and low egg-load females although there were no significant differences in their activities in the first foraging attempt when the pheromone was absent. The effect of the pheromone on the searching behaviour of A. ervi within different tritrophic systems was investigated. The results showed variation in this response depending on which host aphid and/or the host plant they had been reared, showing that "conditioning" may have an influence on this response. Using two isofemale lines and an insectary-maintained laboratory population of A.ervi, the genetic basis of this response was investigated. The behavioural experiments showed no significant differences between the three different populations in their response to aphid sex pheromones. A supportive molecular study using DNA microsatellites v/as also conducted, which revealed low genetic variability among the three studied populations. The results are discussed in the context of using the response of A. ervi to aphid sex pheromones in a strategy to manipulate natural populations of the parasitoid for the biological control of aphids and the importance of studying the variation in this response to increase the effectiveness of such strategy.
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