Host discrimination in the aphid parasitoid Aphidius nigripes

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Abstract
Females of the solitary aphid parasitoid Aphidius nigripes do not consistently avoid superparasitizing their hosts, the potato aphid Macrosiphum euphorbiae. Tests were conducted in which two attacks separated by a time interval ranging from 1 to 24 h were allowed per host. No evidence of host discrimination was found when the interval was <2 h. For longer intervals, host discrimination was an increasing function of the length of the interval up to 24 h after the initial attack. Various mathematical models, permitting time-related rejection of parasitized hosts, were used to simulate the parasitoids–host distributions following two attacks. The best fitting models also had to incorporate time-dependent discrimination against hosts that had been attacked but not successfully parasitized. The results are discussed with respect to possible mechanisms of host discrimination in A. nigripes and its significance in intraspecific competition.

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