Does parasitoid state affect host range expression?

• 3-d-old & 10-d-old Diadromus pulchellus were offered 12 non-targets in no-choice & choice trials.
• 3-d-old & 10-d-old parasitoids attacked four non-target species & offspring emerged from three.
• Increasing non-target density altered attack by 10-day-old, but not 3-day-old, parasitoids.
• Parasitoid state influenced frequency of non-target attack but did not affect species attacked.

The pre-release risk assessment of parasitoids for classical biological control generally involves non-target testing to define the agent’s host range. To ensure that no suitable host species are falsely rejected in these tests, it has been suggested that the physiological and informational state of parasitoids be manipulated to enhance their “motivation to oviposit”. However, the effects of such factors on host acceptance are not consistent across parasitoid species, making it laborious to identify the conditions necessary to maximise host acceptance. Our objective was to determine whether changes in parasitoid state could alter host acceptance behaviour sufficiently to affect host range expression. In addition, we tested the assumption that a state-dependent shift in motivation to oviposit on the target host will translate to a similar change in responsiveness to lower-ranked host species. Three-day-old and 10-day-old females of the candidate classical biological control agent, Diadromus pulchellus, were offered 12 non-target species of varying relatedness to the target pest, Acrolepiopsis assectella, in a series of no-choice and choice oviposition trials. Younger D. pulchellus females had previously demonstrated greater motivation to oviposit in the target pest and were, therefore, predicted to express a broader host range than older females. Parasitoid age had a minor effect on host range expression that was contrary to expectations. Older females more readily attacked one of the non-target species in no-choice tests and inflicted higher mortality in one of the choice tests. Ultimately however, young and old parasitoids still attacked the same four non-target species and their offspring emerged from the same three. There was an interaction between the effects of parasitoid condition and experimental design on responsiveness to low-ranked hosts: increasing non-target density in choice tests significantly altered attack rates by 10-day-old, but not by 3-day-old, parasitoids. The implications of these findings for host specificity testing depend largely on the specific aims of a host range assessment. Parasitoid state influenced the frequency of non-target attack but did not affect which non-target species were attacked.

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