The Importance of the Larval Parasitoid <scp> <i>Actia interrupta</i> </scp> (Diptera: Tachinidae) in Low Density Eastern Spruce Budworm Populations

ABSTRACT Actia interrupta is a tachinid parasitoid attacking several species of Lepidopteran tortricids, geometrids and noctodontids. It is an important parasitoid of the eastern spruce budworm, Choristoneura fumiferana , but only during the low‐density phase between two outbreaks, probably because of its need for alternate hosts. In this study, we followed five sites in Quebec, Canada, during the collapse of the 1980s spruce budworm outbreak. Foliage sampling showed that the spruce budworm population density decreased and that this decrease was accompanied by a change in the parasitoid species observed: while Meteorus trachynotus was at first the most important species, its importance declined with the spruce budworm decline, and it was replaced mainly by A. interrupta . Manipulative experiments using the sentinel technique showed that host patch size and host density had no impact on the parasitism rates by A. interrupta , and that the risk of attacks increased with exposure duration. Finally, both field and lab experiments confirmed that A. interrupta is a solitary species, and although there is a tendency to avoid superparasitism, females do lay more than one maggot per host, and it increased the probability of survival of one maggot.