Insects that pupate on the branches of trees and shrubs suffer mortality from both predators and parasitic wasps. Which natural enemy represents the greater threat and therefore the stronger selection force on pupation site selection depends upon the time of year, the relative abundance of predators versus parasitoids, and the availability of alternative prey or hosts. Predation by foraging birds and mammals is likely to occur most commonly in winter when leaves have fallen, cocoons are conspicuous, and higher quality prey are scarcer. Inaccessibility and crypsis of pupation sites may provide protection from visually hunting predators. Attacks by parasitic wasps, which take place only during warmer months, may not be as easily avoided by inaccessibility or crypsis. We studied the patterns and mortality risks of pupation site selection in Diprion similis (Hymenoptera: Diprionidae; Introduced Pine Sawfly). Cocoons that were smaller than average and situated in relatively inaccessible sites (thinner branches, underside of branches) were less likely to be attacked by predators; background matching in terms of branch size proved not to improve survival. In contrast, the probability that a cocoon would be attacked by parasitic wasps (primarily Monodontomerus dentipes; Hymenoptera: Torymidae) was unaffected by location along branches, indicating that parasitoids are more difficult to escape through pupation site selection. Because Pine Sawflies were twice as likely to be killed by predators than by parasitoids during the cocoon stage, inaccessibility may be the most important factor for selecting pupation sites.
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Vol. 24 • No. sp7