Temporal Variation in Three-Trophic-Level Interactions Among Willows, Sawflies, and Parasites

We examined the importance of temporal variation in three—trophic—level interactions among arroyo willow (Salix lasiolepis), a leaf—galling sawfly (Pontania sp.), and sawfly natural enemies (inquilines and ectoparasitodis). Two populations of Pontania sawflies were studied in northern Arizona, USA: one at Oak Creek and one at Flagstaff. The phenology of sawfly oviposition and larval development differed dramatically between the two locations. Enemy—caused larval mortality was also significantly different for the two sites. At Oak Creek, sawfly larval survival was high, the natural enemy attack rate was low, and sawfly oviposition occurred early in the period of willow growth. Sawfly larval survival was low at Flagstaff, enemies killed a high proportion of the larvae, and oviposition occurred late in the period of willow growth. Sawflies that oviposited early at Flagstaff produced larvae with a lower probability of survival than larval in later cohorts. Sawfly natural enemies were responsible for increased Pontania mortality with earlier oviposition at Flagstaff. One mechanism that caused increased mortality was temporal synchrony with the enemies' oviposition period. The rate of mortality from abortion of sawflies or miscellaneous causes did not relate to oviposition phenology in any consistent pattern. We hypothesize that temporal variability in three—trophic—level interactions may have resulted in a phenological shift by the Flagstaff Pontania population to synchronize with "windows of time" that maximum survival and enemy—free space.