HIGH LARVAL PREDATION RATE IN NON-OUTBREAKING POPULATIONS OF A GEOMETRID MOTH

We conducted a two-year predator-exclusion study to assess the magnitude and timing of larval predation in non-outbreaking populations of a geometrid moth, Epirrita autumnata. Laboratory-produced newly hatched larvae were placed on the experimental trees which were assigned to five treatments within two larval densities: (1) all predators, including parasitoids, excluded by mesh bag, (2) birds excluded by cage, (3) ants excluded by glue ring, (4) birds and ants excluded, and (5) control without any predator exclusion. Thereafter, larvae were censused every 3-4 d throughout the five-instar larval period. Mortality of E. autumnata larvae in these populations was high and mostly due to natural enemies. In control trees, only ~10% of larvae survived, while survival was ~90% in mesh bags preventing all natural enemies. Bird exclusion significantly improved larval survival, as survival was almost three times higher in trees with cages than in those without cages. On the other hand, ant exclusion did not have any overall effects on larval survival, mostly because ants were only detected in about half of the trees without glue rings. Larvae survived longer in high-density trees from which ants were excluded, but the effect was masked by high mortality, unrelated to ant exclusion, in the late larval season. The results suggest that the effect of ant predation on survival of E. autumnata larvae may be spatially restricted and not important at a larger scale. The same result applies for crab spiders, as they caused high mortality in ~20% of the study trees. Our results emphasize the importance of considering the spatial scale as well as assessing the impact of multiple predators in order to detect predators affecting survival at the population level. Exclusion of all predation had a significantly stronger effect on larval survival than exclusion of birds alone. Further, mortality was highest during the late larval period, when parasitoids emerge. Thus, a large proportion of larval mortality was most likely due to parasitism. Our results suggest that predation by passerine birds and parasitism may contribute to maintenance of low E. autumnata densities by strong suppression of the number of larvae entering the pupal stage. Key words: ants; birds; crab spiders; Epirrita autumnata; field experiment; insect herbivore; larval predation; parasitism; predator exclusion; stability; suppression; survival analysis.