Abstract 1. Field surveys and a manipulative experiment were conducted to examine the hypotheses that intra-tree heterogeneity in natural enemy activity, foliar quality (independent of phenology), or phenology influence the intra-tree distribution and performance of Ennomos subsignaria on mature sycamore maple, Acer pseudoplatanus.
2. Ennomos subsignaria intra-tree distribution was distinctly clumped. Egg mass density was 85% higher on the lower bole than in the crown. Most early instars were found on lower crown proximal branches while most late instars and pupae were found on lower crown distal branches. This resulted in high levels of defoliation in the lower crown, especially on proximal branches.
3. No parasitoids were reared from eggs or late-instar larvae and only one pupa was parasitised, suggesting that preference for the bole and lower crown was not a response to parasitism. Similarly, E. subsignaria performance was not influenced by variable foliage quality (independent of phenology) within the crown. However, sycamore maple phenology had a large influence on E. subsignaria survival. More than 90% of newly emerged larvae survived to the adult stage when they fed on foliage with three pairs of leaves expanded per bud, whereas survival on younger foliage was reduced by >45%.
4. The peak period of E. subsignaria egg hatch was approximately 2 weeks after the peak period of sycamore maple budburst, which occurred acropetally. Egg hatch was closely synchronised with the availability of most suitable leaves for insect development on proximal branches of the lower crown, the location where most larvae initiated feeding.
5. The results support the phenology hypothesis and suggest that intra-tree variation in oviposition site and host phenological development influence the intra-tree distribution and performance of this generalist herbivore.