CAUSES OF VERTICAL STRATIFICATION IN THE DENSITY OF CAMERARIA-HAMADRYADELLA

1. The density of Cameraria hamadryadella, a leaf-mining moth, is vert ically stratified within the crown of oak trees. It occurs at higher d ensities on foliage in the lower crown. 2. Oviposition preference test s indicate that females display no preference to oviposit on foliage f rom the lower tree crown over foliage from the upper tree crown. 3. Ex periments in which potted trees were placed at various heights indicat e that foliage nearest ground level receives more oviposition, and tha t the higher rates of oviposition on foliage near the ground is not ca used by differences in quality between foliage from low or high in the tree crown. 4. Host-plant- and natural-enemy-mediated juvenile mortal ity and the abundance of parasitoids did not differ between the upper and lower crown. 5. Vertical differences in the timing of leaf product ion within the tree crown are unlikely to account for the observed gra dient in the abundance of C. hamadryadella. Furthermore, the mean date of leaf fall does not differ among heights within the tree crown. 6. It is argued that microclimatic gradients and interspecific competitio n are also unlikely to account for the observed gradients in the densi ty of C. hamadryadella within the tree crown. 7. Because of the absenc e of effects of other potentially causal factors, the most likely expl anation for the gradient in density of C. hamadryadella is a lack of m ovement by females into the upper tree crown from overwintering, emerg ence, or resting sites located in the lower tree crown, combined with egg depletion by females as they move from the lower to the upper tree crown. 8. It is suggested that the lack of movement and egg depletion hypothesis should serve as the null hypothesis in studies of vertical stratification of tree-feeding insects. In the absence of evidence of an effect of foliage quality, natural enemies, plant phenology, inter specific competition, or microclimate, the movement and egg depletion hypothesis is the most reasonable.