We studied patterns of association within and between two species of Pachypsylla gall insects (P. celtidismamma and P. sp. A) on hackberry (Celtis occidentalis L.) trees in eastern Iowa. Pachypsylla is a recently radiating clade of jumping plant lice (Homoptera: Psyllidae). Exploitative competition has been invoked to account for niche diversification in Pachypsylla as new lineages switched from leaves to feeding on different plant parts. Our results suggest that competition did not drive diversification, but could have reinforced it. Focal P. celtidismamma gallmakers grew larger when they shared a leaf with more conspecific gallmakers, which is consistent with intraspecific facilitation. Facilitation may result because gallmaker aggregations act as physiological sinks for photosynthate originating elsewhere in the plant. Pachypsylla celtidismamma gallmakers were smaller when they shared their gall with more Pachypsylla sp. A, which live as inquilines inside P. celtidismamma galls; this is consistent with the occurrence of interspecific competition. At spatial scales larger than a single leaf, we detected no significant relationships between larval densities and performance.
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Vol. 140 • No. 2