Bottom-up density regulation in the holly leaf-miner Phytomyza ilicis

1. The holly leaf-miner Phytomyza ilicis Curtis shows density patterns typical for non-eruptive leaf-miners, with stable mean densities, but consistently different infestation rates on individual trees. 2. To identify the mechanisms responsible for these patterns the population dynamics of P. ilicis were studied during an 8-year period on 92 host trees in 15 holly patches. The patterns of oviposition, mining and survival and the dominant sources and rates of mortality were monitored. The impact of tree and patch parameters on leaf-miner population dynamics was analysed, and the dispersal ability of P. ilicis was evaluated experimentally. 3. Infestation differences among trees were due to host selection, which determined egg numbers on individual trees and was explained partly by holly variety and leaf availability. 4. Egg distributions on medium to highly infested host trees were aggregated, suggesting limits in the availability of young leaves for oviposition. 5. Due to a suite of mortality factors mean survival rates of P. ilicis were below 10% and did not show a relationship with host choice. 6. There was strongly density-dependent pre-mine mortality most probably caused by intraspecific competition in the leaf midrib. This lead to the formation of a single mine on most leaves. There was a weak density-dependent effect of mine density on miscellaneous larval death and on the attack rates of the larval parasitoid Chrysocaris gemma. 7. In the holly leaf-miner system phenology effects set a population ceiling delimited by the availability of oviposition sites. The resulting egg distribution determined the impact of density-dependent intraspecific competition in the leaf midrib. The incidence of larval competition was therefore not a function of density per se, but density-relative to the availability of a limiting resource, namely suitable young leaves for oviposition.