Population regulation of the native holly leafminer, Phytomyza ilicicola Loew (Diptera: Agromyzidae), on American holly.

Three sources of mortality, intraspecific competition, parasitism, and early leaf abscission, were investigated to determine their importance in regulating populations of the leafminer Phytomyza ilicicola Loew (Diptera: Agromyzidae) on cultivated American holly trees. Spatial aggregation of leafminers among leaves resulted in density-dependent mortality from interference and from intraspecific competition for nutritional resources. Pupal weight of survivors decreased significantly as within-leaf density increased. Within-tree parasitism of P. ilicicola by Opius striativentris (Hymenoptera: Braconidae) ranged from 3%-58%, but parasitism was independent of host density both for individual leaves and on a tree-wide spatial scale. The incidence of early leaf abscission was significantly higher for mined than for unmined leaves, with about 14% of the mined leaves abscising before the adult leafminers emerged. There was high mortality of both P. ilicicola and O. striativentris within fallen leaves. These results are discussed in relation to recent hypotheses concerning the role of competition, parasitism, and early leaf abscission in population regulation of leafminers.