Seasonal abundance of Eriosoma lanigerum (Hausmann) (Hemiptera: Aphididae) and its regulation by Aphelinus mali (Haldeman) (Hymenoptera: Aphelinidae) from a 3-year survey in apple orchards.

Eriosoma lanigerum has become an increasing threat to apple orchards, partly due to the withdrawal of some insecticides such as neonicotinoids and organophosphates. Native to North America, this aphid completes its life cycle on apple trees, infesting roots, trunks, and branches, and causing yield losses. Among its natural enemies, the specialist parasitoid Aphelinus mali plays a key role in biological control and represents a successful case of classical biological control. However, data on the seasonal abundance of E. lanigerum and A. mali, and in particular on their interactions under temperate European conditions and different management systems, remain limited. Therefore, 3-year surveys were carried out in 5 apple orchards in northwestern Italy to assess the population dynamics of E. lanigerum and its regulation by A. mali under temperate climatic conditions. Aphid infestations increased in spring, peaked during summer, and declined toward autumn but varied considerably across years, orchards, and management systems. Infestation pressure was generally higher in organic orchards than in IPM orchards, reflecting differences in pesticide use. The abundance of A. mali broadly mirrored aphid dynamics, and parasitism rates generally increased after pest peaks, although timing differed among orchards. Analyses indicated that A. mali responds strongly to rising host density until a certain infestation threshold is reached, after which its control effect is expressed mainly through increased parasitism rather than higher adult abundance. These findings emphasize the density-dependent regulation exerted by A. mali and provide a quantitative basis for integrating parasitoid activity into decision thresholds for sustainable apple management.