Numerical Response and Impact of Laricobius nigrinus (Coleoptera: Derodontidae) on Adelges tsugae (Hemiptera: Adelgidae) in Their Native Range

To determine if key attributes for a successful biological control agent are possessed by the predator, Laricobius nigrinus Fender, field studies were conducted in its native range of Seattle, WA. The relationship between adult and immature L. nigrinus abundance to different densities of its prey, Adelges tsugae Annand, were determined. In a second study, predator and prey densities, and survivorship of each sistens A. tsugae stage were determined to gauge the impact of predation. The predator strongly aggregated and increased its reproduction when prey density increased, the two mechanisms of a numerical response. Immature predator-prey ratios were high and average prey density was low in comparison with invaded areas of the eastern United States. Survivorship of aestivating first-instar sistens A. tsugae was low and survivorship of each instar (second, third, and fourth) and adults was high and increased with each stage. When pooled, however, the survivorship of sistens second instar-ovisac stages was low primarily owing to L. nigrinus larval consumption of ovisacs. In its native range, L. nigrinus has key attributes of a successful biological control agent, such as a strong numerical response, high predator-prey ratios, and an important larval impact on A. tsugae populations. Demographic data could serve as important benchmarks for future studies to determine if L. nigrinus and other predators can regulate densities of A. tsugae below eastern hemlock's physiological damage threshold in the eastern United States.