The abundance and phenology of spruce aphids Elatobium abietinum (Walker) and natural enemies in plantation forests of Sitka spruce (Picea sitchensis (Bong.) Carr.) suggested that predators and fungal pathogens contributed to the depression of aphid numbers in summer. This was confirmed by exclusion of insect predators. The mortality attributable to pathogens estimated from prevalence, increased as the population density of aphids increased during the spring, killing at least 7% of aphids at the population maximum. Estimated parameters of Type 2 functional responses for the main insect predators, particularly handling time, were scaled in accordance with aphid prey size. The most effective of the four tested predators in the laboratory was the brown lacewing, Hemerobius stigma Stephens, which spent the least time consuming prey, had the shortest handling times, and consumed the highest prey number. In order of consumption rate were the hemerobiid H. stigma > the cantharid, Rhagonycha lignosa (Muller) > the coccinellid, Aphidecta obliterata (L.) > the syrphid, Syrphus ribesii L. The tendency of first instar green spruce aphids to drop from a host plant while dispersing, was investigated under experimental conditions on spruce needles. The proportion of displaced aphids among all aphids surviving, increased markedly in treatments with predators which clearly disturbed many more aphids than they consumed. The proportional effect was greater for some predator species and increased with aphid density; H. stigma and Ap. obliterata had a more pronounced effect than either S. ribesii or R. lignosa. (c) 2006 Elsevier Inc. All rights reserved.
Day, KR., Docherty, M., Leather, SR., & Kidd, NAC. (2006). The role of generalist insect predators and pathogens in suppressing green spruce aphid populations through direct mortality and mediation of aphid dropping behavior. Biological Control, 38(2), 233-246. https://doi.org/10.1016/j.biocontrol.2006.01.002