Pre-release Host Range Assessment for Classical Biological Controls: Experience with Predators for the Hemlock Woolly Adelgid

There are few regulations for the release of parasitoids and predators, compared to herbivorous arthropods and pathogens, used as classical biological control agents in the United States The types of tests conducted prior to release of the predator or parasitoid into the environment are often up to the individual wishing to introduce the new agent. Ethical researchers understand the importance of host range testing to examine non-target impacts before releasing exotic arthropods into the environment. We use classical biological control of the hemlock woolly adelgid (HWA), Adelges tsugae, and host range testing of natural enemies for control of HWA as an example. The hemlock woolly adelgid is an exotic pest native to Asia that can cause severe damage to hemlocks in the eastern United States Several natural enemies have been imported, and two agents with good potential are the coccinellids, Pseudoscymnus tsugae from Japan and Scymnus ningshanensis from China. We included Harmonia axyridis in the feeding preference trials to show the nontarget impacts of a generalist predator. The host range tests for P. tsugae were inconclusive because the beetles ate significantly fewer prey items, including HWA, than the other beetle species; but this species seemed to prefer HWA over other adelgids and aphids. Scymnus ningshanensis preferred hosts within the family Adelgidae over Aphididae or other alternate prey items (p < 0.05). Harmonia axyridis was the only species that consumed more woolly alder aphid nymphs than HWA nymphs. Scymnus ningshanensis and P. tsugae have narrow host ranges in the laboratory and may be more host-limited in the environment, so releases of the beetles may have few ecological consequences. Harmonia axyridis, however, is a known generalist and mass releases of the beetles are known to have negative impacts on non-targets.