Invasive and Exotic Species

Chapter 2: Outlook

Biological Control of Hemlock Woolly Adelgid

From: Cheah, C., M. E. Montgomery, S. Salom, B. L. Parker, S. Costa, and M. Skinner, 2004. Biological control of hemlock woolly adelgid. USDA For. Serv. FHTET-2004-04, Reardon, R. and B. Onken (Tech. Coordinators), 22pp

Developing a successful biological control program for HWA is a difficult task. The performance of natural enemies is rigorously density-dependent on the quality of both the HWA population and health of the tree. For example, predators are most effective against HWA if introduced to and allowed to become established on, small, healthy, developing HWA populations that have yet to have a severe impact on their hemlock hosts. Conversely, fungi might be most effective if applied to stressed HWA populations. In effect, predators and fungi could work in concert to maintain HWA populations at levels less damaging to hemlocks.

Half the range of hemlock in the eastern United States is now infested with HWA, and the entire range is at risk. It appears that a classical biological control program that uses both predators and pathogens will be required to maintain HWA populations at non-damaging levels. To be considered in such a program, candidate species must:

• show a significant impact and close association with the target pest;
• have a host range limited to that of HWA (or similar pest);
• originate in a climate similar to the proposed area of release;
• be tolerant of a wide range of environmental variables;
• be phenologically synchronized with HWA’s life cycle.

The predators, Sasajiscymnus tsugae, Scymnus sinuanodulus, Scymnus ningshanensis, and Laricobius nigrinus, as well as several fungi, meet many of these attributes. However, neither individually nor as a complex, should they be considered an immediate remedy to the HWA problem, but rather as parts of a long term solution.

The rearing of predators in laboratories must continue, along with the release of predators into numerous geographical areas to establish and promote their natural spread. Field insectaries need to be established in an effort to supplement the release of laboratory-reared individuals with “wild” individuals, which are more adapted to climate and other local variables.

The development of new isolates of insect-killing fungi is a long term process. The most active isolates of such fungi require further field testing in larger-scale trials and in multiple geographical areas. The methodology to mass produce these fungi for field trials has been developed and will support research on identifying optimal delivery strategies.

Integrating these and other biological approaches is an underlying theme that must be continuously considered while developing an HWA management strategy. Efforts must continue in Asia and along North America’s west coast to identify more natural enemies of HWA. The addition of species to the natural enemy complex will improve chances for successful biological control of HWA as it spreads throughout the range of eastern hemlock.

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