The Hemlock Woolly Adelgid (HWA) (Adelges tsugae), an aphid-like insect, was first described in western North America in 1924 and first reported in the eastern United States in 1951 near Richmond, VA.
HWA is not native to eastern North America, and in the absence of a natural predator, HWA, an introduced insect pest, attacks both Eastern Hemlock (Tsuga canadensis) and Carolina Hemlock (Tsuga caroliniana) which are often damaged and killed within a few years of becoming infested. HWA is now established from northeastern Georgia to southeastern Maine and as far west as southeastern Ohio and Tennessee.
Description
HWA is tiny, less than 1/16-inch (1.5-mm) long, and varies from dark reddish-brown to purplish-black in color. As it matures, it produces a covering of wool-like wax filaments to protect itself and its eggs from natural enemies and prevent them from drying out. This “wool” (ovisac) is most conspicuous when the adelgid is mature and laying eggs. Ovisacs can be readily observed from late fall to early summer on the underside of the outermost branch tips of hemlock trees. HWA can spread 20 Km per year.
Control
Control of HWA can be accomplished through a variety of methods and procedures. The most effective and appropriate treatment is dependent on a variety of factors including, but not limited to
a. The size of the overall hemlock stand
b. The size of the individual trees within the stand
c. The density of trees within the stand
d. The specific health (vigor) of the stand and individual trees
e. The current extent of HWA within the stand
f. The terrain and environment in which the trees are growing
In general, there are two primary ways we treat hemlocks for HWA; Chemical and Biological.
Chemical control involves the use of chemicals or insecticides to kill the HWA insects. Common chemical treatment methods for HWA include:
a. Foliar spray of insecticidal soaps/ horticultural oils
b. Soil drenches and injections of systematic insecticides
c. Trunk spray of systematic insecticides.
For more information on these specific chemical treatment techniques please check back soon for our “fact sheets”, or contact our Stewardship Staff!
Biological control methods exclusively involve releasing other insects that pray on the HWA. While a host of different biological control agents are being researched as viable options, the two most researched and readily available of the options are, the beetles Sasajiscymnus tsugae (St) and Laricobius nigrinus (Ln).
- SASAJISCYMNUS TSUGAE (St)-
Description: This beetle species is native to Japan and was first discovered in 1992. While the beetle will eat other conifer adelgids, it highly prefers the hemlock woolly adelgid. Studies have found that this species has a better generation-time ratio than hemlock woolly adelgid, meaning it reproduces at a much faster rate. Studies have also found that this species is highly mobile and readily disperse into adjacent crowns after release and that overall recovery of crown health has persisted in sites where beetles were released. Adult St beetles are active from around March/April until early-mid Fall, and feed on hemlock woolly adelgids during this time. While hemlock woolly adelgids remain active during the winter, St beetles do not.
- LARICOBIUS NIGRINUS (Ln)-
Description: This beetle species is native to western North America and was approved for release in the east in 2000. Field releases of Ln began in 2003 and populations have established from the southern Appalachians to New England. Ln is an adelgid feeding specialist and has an estimated dispersal distance of 320 meters/year in most of its range. While studies have found that hemlock woolly adelgid densities are significantly reduced at sites where Ln has been released, there is a lack of evidence showing that the decline of hemlock trees has changed. Unlike St, the life-cycle of Ln is synchronized to that of the hemlock woolly adelgid. This means that adult beetles are active from fall to spring and are dormant in the summer.
It is generally recognized that neither one nor the other control method will provide adequate control for all of our hemlock forests; and that an integrated approach of chemical and biological treatments are the best hope. Research is consistently going on to find the best management and control procedures to combat this annoying insect.
Conserving Carolina’s Effort
As active land managers and responsible stewards of our land, Conserving Carolina has worked with a variety of partners to control HWA on conservation easement lands, and lands held by the organization. Thanks to a large list of community partners, and volunteers, we have accomplished the following within our service region.
- Released 20,000 St beetles between 2010-2016
- Released 200 Ln Beetles
- Number of Hemlock Trees Chemical treated = 467
- Established 4 predator beetle insectaries
- Released beetles on 51 conservation easement properties
- Conducted 54 hemlock stand assessments
- HWA control programs: (Check back soon for information) -Transylvania Hemlock Program -Beetle program for Conservation Easement Landowners -Grant from HRI to assess and treat hemlocks