RECOMMENDATIONS FOR HEMLOCK WOOLLY ADELGID CONTROL
IN THE LANDSCAPE
by Dr. Jill R. Sidebottom, Area
Extension Specialist and Christy Bredenkamp, Swain and Jackson Horticultural
STEP 1: EVALUATE TREES. It may not be possible to save every hemlock. Some may have been growing poorly before being infested with hemlock woolly adelgid (HWA). These trees may be very old or may be growing off-site, and therefore may not be worth trying to save. In some cases, not all hemlocks may be saved because it would be too expensive to do so. Insecticide treatments will need to be applied every few years. Consider the cost! But also consider the cost of removing dead trees that are near homes and other structures. Treat the hemlocks that are the healthiest, most vital to the landscape, easiest to reach with a sprayer, and furthest from sources of water. Removing trees that will not be treated will eliminate a nearby source of the insect to reinfest treated trees, though if you are near natural stands of infested hemlocks or neighbors with infested and untreated trees, this may not make a difference.
If trees do not have any HWA on them at all, they do not need treating. However, if the infestation is very light, or if adjacent trees are infested, it is best to start making insecticide applications to keep the tree from going into decline. If the tree is severely defoliated, soil or trunk injections with an insecticide will probably not work well. Spraying lower branches will give the tree the best chance at recovery. Treating HWA aggressively while the tree is still in good health is the best way to maintain a healthy tree.
STEP 2: DECIDE INSECTIDE TREATMENT. The use of any insecticide can have unwanted consequences to the environment. Be sure to follow all label directions. Do not apply pesticides through sprays or soil injections near surface water such as streams or ponds. Do not exceed labeled rates of products. Applying a higher rate than what is labeled will not increase control. The following are the current recommendations of insecticide applications.
Note about foliar spraying: A homeowner may be able to spray trees eight foot or smaller with backpack or some other type of hand held sprayer. To get complete coverage, spray until you observe droplets running off. Make it a point to spray on the underneath side of the limbs as well as on top. Every branch must have thorough coverage to get control. Larger trees require a high pressure sprayer and may require hiring a commercial pesticide applicator.
Note about soil and trunk injections: For soil or trunk injections to work, the trees must be healthy enough to move the product from the roots or trunk up into the foliage. If trees are already in a state of decline due to HWA, spray as much of the tree as possible to kill as much of the pest as possible with either horticultural oil or insecticidal soap so that the tree can take up the imidacloprid. Even if the products are working properly, the soil and trunk injections of imidacloprid products may take a year or longer to show control. Do not expect instant results.
Note on commercial pesticide applicators: Anyone that you pay to apply a pesticide to your property, even if it is only horticultural oil or Roundup, requires a commercial pesticide applicators license from the North Carolina Department of Agriculture and Consumer Services. You can search on-line to determine if a person is properly licensed by typing in their name at: http://www.ncagr.com/aspzine/Fooddrug/data/search.asp
STEP 3: DETERMINING CONTROL. It is not always easy to know if HWA has been successfully controlled. The white spots may still be on the foliage following pesticide treatments. Soil and trunk injections can take several months to be effective. In many cases, it takes examination of the insect under magnification to see if it is dead. The best way to learn if the trees are recovering is to wait until the next flush of hemlock growth to determine if growth has improved. Contact your County Extension Agent to help in determining control. Many commercial pesticide applicators provide monitoring as a service to determine how well controls are working and any need for follow-up treatments. Between the systemic insecticides, Safari has a faster uptake and should be considered where time is more crucial. Imidacloprid products have a slower uptake, but offer longer residual protection.
STEP 4: PLANNING FOLLOW-UP TREATMENTS. Any of these treatments can last anywhere from 1-5 years depending on their success and the proximity to untreated, infested hemlocks. Success of treatments is usually a function of the initial health of the tree and the amount of soil moisture when treatments were made. Keep monitoring the new growth of hemlocks to find the small, pepper-like nymphs or white, waxy wool of the adults. Retreatment is necessary when more adelgids are commonly found on many of the branches.
STRATEGIES FOR LARGE TRACTS OF HEMLOCKS: In wooded areas with many hemlocks, it is not possible to save every tree. If you want to treat with a soil drench, determine your budget for materials and market trees that are most important to the landscape, or important seed trees. Measure and mark trees to be treated. Small understory trees can be sprayed with oil in the fall.
The predatory beetle, Sasajiscymnus tsugae, is commercially available from a small rearing lab in western North Carolina. Success of beetles is still being researched and has varied greatly. Released beetles can fly and may leave your property. But some landowners are interested in releasing these beetles in large tracts of land as biological control is the one hope of saving these trees. Currently the rearing lab will only sell no less than 1,000 beetles. These are sent on infested hemlock branchlets that are then attached to branches of the hemlock trees. Releases have worked best where trees are evenly infested but have not started to decline in health. Releases are made in the spring when adelgid eggs are plentiful. This will vary with region and elevation. If interested in releasing beetles, call your County Extension Agent. Beetles will not save individual trees, nor are they appropriate for trees in the home landscape. To save individual trees, use an insecticide.
For more information on HWA:
US Forest Service HWA web site at: http://na.fs.fed.us/fhp/hwa/
Save Our Hemlocks web site at: http://www.saveourhemlocks.org/
The Partnership for Saving Threatened Forests: http://www.threatenedforests.com/
For more information on trunk injection systems:
Arborjet systems: http://arborjet.com/
Mauget micro-injections: http://www.mauget.com
For more information on imidacloprid, see: http://npic.orst.edu/factsheets/imidacloprid.pdf
Recommendations for the use of chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention of commercial products or services in this publication does not imply endorsement by the North Carolina Cooperative Extension Service nor discrimination against similar products or services not mentioned. Individuals who use chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage and examine a current product label before applying any chemical. For assistance, contact an agent of the North Carolina Cooperative Extension Service in your county.
June 2009 . Reviewed January, 2011.