What can I do on my property? (Updated April 2005)We are fortunate that we have available to us a number of means of treating our hemlocks—killing the adelgids that infest them to prevent the damage and tree death that the hemlock woolly adelgid can cause in just three to five years. Here we lay out how to determine if your trees have become infested with the hemlock woolly adelgid, how to treat the trees to kill the adelgids, and how to maintain your hemlock trees’ general health.
Hemlock Woolly Adelgids in Your Trees?
The adelgids in all of their life-cycle stages (eggs, nymphs, and adults) are near microscopic in size so it is difficult to spot them. Fortunately, the sacs that protect the eggs are woolly and white, looking something like tiny cotton balls attached to the base of the needles on the underside of the twig. This “wool,” most visible from late fall to early summer, is a sure sign that the adelgids are making a home in your hemlock. Once the infestation has affected the general health of the tree, you also will notice that the tree produces no new growth on certain limbs or throughout the tree. Limb die-back will then begin to occur.. Treating Your HemlocksThere are two treatment types available to the homeowner or private landowner for treating adelgid-infested trees. They are soap spray and systemic treatments (by soil drenching, soil injection, or trunk injection). Selecting the appropriate treatment method for your hemlocks depends on factors such as the trees’ accessibility (can you get to it easily), whether it is near water or in rocky soil, and the expense of the treatment. Each treatment option has benefits and drawbacks relating to cost, effectiveness at killing the adelgids, and ecological effects. None of these treatments have any preventative effect, so treating uninfested hemlocks will not protect them. Soap or Horticultural Oil Spray – In your home landscape, the application of soap spray or horticultural oil is one of the most effect treatment methods and has the least potential for negative environmental consequences. Soap and oil have been used in gardens and home landscapes for more than 20 years and they cause no or minimal harm to other, beneficial insects. Insecticidal soaps and horticultural oils are sold at your garden center under many different name brands. Follow label directions carefully, mixing a 2% solution (2 ounces of soap to 98 ounces of water) in your sprayer. Any pump sprayer will work, from a one-gallon garden sprayer to a power sprayer. Thoroughly drench the tops and bottoms of needles, covering the entire tree and thoroughly coating the insects. Application will be most effective in late March through April when adelgids have hatched from their eggs, and in late August to October before the nymphs produce their woolly covering. To prevent leaf burn, do not spray in the spring when new twigs are growing or when temperatures are above 75°F or below 45°F. Also, a 1% solution of horticultural oil may be advisable between May and September. Insecticidal soaps and horticultural oils have no residual effects, so reapplication may be necessary if rain occurs within 24 hours of application. If your tree(s) are so tall that you cannot apply soap or oil to the entire tree, select another treatment option or contact an arborist who has equipment that allows the entire tree to be covered. Systemic Treatment Systemic treatment of adelgid-infested hemlocks is accomplished with the chemical imidacloprid. The product is applied either to the soil and tree’s root zone by drenching or injection, or to the tree itself through a trunk injection system. Imidacloprid has been available for some time for the treatment of insects on trees and shrubs, and it is the key ingredient in some flea control products for pets. Imidacloprid is sold under the brand names Bayer Advanced ®, Merit ®, and Marathon ®. Most garden centers stock the Bayer product, and the Merit and Marathon products are either stocked by or can be ordered by farmers’ cooperative stores. Other brand names may be available. The product labels provide directions for mixing the product and indicate how much should be applied. One should be mix the solution carefully according to label directions and apply either in the spring or fall when the tree is taking up water and nutrients. Do not exceed the recommended solution strength. Research is beginning to suggest that imidacloprid may be effective against the hemlock woolly adelgid at even weaker solutions than those recommended on the product label, but current advise is to follow the label directions. Research also suggests that imidacloprid taken up by the tree’s roots may be effective for up to 24 months. Root zone application is most effective from September through November. While the benefits of using these products—saving your hemlocks—may far outweigh the environmental consequences of using them, imidacloprid can kill aquatic species and, potentially, some beneficial insects. One should never use either soil drenching or soil injection near open water or in rocky soils that will drain quickly to open water. Soil drenching is accomplished by slowly pouring a light stream of the solution over the soil or into a set of small holes in the soil spaced evenly around the tree and about 6 to 12 inches from the base of the tree. The product label provides directions on determining how much solution to apply. The soil should be raked bare before application and every effort to prevent runoff or washoff should be made. Thus, one should not apply the solution to completely dry soil, completely drenched soil, or if considerable rain is expected because the product may wash away from the tree’s root area. This potential for runoff is the primary drawback to soil drenching. Soil injection uses a pressurized injection device to deliver the imidacloprid solution several inches beneath the soil’s surface, thus reducing the possibility of runoff. A commonly used system is the Kioritz Soil Injector ® available from agricultural chemical supply houses like Helena Chemical or Wilbur Ellis. An individual could obtain the necessary equipment or seek out an experience arborist to do the soil injection. The third method, trunk injection, has not been shown by research to be any more effective that the soil application methods and it must be done by a trained arborist. It is the only method available for trees near open water because trunk injection prevents contact between the water and the chemical. However, there are a number of drawbacks to trunk injection. First, it is two- to four-times more expensive than soil injection. Second, trunk injections actually wound the tree. It is important when picking from among the injection systems available to consider the size wound that the injector makes. Third, the residual effect—the number of months that the treatment is effective—is shorter with trunk injection than with soil application. Fourth, because the process has such a short window for the tree to take up the chemical, the application must occur in the spring when the tree is just leafing out or in September when the tree is moving additional water and nutrients before its dormancy. Application must also occur before mid-afternoon (about 3:00 p.m.) because trees cease uptake then. There are several tree injection systems being marketed by arborists. Preliminary research is showing that tree injection systems that use pressure and a higher volume of liquid delivered over a longer period of time are more effective than other systems. Summary of factors to consider when selecting a method to treat adelgid-infested hemlock trees
Hemlock HealthMaintaining the general health of your hemlocks will help their survival. You should irrigate your trees to insure they get one inch of water per week and prune dead and dying limbs. Although fertilizing may benefit a healthy hemlock, you should not fertilize adelgid-infested hemlocks because doing so improves adelgid survival and reproduction. Identifying a Knowledgeable ArboristThe International Society of Arboriculture and the Tree Care Industry Association, formerly the National Arborists Association, are two great sources of information about what to look for in a tree care specialist. At both Web sites, you can find contact information for arborists in your area who are members of or certified by these organizations. Your county agricultural extension agent can also refer you to qualified arborists.
In general, good arborists will
Don't hesitate to ask what experience the arborist has with hemlock woolly adelgid. Sources:
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