Bulletin #56: Help Stop Insect and Disease Invasions


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The cast of infamous and unwelcome insects and diseases is constantly changing, but the USDA Forest Service annually monitors about 20 of the more serious forest pests. Here are five that have been introduced and are destroying urban trees.


This is a shortened, digital version of Bulletin #56. This is a sponsored bulletin funded by the USDA Forest Service, Forest Health Protection. Download the PDF for the complete content or purchase a printed bulletin.

Emerald Ash Borer

emerald ash borer

First reported in Detroit in 2002, this shiny green beetle is only about 1/3-inch long in the adult stage. It targets all ash trees, with a preference for green ash, white ash and blue ash in that order. Tens of millions of ash trees in urban and forested settings have been killed and the insect is spreading.

Biology: Adults emerge in summer and feed on ash leaves. Soon they lay eggs in bark crevices and after their eggs hatch tiny larvae chew into the inner bark and feed in the phloem layer. Pupation occurs over winter and the adults chew their way out making D-shaped holes.

Recognition: Sudden fading of an ash tree’s crown in summer. D-shaped holes where the adults emerged. Direct observation of the adults feeding on leaves.

Control: On the large scale, quarantines have been used, including removal of ash trees in a swath surrounding infected trees. For individual trees, there is some hope, although expensive. Insecticides applied under the bark (systemic treatment) and/or sprayed on the bark is proving effective. In some cases, trees that have less than 50 percent crown damage can still be saved. Use of insecticides is a job for professionals, but brand names can be found at a link in the supplimentary resources.

Asian Longhorned Beetle

asian longhorned beetle

This infamous pest was first observed in the Brooklyn, New York area in 1996. However, it is believed it arrived there in the 1980’s, probably in solid wood packing material. It is a huge insect that bores into a wide variety of hardwood trees. These include its favorite – maples – as well as willows, horsechestnut, elms, birches, poplars, mimosa, hackberry, ashes, London plane and mountain ash.

Biology: Adults emerge throughout the summer and early fall. They disperse only a short distance to new host trees where the females lay up to 160 eggs. In 10 – 15 days, the eggs hatch and larvae then tunnel into the tree and pupate. When the adult condition is reached, the mature insect chews an exit hole.

Recognition: It is the large, 3/8-inch exit hole that gives away this invader, often before the tree even shows signs of declining because of the tunneling activity. The adult is spotted black and white and is about 1-inch long with antennae that spread up to 3 inches. They usually feed on the midrib of leaves. Unfortunately, there are several harmless, native look-alikes.

Control: Through early detection, quarantines and tree removals, there is some hope this insect can actually be eradicated. Susceptible trees near infested areas have been successfully treated with a group of chemicals under the generic name, imidachloprid.

This is a shortened, digital version of Bulletin #56. This is a sponsored bulletin funded by the USDA Forest Service, Forest Health Protection. Download the PDF for the complete content or purchase a printed bulletin.

Hemlock Woolly Adelgid

hemlock wolly adelgid

Shady hemlock groves are dear to the hearts of many who enjoy our eastern forests. Hemlocks also add beauty to parks and home landscapes. Unfortunately, this species is attractive to a tiny, white insect introduced from Asia. Decline and mortality in hemlocks began to appear in Connecticut, New Jersey and Virginia in the late 1980’s and is being spread by wind, birds and at least in one case, on nursery stock.

Biology: According to the Forest Service, this insect has a complex and unusual life cycle. For example, it has two generations per year, affects hemlock trees of all ages, goes through six life stages (eggs through maturity), is parthenogenic (all-female population that does not require sexual reproduction), and feeds primarily in winter. In the crawler stage, the insect feeds on stored nutrients by penetrating xylem cells at the base of needles. This stunts shoot growth, resulting in morality over a 4–10 year period.

Recognition: Large numbers of fluffy white specks that line twigs at the base of needles on a hemlock is a sure giveaway that the tree is infected by mature hemlock woolly adelgids. Immature stages are more difficult to detect because they are smaller than the diameter of a needle and dark colored.

Control: Nature’s control is very cold weather. Cold periods can reduce a local population from 20–100 percent. Otherwise, in home and park landscapes, this pest is easily controlled on small trees or hedges with the use of any of several insecticides, including environment-friendly oils and soaps. On larger trees, injections have proven effective. In forest areas, control is more problematic and research on practical methods is underway.

Sudden Oak Death

sudden oak death

We have lived with oak wilt disease in eastern forests for years and have developed a number of preventative methods. Now there is a west coast disease that kills western oak species in the red oak group and can damage the foliage of other species such as California bay laurel, Douglasfir and coast redwood. Some eastern species may also be vulnerable. The disease was discovered in the 1990’s and spreads by infected nursery stock, wind-blown rain and contaminated irrigation water.

Biology: This disease has been identified as Phytophthora ramorum, a water mold. When its spores land on the bark of a susceptible species, they germinate, penetrate the bark, form cankers and reduce water flow in the tree.

Recognition: This one requires laboratory analysis for positive identification. However, the diagnostic symptoms on susceptible species are red-brown to black cankers and seepage on the lower trunks.

Control: Government efforts include intensified inspection of nursery stock and soils, and the clearing of infected and susceptible trees to develop host-free zones. A preventative fungicide is used to spray potential host trees in an effort to induce resistance to the disease.

This is a shortened, digital version of Bulletin #56. This is a sponsored bulletin funded by the USDA Forest Service, Forest Health Protection. Download the PDF for the complete content or purchase a printed bulletin.

Thousand Cankers Disease

thousand cankers disease

Eastern black walnut trees are the victim of this fungal disease. Mortality in black walnuts growing in the western states was at first attributed to drought but a new fungus, Geosmithia morbida, has now been identified as the culprit. Unfortunately, the disease may be spreading eastward into the native range of black walnuts.

Biology: The fungus is spread from tree to tree by the walnut twig beetle. Once established, the pathogen forms multiple circular or oblong cankers in the tissue just under the bark. The cankers kill the living bark and disrupt the flow of nutrients in the phloem. When thousands of the cankers coalesce, the tree declines and usually dies within 3 years from symptom to recognition.

Recognition: Crown dieback and thinning in walnuts. Removal of bark will reveal the cankers but laboratory cultures in agar and microscopic examination are necessary to positively identify the disease. The insect that carries the disease (vector) is smaller than a pencil point and difficult to detect. Sometimes there is seepage from the tiny hole it makes as it enters the tree.

Control: One method of control is to prevent entry or reproduction of the beetles. This is attempted with insecticides that are taken up by the tree’s roots after drenching the soil with them. Another treatment is the trunk injection of fungicides combined with the insecticides. Results have been marginal.


What You Can Do

There is a role for everyone to help fight against the spread of insects and diseases that destroy trees. Whether you are an individual working alone or a member of an organized group, there is a need to heighten awareness about pests, be on the watch for invaders, and know who to contact for help or to report findings.

This is a shortened, digital version of Bulletin #56. This is a sponsored bulletin funded by the USDA Forest Service, Forest Health Protection. Download the PDF for the complete content or purchase a printed bulletin.

Learn More About Insect & Disease Pests

Whether working alone or as part of an organization, a first step is to become knowledgeable about the pests that threaten tree species in your area. An excellent guide to this information is an annual publication produced by the USDA Forest Service titled Major Forest Insect and Disease Conditions in the United States. For more about this publication as well as links to specific websites about the major pests, visit arborday.org/bulletins. There are also workshops conducted in many communities. Your state forester, county Extension office or local branch of Master Gardeners would be a starting point to see what might be available.

Include Monitoring and Information as a Tree Board Activity

Tree boards can expand their responsibilities by providing information about pests and pest control through local media, workshops, publications and information booths at public events. Also, ordinances can be reviewed to assure content that allows for inspection and control of infected trees on private property as well as in rights-of-way and on public land. Suggested language for a section on “Tree Protection” can be found in Tree City USA Bulletin No. 6. Another activity potentially spearheaded by tree boards and/or the urban forester is to include pest monitoring as part of the tree inventory with a goal of using this information to guide future management decisions. IPED, part of the i-Tree suite of programs, is a free inventory tool to make this job easier.

What to do if you Find Something Suspicious

  1. Carefully compare the symptoms or actual insect with photos in reference materials.
  2. Collect samples. The insect is best when properly euthanized and contained in a tight container such as a pill box. Chewed leaves, bark samples, etc. may also be helpful. In the case of diseases, the collected sample should be contained in a tightly sealed plastic bag or other container.
  3. Take photos of the infected tree, both distant and close-up, and note exact location and date.
  4. Describe the extent of damage, position on the tree(s) and other helpful observations.
  5. Submit your findings to a specialist, such as your local county Extension specialist, certified arborist or consulting forester.

Help is Available

For examples of volunteer organizations that monitor invasive pests, obtain a complete edition of bulletin #56 by downloading the complete PDF developed with support from the U.S. Forest Service or purchase a printed version in our online store.

More Information

Call 1-888-448-7337 Monday–Friday 8:00 AM to 5:00 PM CST


Tree City USA is an Arbor Day Foundation program in cooperation with:
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