Invasive species in the U.S. – 5 Insect Pests That Damage Trees

There are many invasive insect species in the U.S. that damage trees. Their impacts are widespread. Among these invasive insect species are a variety of borers, beetles, flies, moths and aphid-like adelgids. Let’s look at five of the most destructive of these pests, the damage they do and what, if anything, we can do to stop them.

What, exactly, are “invasive insects” and what is their impact? According to the U.S. Forest Service, invasive creatures are those that are not native to the ecosystem where they’re found. Their presence can be harmful to the native species of trees and plants that grow there.

Most invasive insect species arrive in the U.S. as a result of “human activities.” Insects and their eggs often end up in shipping crates and pallets delivered here from other parts of the world. Also, as people travel, they sometimes pick up uninvited passengers in their baggage, clothing, cars, campers or the products they buy.

Proper identification is the first step to figuring out if an insect on one of your trees is an invasive species and if so, what can be done about it. Here’s information to help you identify five of the most common invasive insects along with details about the damage they cause and control measures.

Identification: This beetle ranges from 0.75 to 1.5 inches (19 – 38 mm) in length, is black with white spots, and has very long black and white antennae. It attacks hardwood trees and creates round boreholes that range from pencil to dime-sized in diameter. It will also leave shallow scarring on the tree bark and piles of sawdust near the tree’s base. The leaves of a tree infected by this beetle will turn yellow and droop when they should be green.

Damage: Per the USDA, this beetle is destroying a variety of hardwood trees across millions of acres in the U.S. Also per the USDA, there is “no cure” for infected trees.

Control: The USDA has created a special website for reporting any evidence of this beetle’s presence in your area so that containment measures can be implemented. They also request that you photograph any damage and that you try to capture one of the beetles. If you do, place it in a jar and put it in your freezer to preserve the insect for inspection.

Although there are some DIY articles available online that include step-by-step instructions for dealing with this pest, there are no known effective methods for treating an infected tree. The only solution is the removal and destruction of the tree. Government agencies have used imidacloprid insecticides to pre-treat trees to prevent infection. Your local licensed pest control expert may be able to provide more information about pre-treating your trees.

Identification: As its name implies, the emerald ash borer attacks ash trees. All ash varieties are vulnerable. Signs of an emerald ash borer attack include an unseasonable loss of leaves, D-shaped holes in the tree (holes are left when the adult beetles emerge from beneath the bark) and cracks in the bark that result from the tunneling activity of the insect larvae. The emerald ash beetles are metallic in appearance, displaying a shiny emerald-green color with specs of gold and black. They range in length from 0.4 to 0.5 inches (10 – 13mm).

Damage: This invasive insect bores into the bark of ash trees causing them to lose most of their leaf canopy and die within three to four years. The larvae feed on the inner bark and destroy the ash tree’s ability to transport water and nutrients. Emerald ash borers have killed millions of trees across America.

Control: For many infested trees the damage may be so severe by the time the infestation is noticed that the only solution is to remove and destroy them. There are insecticides on the market that are effective for battling these bugs. Some of the most effective contain imidacloprid, dinotefuran and/or emamectin benzoate. If you suspect your ash tree may be infested with emerald ash beetles, it may be best to contact a licensed arborist for help with diagnosis and treatment options.

The USDA also maintains an online reporting site for emerald ash borers. If you have ash trees infested with emerald ash borers your report can help track the spread of the insects around the country.

Oriental Fruit Flies (Bactrocera dorsalis) – Usually larger than a housefly, this variety has clear wings and is yellow with a dark “T” shape on the abdomen.

Mediterranean Fruit Fly – Aka “Medfly” – (Ceratitis capitata) – Roughly the size of a large housefly, this insect is black with silver markings, a tan abdomen, and clear wings with brown stripes and gray specks.

Mexican Fruit Fly (Anastrepha ludens) – Larger than a housefly, this one is pale orange to yellow with white stripes on the thorax. The wings are clear with yellow and brown stripes.

Damage: Fruit flies cause fruits to become infested with fly larvae and to decay and drop early. They damage or destroy more than 400 varieties of fruits and vegetables worldwide.

Control: A good starting point for controlling fruit flies on your fruit trees is to hang traps that use pheromones. These attract and trap the male flies so that they can’t breed with the females. Traps can alert you to the presence of fruit flies. When you first notice flies in your traps, apply a gardening insecticide spray to the lower leaves and around the tree’s trunk. Be sure to remove any rotting or larvae-infested fruit. For small infestations of adult fruit flies around the house, you can also make your own fruit fly traps that can be used in the home or outdoors.

The USDA requests that you refrain from transporting any fruits or vegetables across state lines and that you follow any quarantines that may be issued in your area.

Identification: These aphid-like insects are almost too small to see with the naked eye. They cover themselves with a white waxy substance and leave small white “cotton ball” deposits at the base of the hemlock’s needles.

Damage: This insect destroys forest and ornamental hemlock trees by feeding on the sap at the base of the needles. This disrupts the sap flow causing the needles to drop. Hemlocks starve without their needles and will die within three to five years of infestation.

Control: Smaller hemlocks can be sprayed with insecticidal soaps or horticultural oils that you can find at most garden centers. You can apply these, yourself. If your trees are too large to spray, you can clear the ground around the base of the tree and drench the soil with an imidacloprid pesticide.

Identification: These moths are light in color and covered in hair. They have five sets of blue dots and six sets of red dots lining their backs. Females have a wingspan of about 2 inches (5.8 cm) and have gold hairs on their heads. Males are smaller, with a 0.75-inch (1.9 cm) wingspan.

Damage: Spongy moth caterpillars feed on the leaves of over 300 species of trees and shrubs. They prefer oaks. Because they start with the uppermost leaves, their presence in large trees often goes undetected until significant damage is done. A tree can be completely defoliated by these pests. A healthy tree can only withstand about two years of this before it dies.

Control: Keep an eye out for the caterpillars and try to control them before they have a chance to climb into the tree canopy to feed on the leaves. Because the caterpillars will move up and down the tree’s trunk, creating a physical barrier by tree banding can help reduce the population.

Insecticides containing the bacterium, bacillus thuringiensis, are widely available at garden centers and very effective at controlling the spongy moth caterpillars. For very large trees, a well-timed treatment by a professional is a good preventative measure.

Invasive species do a tremendous amount of damage to trees and plants in the U.S. Knowing how to identify them and what to do when you encounter one of these pests could be the difference between losing or saving valued trees, shrubs and plants. Reports from individuals are helpful for monitoring the spread of invasive species on a local and national level. The USDA offers a wealth of information on where you can report invasive species in your area. Local outbreaks can also be reported to your nearest university extension office.

Check out our Common Houseplant Pests article to learn more about some of the bugs targeting your favorite indoor plants. When you’re battling these pests – indoors or out – it’s also important to remember that there are beneficial insects that shouldn’t be subjected to insecticides.