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How to identify it
- Adults are big, robust longhorned beetles, usually 1 to 1 1/2 inches long and 3/8 to 1/2 inch wide; Clemson notes they can reach about 2 inches.[2][1][4]
- The back is patterned in black and white, with rows of black, angular patches bordered by white stripes. The antennae are solid black, about as long as the body in females and slightly longer in males, and there is a strong spine on each side.[1][2][4]
- Eggs are white, elliptical and about 1/8 inch long. Larvae are legless, yellowish-white grubs that grow to 1 1/4 to 1 1/2 inches.[2][3]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Asian longhorned beetle | Clemson names the cottonwood borer as the large longhorned beetle most often confused with it. The Asian longhorned beetle is black with white spots, 1 to 1 1/2 inches long, with black-and-white striped antennae and bluish feet; the cottonwood borer is slightly bigger, mostly white with black spots, has solid black antennae and lacks the bluish feet.[4] |
| Poplar borer | It attacks the same trees, but the adult is about 1 inch long and gray-tan with small orange spots, and its larvae work in the trunk and larger branches, leaving swollen areas and exit holes where branches meet the trunk rather than working in the base and roots. Its fibrous frass can also pile up around the base of the tree.[1] |
| Clearwing borer (Paranthrene dollii) | This moth borer of cottonwood attacks the stem above ground near the base, with sap ooze and granular frass instead of fibrous frass. Its larvae are white to pinkish caterpillars with brown heads, and the adults are dark rusty-red clearwing moths that look like wasps.[3] |
Where it lives
It follows the range of eastern cottonwood in the eastern United States and reaches west into the Plains States, with reports as far west as New Mexico and Montana. The largest populations are in the Southern and Central States, and the 1975 Forest Service guide calls it a pest throughout the Southeastern States.[3][2]
Biology and life cycle
Eastern cottonwood is the main host. Other poplars, including aspen, are attacked too, and willows to a lesser extent.[3][1]
Adults come out from late spring through summer, mainly in June and July (Nebraska lists June to August), and feed on the bark of tender new shoots and terminals. Their size and color make them easy to spot while they feed and lay eggs.[2][3][1]
To lay eggs, the female digs the soil away from the base of the tree to a depth of 3/8 inch or more, chews a niche in the bark, and lays one or more eggs at or just below the soil line.[2][3][1]
Larvae mine downward in the inner bark and then into the wood of the root collar and roots, pushing frass out at the entry holes. They can hollow out the taproots of small trees; in large trees some carve irregular cavities and others make long tunnels, partly packed with excelsior-like frass.[2][1]
Larvae overwinter in the roots and pupate in their galleries, and the new adult chews out of the pupal chamber and digs up through the soil. Nebraska Extension and the 1975 Forest Service guide give a two-year life cycle; the Forest Service nursery handbook says 1 to 2 years.[3][2][1]
Damage and signs
- Larvae hollow, partly sever or girdle the roots and root collar. Young trees may be girdled or weakened until they break near the ground, and severely infested young trees can die; larger trees usually tolerate light to moderate infestations without obvious serious harm.[2][3][1]
- Look for light brown, fibrous frass pushed out of bark openings at or just above the ground line and piling up at the base of the tree. Because most attacks are at or below the soil surface, infestations are often missed until a tree breaks; pulling the soil back from the root collar and shallow roots shows frass-filled wounds, and infested roots are usually swollen and galled.[2][3]
- In nursery stock, adult feeding often kills the terminal shoot, which leads to excessive branching, forking and crooked stems. The Forest Service has also observed severe damage in young natural stands on sandy soils along the Mississippi River.[3][2]
Health risk
Nebraska Extension and the Forest Service describe it only as a tree pest and mention no risk to people or pets.[1][2]
Control
Prevention
Keep cottonwoods and poplars vigorous: water and fertilize them properly and protect them from other pests and injury, especially in the first two or three years after planting and during drought. The Forest Service notes that good planting sites and care that keeps trees healthy usually hold damage to a minimum.[1][2]
Nebraska Extension lists cottonwood among the trees more prone to borer injury. Where you are choosing a new tree, it suggests well-adapted oaks, lindens, crabapples or conifers, or borer-resistant species and cultivars.[1]
Sanitation
Dispose of dead or dying host trees, and consider removing related wild hosts close to valued trees to reduce borer numbers. Wrapping trunks is not recommended; Nebraska Extension says it typically does more harm than good.[1]
In nurseries and cutting beds, the Forest Service advises a site at least half a mile from wild or planted poplars and willows, borer-free cuttings, burning trimmings and culls, and digging and burning sprout stools every three years, since infested stools are the main source of reinfestation.[3]
Biological control
Larvae below the ground line are well protected from predators and parasites. Woodpeckers take a few that are above ground, a fungus disease has been found but doesn't seem to be common, and extended flooding kills many larvae.[2]
Pesticides
Borers are best prevented, because little can be done once larvae are deep beneath the bark. For borers in general, Nebraska Extension lists permethrin or bifenthrin sprayed on the trunk and lower branches, timed to cover adult emergence, egg laying and early larval activity (June to August for this species in Nebraska); several applications may be needed. Read and follow the label.[1]
Common questions
- Is the big black-and-white beetle on my cottonwood an Asian longhorned beetle?
- Check the antennae and feet. Clemson names the cottonwood borer as the beetle most often confused with it: it is mostly white with black spots, has solid black antennae and lacks bluish feet, while the Asian longhorned beetle is black with white spots and has black-and-white striped antennae. If you think you have found an Asian longhorned beetle, Clemson advises catching it or photographing it and reporting it (in South Carolina, to the Clemson Department of Plant Industry).[4]
- What is the sawdust-like material piled at the base of my cottonwood?
- Light brown, fibrous frass at or just above the ground line can be a sign of cottonwood borer larvae working in the root collar and roots, though poplar borer frass can also collect at the base. Pull the soil back from the base to look for frass-filled wounds in the bark.[2][1]
- Will cottonwood borers kill my tree?
- Young trees are the ones at risk: they can be girdled, weakened until they break, or killed. Larger trees usually tolerate light to moderate infestations without obvious serious harm.[1][2]
Sources
- University of Nebraska-Lincoln Extension, “Insect Borers of Shade Trees and Woody Ornamentals, EC1580 (J. A. Kalisch and F. P. Baxendale, 2010).” extensionpubs.unl.edu/publication/ec1580/2010/pdf/view/ec1580-2010.pdf (accessed 2026-11-08)
- USDA Forest Service, Southern Forest Experiment Station, “Cottonwood borer, in Insects and Diseases of Cottonwood, Gen. Tech. Rep. SO-8 (R. C. Morris, T. H. Filer, J. D. Solomon and others, 1975; archived on the Bugwood wiki).” wiki.bugwood.org/Archive:Cottonwood/Cottonwood_Borer (accessed 2026-11-08)
- USDA Forest Service, “Cottonwood Borers, chapter 29 (F. L. Oliveria and J. D. Solomon), in Forest Nursery Pests, Agriculture Handbook 680 (revised edition).” www.fs.usda.gov/sites/default/files/legacy_files/Forest%20Nursery%20Pests-web.pdf (accessed 2026-11-08)
- Clemson Cooperative Extension Home & Garden Information Center, “Asian Longhorned Beetle (HGIC 2021).” hgic.clemson.edu/factsheet/asian-longhorned-beetle/ (accessed 2026-11-08)
- iNaturalist, “Plectrodera scalator taxon page (order and family).” www.inaturalist.org/taxa/205777 (accessed 2026-11-08)