Cottonwood Borers Identification Guide

Cottonwood Borer Identification Guide

The cottonwood borer (Plectrodera scalator) is one of the largest and most striking longhorn beetles in North America. Its bold black-and-white pattern is hard to miss. In early summer, one may turn up on a sidewalk or porch.

Many wood-boring beetles attack the wood in houses. This one does not. Its grubs live underground, inside the roots of living cottonwoods, poplars, and willows.

Physical Characteristics

Adults are 25 to 40 mm long, or about 1 to 1.6 inches. That does not count the antennae. They are stout and heavy for a longhorn beetle.

What to look for:

• A black body with thick patches of white or cream hair. It looks like rows of black blocks on a pale background.
• A sharp spine on each side of the pronotum, the section just behind the head
• Solid black antennae, about as long as the body or a bit longer
• Black legs, often with a gray or white dusting of hair

The pale color comes from tiny hairs, not from the shell itself. On older beetles the white can look yellowish or cream.

The grubs do the damage. They are legless, cream-colored, and cylindrical, with a darker head. A full-grown grub reaches about 1.25 to 1.5 inches long.

Common Species

Several other longhorn beetles share the same trees or a similar look:

• Poplar borer (Saperda calcarata). Tunnels in the trunks of aspens and poplars, higher up the tree. Adults are gray with yellow flecks.
• Cottonwood twig borer (Oberea quadricallosa). A slim beetle. Its grubs tunnel in cottonwood twigs.
• Asian longhorned beetle (Anoplophora glabripennis). An invasive pest that must be reported. It is shiny black with white spots and has banded antennae. The cottonwood borer is mostly white with black blocks, and its antennae are solid black.
• White oak borer (Goes tigrinus). Also boldly marked with white, but it attacks young oaks, not poplars.

Cottonwood Borer Behavior and Biology

Habitat and Range

Cottonwood borers live where their host trees grow in large numbers. They are most common in the Great Plains and south-central states. That runs from Texas and Oklahoma north through Kansas and Nebraska. River bottoms, windbreaks, and poplar plantings are prime habitat.

The range reaches east through the Mississippi Valley and into the Southeast. In Virginia and Maryland, the beetle is rare. Range maps include the Mid-Atlantic, but very few real sightings come from our area. Most homeowners in Northern Virginia and suburban Maryland will never see one.

Diet and Host Trees

• Grubs feed inside the base and main roots of cottonwoods, other poplars, and willows. Hybrid poplars are hosts too.
• Adults chew the bark of tender shoots and leaf stems. A few leaves or twig tips may wilt. The grubs cause the real harm.

Life Cycle

In most of its range, the life cycle takes about two years.

1. Egg laying. In early summer, females chew small pits in the bark at the base of the tree. The pits are at or just below the soil line. They lay one egg in each pit.
2. Young grubs. Eggs hatch in 10 to 14 days. The grubs bore into the inner bark, then down into the roots.
3. Older grubs. Over two winters, they carve wide tunnels in the tree base and larger roots. Some sawdust gets pushed out.
4. Pupation. Grubs turn into pupae inside the root or trunk base in spring.
5. Adults. New beetles chew their way out near the ground. They are active from late May through August. They live a few weeks and are often seen walking near host trees.

Signs on a Tree

• Coarse, stringy sawdust at the base of the trunk or pushed up through the soil
• Small chewed pits in the bark near the ground, where eggs were laid
• A loose, cracked, or swollen base
• Wilting, yellowing, or leaning in young trees
• Young trees that snap off at ground level in wind, with hollow tunnels inside

Treatment Methods for Cottonwood Borers

Better Termite & Pest Control does not treat cottonwood borers. They live inside the roots of living trees. That is tree care, not household pest control. Think a cottonwood, poplar, or willow on your property is infested? Contact an ISA-certified arborist or your county extension office. They can check the base of the tree, judge the damage, and tell you if the tree is worth saving.

These steps generally help against cottonwood borers:

• Keep trees strong. Water deeply in dry spells. Keep mowers, trenching, and construction away from roots. A strong tree copes with grubs better.
• Watch young trees. Young and newly planted poplars are the most likely to be girdled. Check the base of these trees during the summer egg-laying season.
• Remove badly infested trees. A young tree that is hollow at the base can break without warning. Remove it. Destroy the stump and big roots before the next summer so new beetles cannot come out.
• Timed trunk-base treatments. On a valuable tree, a licensed arborist may apply an EPA-registered product to the lower trunk. It is timed to the egg-laying weeks and must follow the label. Results on grubs already deep in the roots are mixed.
• Hand-pick adults. Adults are large and slow. Picking them off young trees in June and July means fewer eggs.

Are beetles or sawdust coming out of wood inside your home? That is a different problem. Start with our guides to old house borers and powderpost beetles. Then reach out about our pest control services. We also handle termites, which eat structural wood but leave no round exit holes.

Similar Species and Look-Alikes

• Asian longhorned beetles. The most important look-alike. Shiny black with white spots and banded antennae. It attacks maples and many other hardwoods. Report a suspected sighting to your state department of agriculture.
• Round-headed borers. The general name for longhorn beetle grubs, including this one.
• Prionus root borers. Their grubs also feed on roots, but the adults are plain reddish-brown to black.
• Locust borers. A black longhorn with bright yellow bands. It attacks black locust trees in late summer.
• White oak borers. Smaller, with a mottled white pattern. Found in oaks.

References

• Oklahoma State University Extension: Cottonwood Borer
• Oklahoma State University Extension: Shade Tree Borers
• Kansas State University Research and Extension: Cottonwood Borer
• LSU AgCenter: Plectrodera scalator, Cottonwood Borer
• USDA Forest Service: Forest Nursery Pests, Agriculture Handbook 680
• University of Maryland Extension: Borer Insects on Trees