The cottonwood borer (Plectrodera scalator) is a large long-horned beetle primarily known for its association with trees in the Populus and Salix genera, including cottonwood, poplar, and willow species. This insect spends most of its life cycle hidden within the wood and root systems of its hosts, where its feeding can cause significant damage. The danger they present is almost entirely directed toward the health and structural integrity of susceptible trees. This article examines the appearance of this beetle, the nature of its threat, the evidence of its presence, and control methods.
Identifying the Cottonwood Borer
The adult cottonwood borer is one of the largest beetles in North America, measuring between 1 and 1.5 inches in length. Its body features a bold, recognizable pattern of black and white or creamy-yellow patches, formed by dense, pale hairs. Like other long-horned beetles, it has long black antennae, which can be nearly as long as the body in females and slightly longer in males.
The damaging stage is the larva, a legless, cylindrical grub that is creamy-white with a brown or black head. Larvae tunnel beneath the bark and into the wood, reaching up to 1.5 inches when fully mature. The cottonwood borer’s life cycle typically spans two years, beginning when the female deposits eggs in a niche chewed into the bark near the base of the tree in late summer. Adults emerge after pupating within the root system and are found on host trees during the summer months.
Assessing the Danger: Threat Level to Humans and Trees
The threat posed by the cottonwood borer to humans and pets is negligible, as the insect is not venomous and does not sting. While the large adult beetles possess chewing mouthparts and can deliver a defensive pinch if mishandled, this bite is non-toxic and not medically harmful.
The primary danger is directed toward host trees, particularly young specimens and those under environmental stress. The larval stage feeds on the phloem and wood at the base and roots, severely compromising the plant’s vascular system. This internal tunneling obstructs the movement of water and nutrients, which can lead to the girdling and death of smaller trees. In larger trees, extensive larval mining causes structural degradation, making them susceptible to breakage during high winds.
Specific Signs of Infestation and Tree Damage
The most telling sign of an active infestation is the presence of coarse, fibrous sawdust, known as frass, which collects at the base of the tree trunk. Larvae push this material out of their tunnels, causing it to accumulate in small piles near the ground line. Adult beetles emerge through characteristic large, circular or oval exit holes chewed through the bark, usually appearing in the lower trunk or root collar area.
Chronic larval activity within the root collar zone can cause swollen or calloused areas on the trunk as the tree attempts to grow over the internal damage. Repeated attacks result in the root collar being riddled with galleries, which weakens the tree’s anchorage and makes it prone to snapping off at the ground level.
Prevention and Control Measures
Prevention
Maintaining the health of susceptible trees is the most effective preventative measure, as borers tend to target stressed or weakened plants. Ensuring proper site selection, adequate irrigation, and appropriate fertilization helps trees resist attack. For young, high-value trees, physical barriers such as tree wraps can be applied to the lower trunk to deter females from laying eggs near the base.
Direct Control
Direct intervention methods include hand-picking the conspicuous adult beetles during their active summer period to reduce egg laying. Small, recently hatched larvae can sometimes be physically removed by carefully digging them out of the bark near the soil line or by probing larval tunnels with a flexible wire.
Chemical Management
For chemical management, residual insecticides containing ingredients like permethrin can be applied as a trunk and soil drench in late spring. This deters adults from laying eggs and kills newly hatched larvae. Systemic insecticides are also an option, which are absorbed by the tree and target internally feeding larvae. These applications are often best handled by a certified arborist to ensure proper timing and technique.