By Judy Sedbrook, Colorado Master GardenerSM, Colorado State University Cooperative Extension, Denver
Poplar borers are a serious pest along the Front Range. This roundheaded borer is the larva of a long-horned beetle, Saperda calcarata, that primarily attacks aspen but can also damage poplar, cottonwood and willow trees.
Adults are approximately 1 inch long and are dark grey with small orange spots on the wing covers. Their antennae are nearly as long as their body. They emerge in summer, from June until August, and lay eggs in slits they have cut in the bark, usually near the middle portion of trees. Egg-laying females are most likely to be attracted to open-grown and single trees in partial to full sun. Over-mature and stressed trees are also likely to be attacked..
Long-horned Adult Poplar Borer
Larvae are white and about 1 1/4 inch long, legless, and yellow-white in color. The eggs incubate for about two weeks and hatch, after which 1¼-inch long larvae, enter the bark and move into the heartwood.. The larvae can take from two to five years to mature, generally three years in our area.
Roundheaded borer larva
Damage appears as swollen areas on trunks and larger branches. Exit holes where adults emerge, and woodpecker activity, are other signs of infestation. Chronically infested trees show a varnish-like stain on the bark below the points of attack, with reddish sap running down the trunk (see above). These repeated infestations may not kill larger trees, but the trunk or branches may be weakened and prone to wind damage which in turn allows for the introduction of disease pathogens and decline of the tree. Smaller trees can be killed by the girdling effect of numerous galleries in the trunk.
The poplar borer is difficult to control because of its long life-cycle.
Cultural practices include the planting of trees in an appropriate site, planting trees in groups where the trunks are shaded, and maintaining good health by applying proper amounts of water and fertilizer. Heavily infested trees should be removed as they can harbor a large number of poplar borers that may infest other nearby trees.
There are at last three naturally occuring insect parasites that impact poplar borers. Insect-attacking nematodes injected into borer holes have given partial control of poplar borer larvae. These nematodes belong to the genus Steinernema and are sold under the trade names "Biosafe", "Biovector", and "Exhibit". In addition to these, woodpeckers may consume up to 60% of larvae.
Most insecticide treatments are directed at the adults and small larvae at the time of egg-laying or hatch. For this reason, proper timing of insecticide treatments is essential. Insecticides currently labeled for borer control on poplars include certain formulations of carbaryl, acephate, dimethoate, and permethrin. Systemic insecticides, such as acephate and dimethoate, have shown some promise in controlling larvae that are already in the tree, but satisfactory control of larvae is difficult.
For maximum protection, the first treatment should be applied around the first of June. Some recommendations call for repeated treatments at two-week intervals through August 1, making a total of four treatments. However, monthly treatments may be sufficient given the residual effect of the insecticides. Applications should be most thorough on existing areas of attack, if any, and on the middle areas of the tree, including the trunk and large branches.
Be sure to read and follow the directions when using any pesticide.
Photos: Linda Cantrill (Damage to Poplar Tree) and Judy Sedbrook (Adult poplar borer, larva)
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010