Common Name: Cottonwood borer
Scientific Name: Plectrodera scalator Fabricius
Description: Adult beetles can be found on and around host plants during the summer. They are large (1 1/4 inch long), robust longhorned beetles with black antennae as long or longer than the body. The body is beautifully marked with a bold pattern of black rectangular areas on a creamy white to yellow background. Larvae are legless, cylindrical (oval in cross section), creamy-white bodies and brown to black headed, growing to 1 ½ inch long.
There are a great many other cerambycid beetles that attack trees and other plants, but few others have larval stages that feed on roots or are larger than the cottonwood borer. Larvae of root borers, such as the tilehorned prionus, Prionus imbricornis (Fabricius), attack apples, Prunus (peaches, plums, etc.), pears, pecans, poplars, and other trees, by feeding in the crown and larger roots. They develop over a period of 3 to 5 years. Heavily-infested mature trees, particularly in sandy soils, slowly decline and die. Adult beetles are dark brown, over 1-1/2 inches long and their antennae are not as long as the body. They do not feed, are active at night and emerge in June and July.
Life Cycle: Adult beetles emerge from mid-May through early-July. Mated females dig burrows at the base of the tree and lay yellowish-white elliptical eggs in niches of chewed, shredded bark around the crown and buttress roots. Development requires 1 and occasionally 2 years before larvae pupate within larval galleries.
Habitat and Food Source(s): Mouthparts are for chewing. Cottonwood borers primarily infest cottonwood, but also occur on poplars and willows. Larvae (grubs) tunnel around the crown and buttress roots. Galleries, at and below the soil line, vary in length and form tunnels up to 8-inches long to 2- to 3-inch diameter oval areas, depending on tree size and infestation site. They are often packed with wood shavings (frass). Adults can be found on infested host plants during the daytime.
Pest Status: Adults are commonly encountered on trunks and branches of cottonwood and willow trees and other host plants during the summer months. Infested mature trees are usually not seriously injured. Larval stages are rarely encountered unless heavily infested young trees are killed or fall over; medically harmless.
Damage: Young trees may be killed when larvae tunnel under the bark (through the xylem tissue) all the way around the base of the tree, girdling it. More commonly, they structurally weaken the tree causing it to fall over in high winds. Adults feed on leaf stems (petioles) and bark of tender shoots, occasionally causing shoots to break, wilt and die, a symptom called, “flagging.”
Literature: Johnson & Lyon 1988.