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Andie Diaz Gonzales, Alejandra Velez, Hudnall, Chance D., Carlton, Christopher E.
Plectrodera scalator, the cottonwood borer, is a species of beetle in the family Cerambycidae (longhorned beetles). Adults are large (40 mm or more body length) beetles bearing bold black and white markings, with long black antennae often exceeding the length of the body. Larvae are elongate, legless, obviously segmented and broader near the head. The bodies are off-white with brownish-black heads. Mature larval body length can reach 50 mm. Pupae are translucent white, with developing adult characters such as the long, curled antennae visible.
Cottonwood borer adults are often mistaken for Asian longhorned beetles, Anoplophora glabripennis. Asian longhorned beetles bear a superficial resemblance in that the adults of both species are large black beetles with white markings and long antennae. Asian longhorned beetles have fewer white spots, striped antennae and differ in other details. The Asian longhorned beetle is not yet recorded in Louisiana.
Cottonwood borers are found primarily east of the Rocky Mountains in the central United States, with isolated records as far west as California and east in New Jersey. Adult female cottonwood borers lay eggs in pits that are chewed into the base of the tree by the females. After two weeks, larvae hatch and feed on phloem within the root collar of the tree, then tunnel deeper into the roots as they mature, taking up to two years to completely mature. Although the larvae eat wood, wood-feeding insects do not digest the cellulose they consume but instead rely on symbiotic microorganisms in their gut to convert cellulose into nutrients. After pupating in the roots for three weeks, adults emerge and tunnel their way to the surface during late spring to midsummer, living up to one month.
Cottonwoods (Populus spp.) are the primary host trees, but other poplars and sometimes willows (Salix spp.) can be utilized. In areas where the cottonwood borer is abundant, adults can often be seen on the trunks and limbs of host trees during daytime searching for mates.
Cottonwood borers are native to Louisiana and are usually not regarded as a pest species. Adult feeding can damage shoots, causing them to blacken, shrivel and die. This damage is usually not extensive and does not pose a serious threat to trees. Larvae can weaken young trees, making them more susceptible to windthrow, and may kill young trees by girdling. Larvae hollow, partially sever or girdle roots, leading to potential loss of vigor and structural weakening. Such damage is mainly a threat in nursery situations and reforestation efforts that utilize species of poplars such as cottonwood. In Louisiana, the cottonwood borer is not considered a serious pest .
In rare cases where high value trees are susceptible to cottonwood borer damage, prevention and prophylaxis is critical to preventing spread. Once larvae begin feeding, their deep tunneling makes detection and control extremely difficult. Tunneling activity can render trees more susceptible to disease in addition to direct damage. Regular inspections may reveal small round holes from borers and the buildup of sawdust (frass) around the base, which can indicate tunneling. Trees showing visible borer marks should be removed, especially if heavily infested. Trees under stress, or those planted in poor sites, are more vulnerable to borer attacks. Proper site selection, along with adequate irrigation and fertilization, is key to prevention. Chemical control can be effective for high value trees. Application of permethrin spray on the lower trunk and adjacent soil, ideally in early summer, may be effective in preventing infestations. All pesticide applications should be made in accordance with label directions .
Asian longhorned beetle adult for comparison (specimen from China) (night-heron, iNaturalist, Creative Commons BY 3.0).
