A Cottonwood Borer from Round Rock, Texas 2

— BugsInTheNews is a VIEWER-PARTICIPANT WEBSITE. This article by Jerry Cates, with contributions from Jean Cochran, first published in June 2001, was revised last on 7 July 2012. © Bugsinthenews Vol. 02:06

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The cottonwood borer (Plectrodera scalator) was frst described by the Danish zoologist Johan Christian Fabricius (1745-1805).

The generic name, Plectrodera, derives from the Greek πληκτρον, plectron, “spear-point”, in combination with the Greek δειρη, dera or deira, “neck, throat, collar”, which together refer to the prominent spines on the lateral surfaces of its neck. The specific name, scalator, is derived from the Latin scalas, “a ladder, stairs, staircase” in combination with the Greek suffix τορ, tor, which–along with a number of similar suffixes–indicate an agent or doer, but is often used for animals that bore in wood and other substrates; the combination, scalator, is a reference to the peculiar behavior of this beetle, both in the way its larvae bore into the wood of cottonwood trees, and in the way the mature females deposit their eggs.

Adult beetles are 1 to 1½ inches (25 to 38 mm) long and about ½ inch (12 mm) wide. They are black with lines of cream-colored hair forming irregular black patches. The adults feed on the tender shoots of young trees, causing them to shrivel and break off.

The adults of this beetle generally appear in midsummer. The first adult specimens in Fern Bluff found in 2001 were reported on June 4th. These photos are of a cottonwood borer found at the base of a Carolina Poplar in Fern Bluff in Jean’s front yard, where three of these trees are planted.

After feeding briefly, the mature beetles descend to the bases of host trees where the female digs into the soil. Note the claws shown in the photo. She then gnaws small pits in the bark to serve as repositories for her eggs. Note the powerful jaws of this beetle, as shown in the photo at left.

The eggs hatch from 16 to 18 days later into small larvae which immediately bore downward into the inner bark. Usually they succeed in entering a large root of the tree before autumn arrives, where they feed and excavate galleries. Pupation occurs in the galleries from April to June and lasts about 3 weeks. The new adults chew exit holes through the sides of the pupal chambers and emerge through the soil, leaving round holes approximately 1/2 inch (13mm) wide. Some larvae complete development in 1 year, while others require 2 years.

The immature beetles, or larvae, bore into the inner bark and wood at the root collar and tunnel downward into the roots. Light brown, fibrous frass is sometimes ejected from bark openings at or slightly above the ground line, accumulating in piles at the base of the tree. The root collar and roots of infested trees may be riddled by larval tunnels.

Jean, a close friend whose home is only a few doors away from mine, called me one afternoon to say she had captured a very large insect and wondered if I might like to see it.

As the photo posted here show, this large, well-marked animal has the requisite number of legs (6) of the insects.

Its posterior dorsum is characterized by a pair of hardened coverings over its wings, placing it in the order Coleoptera (from the Greek adjective κολεός, koleos, “sheath”; and the Greek combining form πτερόν, pteron, “wing” which, together, denote a “sheathed wing”).

This order, known popularly as the beetles, contains a greater number of species than any other order in the animal kingdom, and constitutes some 25% of, not just insects, but all known life-forms. As much as 40% of all described insect species are beetles and new species are constantly being discovered.

It has antennae that are about as long as–and probably slightly longer than–its body. That feature suffices to place it in the Cerambycidae, a cosmopolitan family known popularly as long-horned beetles, though some species have antennae that are so short that they are easily confused with short-horned beetles in the Chrysomelidae family.

Cerambycidae: cottonwood borer (Plectrodera scalator); closeup of neck spine; Jean C., Round Rock, TX–12.13.2008

Cerambycidae: cottonwood borer (Plectrodera scalator), tarsal claw; Jean C., Round Rock, TX–June, 2001

This specimen is not likely to be mistaken for a chrysomelid, not only because of its long antennae, but also because its markings and size immediately fix its identity as a cottonwood borer (Plectrodera scalator).

Many of the Carolina Poplars (which are actually cottonwoods, and members of the Willow Family) that were damaged in the windstorm that struck this area on 20 May 2001 had been previously damaged by this borer. The potential for future damage, if control measures are not taken, is very great. If you have a willow or a poplar tree in your yard, annual inspections should be performed to determine if these beetles are infesting them. If they are, control measures should be begun at once to prevent additional damage.

The cottonwood borer is found throughout the eastern United States, but the highest populations and greatest damage have been observed in the South. It is common in central Texas, including the Round Rock and Austin area. It attacks cottonwood and willow trees which, weakened by severe infestations, may collapse during wind storms.