GV Gardening: They’re back! The mesquite twig girdlers

Mesquite and acacia trees don’t normally have so many brown leaves and drooping branches in the autumn.

However, this is the second year in a row that this invasion has occurred in many areas of the desert. It is caused by the return of the mesquite twig girdler (Oncideres rhodostricta).

Have you seen the charcoal gray beetles around outdoor night lights? Large numbers are found in the mornings on the walls and sidewalks beneath lights. These small gray beetles have long antennae at least as long, or longer, than their body length.

Around 80 percent of those found around lights are male, but it is really the female we need to talk about.

Mesquite girdler infestations are localized, and again this year they appear in large numbers throughout the Santa Cruz Valley. These beetles originated in Mexico, Texas, and Arizona. Similarly-working girdlers are found throughout the U.S., but this one is pretty much ours.

The process goes like this. Adults begin to emerge from last year’s girdled branches anytime from late August through November. They feed on the tender bark around buds, thorns, and small stems for several days before finding a mate and egg-laying. Most adults die within 20-30 days thereafter. It has become noticeable that their numbers drop dramatically.

The female most often girdles only a single branch, choosing one the diameter of a pencil or slightly larger. She chews through far enough into the xylem to prevent passage of food and water. The tree’s carbohydrates are stored in these small stems, and photosynthesis is reduced when the branches are girdled.

Meticulous girdling may take the female at least two days. It always results in death to the portion of branch above the girdle, although leaves may remain attached to the branch for some time. The insect’s plan is to not sever the branch, leaving it connected by a center portion of the heartwood. Interestingly, if she is disturbed during the task, she will drop to the ground and temporarily “play dead.”

The little female then makes an incision in the bark of the girdled branch with her strong mouth. She then deposits a single oval, cream-colored egg inside, sealing the incision with an amber secretion. She travels the length of the girdled branch repeating this process every inch or so, for a total of 5 to 25 eggs per branch.

Ninety-eight percent of the eggs will hatch into larvae within 10 to 14 days.

Girdler larvae are small legless grubs, cylindrical and cream colored. They remain in the girdled branch and feed on the sapwood within the branch for about three months. During winter cold, they burrow just beneath the bark. Approximately a third of all girdled branches are broken away from the tree either by wind or wildlife. Up to 75 percent of the larvae are killed by parasites, predators, and unknown causes.

Late next summer, pupation to adult beetles will occur in about 14 days. The life cycle of the mesquite twig girdler is therefore completed in one year when surviving adults will once more emerge from the branches to mate and girdle, preparing for the next generation.

If or how to handle this process is still debated. Destroying the girdled branch ends is the single most effective control. All fallen branches should be gathered from the ground, as well as those remaining on the tree that can be reached. For years, burning this litter was a

recognized solution. Now it is recommended to cut these branches into small pieces for the landfill. This will remove the larvae that may become next year’s girdlers.

It is impractical to use insecticide for large infested trees. Smaller, high-value ornamental trees can be sprayed with products containing

Lindane or DursbanR. Always read carefully and follow directions on the label.

Any insecticide must be used when the adults are emerging but before girdling begins, leaving a small window of opportunity. Recommendations are that if you must, apply the first week of September and again in early October and November.

Very noticeable now are brown leaves hanging in the trees and fallen branches on the ground… not a pretty picture, and producing litter for cleanup.

However, a healthy, vigorous tree is rarely killed by these beetles. In fact, some folks look upon the mesquite twig girdler as another of “Mother Nature’s pruning tools”!

Mary Kidnocker is a Master Gardener who frequently writes about the subject from a local perspective. Her columns are featured each Sunday. She is currently President of The Green Valley Gardeners.