Girdler Bug Attack: No Treatment Necessary

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Southwest Yard and Garden
By Dr. Marisa Thompson & Dr. Carol Sutherland



Reprint from November 2017
Mesquite girdler. (photo credit: Dr. Salvador Vitanza,
previously with TexasAgriLife, El Paso, TX)

Question:
Our mesquite trees are experiencing what appears to be a girdler bug attack. This is the first time we have experienced this. How harmful are they, and what can be done to rid these pests?
-          Jack D., Doña Ana County, NM
Answer:
In order to answer your question, I enlisted the help of NMSU Extension Horticulture Specialist and NM State Entomologist, Dr. Carol Sutherland. Here is her response:
Yep. The culprit here is a type of long-horned beetle known as the “mesquite girdler” (Oncideres rhodosticta).
Adults are about a half-inch long and are several shades of dark gray, with dots or patches of rusty brown, especially on their forewings. Their most obvious feature is the very long pair of antennae, which is characteristic of most beetles in the Cerambycidae family.
The mesquite girdler is generally found from Baja California probably into southern California, and from Arizona to West Texas and south into Mexico, wherever potential woody hosts occur, including mesquite, acacia, mimosa, palo verde, and ebony. Honey mesquite may be the beetle’s preferred host and is where I have seen the most activity.
Many of the beetles I’ve seen have been resting on utility poles where security lights are located. On that kind of background, they are far easier to spot than on mesquite twigs, where they blend in well.
In natural settings, males can be actively patrolling mesquite twigs where females are located. Females girdle the host twigs before laying their eggs. I would think it makes more sense to lay eggs below the girdle than above; above the girdle, everything is drying up, probably too soon for the egg to hatch and larva to develop. Incubation and larval feeding should take several months, at least. New adults should appear next year.
Females are the ones that girdle host plant twigs. These twigs will usually be about the diameter of one of your fingers. When the beetle is viewed from the side, the axis of the body is at right angles to the beetle’s head. The jaws are perfectly in position to bite the bark; with each bite the female side-steps, going all the way around the twig of choice. This cuts the vascular system leading to the end of the twig, causing the leaves to soon fade and dry. As that happens, the girdled twig dries such that if you pull on it, the twig breaks off cleanly at the girdle mark. Broken twigs become part of the ground litter.
You can see this type of damage on road-side mesquites from Las Cruces to Deming and beyond as well as from Deming almost to Silver City. I-25 has had a nice display again this year from Las Cruces almost to Truth or Consequences. There are probably other areas where damage is noticeable, too.
Like other native insects, populations of this Oncideres go up and down from year to year. This year seems to be one of the “showier” years for beetle activity. Next year, the mesquites will grow again, putting on new twigs and foliage. Mesquites are very tough desert survivors with a variety of insects that feed and/or reproduce on and in them.
No treatments are recommended or needed for Oncideres in New Mexico. The beetles do not stay in an area very long; flights and infestations are “hit or miss” from one year to the next. Land managers hoping for mesquite control by these beetles are disappointed. People with ornamental mesquites see the damage and fear the worst, but, again, this is hit or miss from one year to the next. Developing larvae are well protected by wood and bark.
As for these twig girdlers attacking orchard trees, other species of Oncideres do that in different parts of the country. In the eastern US, another species of Oncideres girdles various fruit and shade trees. In extreme south Texas, still another Oncideres, the “huisache girdler,” is associated with native acacias, both wild and landscape trees.
Special thanks to Dr. Sutherland for answering the question for this week’s column. Check out submitted photos of mesquite girdler damage at https://nmsudesertblooms.blogspot.com/.

For more gardening information, including decades of archived Southwest Yard & Garden columns, visit the NMSU Extension Horticulture page (http://desertblooms.nmsu.edu/), follow us on social media (@NMDesertBlooms), or contact your County Extension office (https://aces.nmsu.edu/county).
Marisa Thompson, PhD, is the Extension Horticulture Specialist for New Mexico State University and is based at the Agricultural Science Center at Los Lunas.


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