Velcro bug, el Torrito, Mesquite Twig Girdler, or Oncideres rhodosticta: the most abundant beetle in Arizona after the monsoon

It’s Velcro-bug time again!

Anywhere between Green Valley and Sierra Vista, Arizona, and probably all over New Mexico and far into Texas, border patrol agents, gas station and supermarket employees, and everyone else who leaves his porch lights on at night is familiar with those half-inch long beetles that cling to every surface as if glued to it. They hold on with little hooks on their feet and even with their mouth parts. For a good reason, as in nature being picked up usually means being eaten by a bird, a grasshopper mouse or a lizard.

Male (left) and female (right) Mesquite Girdler

During the day the beetles rest motionless clinging to the bark of a tree, mostly well hidden by their cryptic wing pattern. Observed closely, the shades of dark-brown and silver-grey and the raised reddish dots are very attractive. The scientific species name Oncideres rhodosticta refers to those markings which distinguish this very common species from the related rarer Oncideres quercus and the much larger cousin Lochmaeocles marmoratus, all found in Arizona.

Arizona members of the tribe Onciderini. Mesquite Girdler on the right

Like most adult longhorn beetles, adult O. rhodosticta feed on plant material. The chew leaf buds and the green bark of fresh mesquite twigs. Sometimes the nightly feeding frenzy of the beetles leaves the ground under a tree littered with chewed-off leaf-matter and twigs.

Life cycle of the Mesquite Twig Girdler Oncideres rhodosticta :
Adult twig girdlers eclose from late August to early November. 

Towards the end of our summer monsoons, the female beetles create the most well-recognized sign of O. rhodosticta 'infestation' when they prepare a nursery for their off-spring: Dead or dying finger-thick mesquite (or sometimes Acacia) branches that stay connected to the tree and usually carry the wilted, bleached leaves like flags into the winter months.

The common name, Mesquite Twig Girdler, hints at the story: Before she lays her eggs, the female chews a precise, complete circle around a finger-thick twigs. This task can take up to two days. She bites all the way through the Xylem and Phloem of the bark and thus disconnects the branch from its water source. Then she chews a separate shallow grove for each of about 8 eggs in the distal, dying part of the branch. The larvae will hatch and live in the wood until they are grown, pupate, go through their metamorphosis and hatch as adult beetles by the end of the next monsoon season to restart the cycle.

Female Mesquite Girdler at her girdling site

Most wood boring insects attack sick, injured or dead wood rather than a living tree. This is partly because of the ability of healthy trees to fight intruders by ‘gumming’ them up, that is by drowning and encapsulating them in sticky resin rich tree sap. So the female Mesquite Girdler protects her eggs by cutting off the tree’s defense lines. A glob of fresh tree sap often hangs from the girdling cut: the trees attempt to fight the parasite which didn't reach its target. The nursery that the longhorn beetle creates is so attractive that several other insect species infest the girdled branches. I have raised more wasps, buprestids, anobiids, bostrichids and dermestids from collected girdled branches than O. rhodosticta adults.

Of course, the pruning-activity of  the beetles comes as a cost to the trees. They lose the carbohydrates stored in the girdled twigs and a part of their photosynthesis-machinery.  However, a study of Texas Tech. University showed no conclusive results concerning the use of the beetles to control the mesquite tree overpopulation of the grass lands.  Natural girdler infestation can cause an over 30% reduction of the canopy, which does not seem to harm the mesquite trees but instead to induce healthy re-growth in the following season.

Living and dead Mesquite Girdlers under the lights of the I 19 border patrol check point
Photo by Joyce Gross Sep. 2008 

Mass occurrences of members of a single species that last for more than a couple of vegetation cycles, as observed in the Mesquite Girdlers in southern Arizona since 2007, usually indicate a disturbance of the natural balance. In this case the overabundance of mesquite trees on former grass lands is certainly a contributing factor. The mesquite tree was introduced into the grass lands through the widespread practice of feeding mesquite beans to cattle. While the nutritious pods are digested, the seeds pass the bovine digestive system not only intact, but with an increased capacity to germinate. But the trees have been with us at least for several decades and the Girdlers seem to proliferate more than ever since 2007. A climate changing to drier and hotter summers seems to have favored this species, while its predators, parasites and diseases are still lagging behind. But there is no doubt among entomologists that these factors will eventually catch up to collapse the population explosion of the girdlers.

PS: The numbers of Mesquite Girdlers did indeed collapse in 2011. Since then I have not seen more than 3 to 5 on my nightly light trap, even during the prime season and in mesquite areas. I was wrong about one thing though: the summers from 2005 to 2009 were not particularly hot or dry in hindsight. It only got hotter and drier from then on.

Literature: The Ecology of the Mesquite Twig Girdler Oncideres Rhodosticta and its Evaluation as a Biologial Control Agent. Thesis in Entomology by K.L. Polk