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Beetles Make Annual Trek to Arizona Mesquite Trees

Swarms of little dark beetles with long antennas and a Velcro-like grasp have been spotted around Sierra Vista in the last couple of weeks. These are the same critters responsible for some unexpected pruning of mesquite trees.

Commonly called the mesquite twig girdler, the Oncideres rhodosticta has been in southern Arizona as long as the mesquite tree itself and is actually quite harmless, said Carl Olson associate curator of the University of Arizona department of entomology. People see dead twigs on the ground and assume the tree is dying. In fact, this pruning only makes it grow back stronger.

After mating, a female girdler chews a ring, or "girdle," around a small mesquite branch to stop the sap flow. She then deposits her eggs along the twig, Olson said. The bugs spend the majority of their lives as larvae, feeding underneath the bark, before undergoing a butterfly-like transformation and emerging as adult beetles at the end of the summer monsoon.

For the past couple of years, there have been high populations of these girdlers in southern Arizona, but this not unusual, Olson said. Ideal weather can sometimes trigger larger populations, but they will typically collapse before coming back in larger numberslater.

A large girdler population comes and goes based on the amount of rain, typically surfacing about every four to six years, said Rob Call, area horticulture agent for the University of Arizona cooperative extension. The adult's lifespan is usually about 30 days but is affected by the temperature. Warmer temperatures lead to a shorter life-cycle and cooler temperature lead to a longer one.

The beetle comes out every year and Call always gets calls from people wondering what they are, he said.

This is the first year Mary Chalifoux, manager of the Express Stop, said she has noticed them, and she has been working at the gas station for three years. "They're aggravating the people who come to the gas station."

She sees quite a few of them at night and a lot of dead ones on the sidewalks, Chalifoux said.

"They bite," she said. " I had one bite me ... it makes a red welt."

Though unusual, there have been reports of the beetles biting people and in one or two cases, they even drew blood, Call said. They only do so as a defensive mechanism, and most bites occur when people are messing with the bugs, Cal said.

Olson said people generally only notice them at gas stations and other places with big lights at night.

Elease Yates, manager of a local Chevron station, said her customers have not been complaining at night but she does have to sweep up a lot of dead beetles in the morning. She has seen some other places where an entire wall was covered with them.

Vic Jury, manager of a local Gas City, said the beetles have been gathering on the north side of his building during the day, in the shade.

"There were hundreds of them out there the other day ... a lot of them in mating session.

"They don't bite at all, but one got on my shoulder while I was having a smoke with one of the maintenance guys," Jury said. "He went to flick it off and had to hit it four or five times. ... They really grab on."

Children sometimes call them Velcro bugs, Olson said. They have little claws that act like grappling hooks, but they're just holding on, not biting.

The only way to manage the population would be to prune the dead, girdled branches before the eggs hatch, Olson said. There's no reason to do this because they do not harm the trees; they only stunt the growth and end up making them grow back healthier, he said.

They would have to girdle the trunk to kill the tree, and they only go for branches that are about half an inch in diameter, Olson said. Trimming the branches would not impact them much anyway.

"Their numbers are outrageous," Olson said.