Ask A Master Gardener

By Judy Curtis,
Master Gardener

QUESTION:
What are the quarter-size holes in the ground around my Palo Verde tree?

ANSWER:
A client called the Master Gardener hotline last week and recounted an amazing story. She was in her yard one evening recently visiting with a neighbor. They noticed holes around the base of a nearby Palo Verde and commented on them. Within a few minutes they saw large beetles begin to emerge from the holes, eventually about three dozen in all. At the same time male beetles, which they later learned had exited a few days earlier, began to fly in and proceeded to mate with the females. No sooner had they mated than the females returned back into the holes.

Most of us have seen the male Palo Verde beetle (Derobrachus geminatus) on monsoon nights as they swoop past our heads like bats, or we may find one on the patio in the morning, crawling into a corner out of the sun. While these beetles are identified with the Palo Verde tree, they will attack other varieties as well.

The eggs laid by the female in the ground hatch into grubs that attach themselves to roots where they feed for the next three years. Large infestations can eventually cause tree death and symptoms of decline in the tree may indicate their presence.

Control is difficult. Insecticides are not usually effective because the grubs are too deep in the soil. At this time of year, if the beetles are ready to emerge, they can be flushed out of the holes with water and dispatched manually. Another suggestion is to lay mesh screening around the base of the tree and cover it with soil. This prevents the beetles from exiting the holes to mate and over a period of years could reduce their numbers. A good deep watering occasionally with a light fertilization can spur new root growth to help the tree compensate for the effects of the beetle.

In spite of the harm they do they are one of our more unusual desert denizens. Whether you are horrified or fascinated by them, they are most interesting to observe.


Maricopa County Master Gardener Volunteer Information
Last Updated November 2, 2004
Author: Lucy K. Bradley, Extension Agent Urban Horticulture, University of Arizona Cooperative Extension, Maricopa County
© 1997 The University of Arizona, College of Agriculture and Life Sciences, Cooperative Extension in Maricopa County
Comments to Maricopa-hort@ag.arizona.edu 4341 E. Broadway Road, Phoenix, AZ 85040,
Voice: (602) 470-8086 ext. 301, Fax (602) 470-8092