The Palo Verde beetle, also known as the Palo Verde Root Borer (Derobrachus hovorei or Derobrachus geminatus), is a large insect found in the Southwestern United States, and is one of the largest beetles in North America. This pest is notorious for the damage its larvae inflict on tree roots, particularly those of the Palo Verde tree and other broadleaf species. Their presence during the summer monsoon season signals a long-term threat to the health of landscape trees. Eliminating an infestation requires addressing both the visible adult population and the destructive, unseen grubs underground.
Identifying Palo Verde Beetles and Their Damage
Identifying this pest involves recognizing the distinct life stages. Adult Palo Verde beetles are large, measuring between two and four inches long. They have a dark brown or black hard shell, very long antennae, and prominent spines on the thorax behind the head. Adults are primarily nocturnal, often seen flying erratically or resting on the ground from late June through August.
The larvae, or grubs, are creamy white, thick, and can grow up to five inches long. These grubs live underground for three to four years, feeding on the woody tissue of the tree’s roots. The primary visible sign of their activity is a decline in the tree’s health, such as unexplained branch dieback. Definitive evidence of an infestation is the presence of quarter-sized, round exit holes appearing in the soil near the base of the tree or directly in the trunk where mature beetles have emerged.
Immediate Physical Control of Adult Beetles
Controlling adult beetles disrupts the reproductive cycle, preventing new eggs from being laid. Since the adult stage is short—lasting only a few weeks—and their sole purpose is to mate, removing them reduces the next generation of grubs. These beetles are slow-moving, making manual removal straightforward, especially during the early morning hours.
When handling adults, wear thick gloves, as they possess strong mandibles and may pinch. Collected beetles can be disposed of by crushing them or dropping them into a bucket of soapy water. Reducing the attraction of outdoor lighting is also helpful, since the beetles are strongly drawn to illumination at night. Limiting the use of exterior lights or switching to yellow “bug light” bulbs during the monsoon season minimizes their presence near the home.
Managing Underground Larvae Infestations
Addressing the larvae is essential, as they cause extensive root damage over several years. One biological approach involves applying beneficial nematodes, microscopic roundworms that hunt and parasitize soil-dwelling insect larvae. The species Heterorhabditis bacteriophora (Hb) is often recommended for treating beetle grubs because they actively seek out and infect larvae in the soil. Nematodes are non-toxic to humans and pets, requiring application to moist soil when temperatures are between 57 and 86 degrees Fahrenheit for optimal activity.
Chemical control is challenging because the grubs are protected deep within the tree roots, making contact insecticides ineffective. Systemic insecticides, applied as a soil drench around the base of the tree, are sometimes used by arborists, but their efficacy against Palo Verde root borers is debated within the scientific community. For heavily infested trees showing significant decline, the best intervention is to remove the tree and grind the stump completely. Removing the dying wood eliminates the food source and prevents remaining larvae from maturing and emerging as adults.
Preventing Future Beetle Activity
Long-term management focuses on cultural practices that increase the tree’s vigor, making it less susceptible to infestation. Palo Verde beetles primarily target trees that are already stressed, weak, or diseased, likely attracted by chemical signals emitted by the struggling wood. Maintaining a healthy tree through proper care is the most reliable long-term defense.
Appropriate watering is important for desert trees, requiring deep, infrequent irrigation rather than shallow, frequent watering, which promotes weak root growth. Avoiding mechanical injury to the trunk and roots prevents open wounds that provide easy access points for egg-laying females. Garden sanitation, such as regularly removing dead wood, fallen branches, and other organic debris near vulnerable trees, helps eliminate potential breeding sites.