Mesquites (Prosopis spp.) are deciduous, large shrubs or small trees that belong to the pea family (Fabaceae). Despite their thorny twigs and messy seedpods, mesquites are commonly used as specimen or shade trees in warm-weather climates. The sweet-scented honey mesquite (Prosopis glandulosa) thrives in U.S. Department of Agriculture plant hardiness zones 10 and 11, while the velvet mesquite (Prosopis velutina), also called the Arizona mesquite, looks striking planted in desert and native gardens in USDA zones 9 through 11. These tough little trees occasionally attract a few insect pests.
Flatheaded appletree borers (Chrysobothris femorata) sometimes attack mesquites. These pests range from 1/2 to 3/4 inch long and have metallic green or brown bodies, short antennae, large eyes and a tooth-like growth on the front legs. Adult borers feed on the foliage from spring through autumn, sometimes eating enough that the mesquite defoliates. Adult females lay about 100 light yellow eggs beneath the bark. The larvae emerge in 15 to 20 days, promptly burrowing just beneath the bark to feed. The boring activity creates tunnels that disrupt the transfer of nutrients and water throughout the plant, resulting in wilted or browning foliage, branch dieback and even death of the mesquite. Mature larvae reach 1 1/4 inches long with pale yellow to light green bodies.
Mesquite plants sometimes attract mesquite twig girdlers (Oncideres rhodosticta), a longhorned beetle species with a brown body, long antennae and three black spots on the head. These pests typically appear from late summer through mid-autumn. The adult females chew V- shaped grooves all the way around small twigs and then lay their cream-colored eggs inside of those grooves. The larvae emerge in about one week and promptly start tunneling under the bark where they remain and feed until they pupate. Fortunately, the larvae doesn't threaten plant health at all, but a female's girdling activity disfigures the mesquite, causing the injured twigs and attached foliage to die and prematurely fall.
The experts at Arizona State University report that various aphid species occasionally feed on velvet mesquite trees. Aphids are tiny insects that congregate in large numbers to feed on new spring growth. They feed by piercing plant tissue and sucking out the sap. While small aphid populations typically don't damage mesquite plants, large numbers cause curled, yellowed and misshapen foliage along with stunted shoot growth. These pests also excrete a sugary, sticky substance called honeydew, which attracts the gray sooty mold fungus. You can easily knock these pests off your plant with a strong stream of water from a garden hose.
Insecticide applications haven't proven effective against borer or twig girdler populations, as of May 2013. If you need to treat large aphid populations, the Montana State University Extension website suggests spraying your mesquites with a 2 percent horticultural oil solution. Thoroughly cover the foliage, making sure to get the underside of the leaves where the pests prefer feeding. Horticultural oils only kill bugs on contact, so you might need to repeat applications. Avoid using broad-spectrum insecticides because they will also kill the aphids' natural predators, such as parasitic wasps, ladybugs and lacewings.
Proper Mesquite Care
Healthy mesquite trees and shrubs are far less likely to suffer from pest damage than those suffering environmental stress. Mesquites thrive in fully sunny locations and prefer dry, loamy or sandy soils, but they tolerate various soil conditions just as long as the site has good drainage. Keep weeds and grass away from your mesquite to avoid attracting bugs and to make it easier for you to spot and remove pests. Prune out and destroy any severely infested foliage, but be careful when pruning because borers are drawn to wood injuries.