Mesquite trees (Prosopis spp.) are deciduous plants native to the Americas, Africa, the Middle East and India. Because they tolerate drought conditions, mesquites are often used as specimen or shade trees for xeriscapes in arid climates. Common species include the honey mesquite (Prosopis glandulosa), velvet mesquite (Prosopis velutina) and the screwbean mesquite (Prosopis pubescens), Southwestern natives that thrive in U.S. Department of Agriculture plant hardiness zones 7 through 11 (honey), 9 through 11 (velvet) and 7 through 10 (screwbean). Damage to the trees from the giant mesquite bug (Thasus gigas) and the mesquite twig girdler (Oncideres rhodosticta) is minor and merely cosmetic, so the University of Arizona recommends no controls for these insects. Other pests that occasionally affect mesquite trees can do more serious damage.
Mesquites occasionally attract flatheaded and roundheaded borers, both larvae of beetle species that attack weakened or damaged trees. Adult beetles feed on mesquite foliage, occasionally eating enough of the leaves for the tree to defoliate. Beetle borer larvae dig shallow tunnels beneath the bark, boring winding paths through the wood and phloem. The tunneling activity interrupts the natural flow of water and nutrients throughout the plants, resulting in wilted and discolored leaves, branch dieback and even death of entire trees.
Mesquite scales and candidula scales can become serious pests on mesquite trees. These small pests sport flat, armored coverings and often look less like actual bugs and more like little, immobile bumps on the foliage. Scales feed by inserting their mouthparts into plant tissue and drinking the fluids, which often results in wilted, yellowed or curled leaves. Infested bark sometimes cracks open and oozes a gumlike substance. Severe infestations can also weaken trees, reduce plant growth and cause branch dieback.
Mesquite trees sometimes attract vine mealybugs, small insects covered with a white, waxy coating. These insects form large feeding colonies among the leaves, but can be hard to spot because they hide in little crevices around the stems. Mealybugs stick their threadlike mouthparts into the plant tissue and withdraw the sap. Feeding activity can cause premature leaf drop, twig dieback and reduced plant growth. They also excrete an abundance of honeydew, a sticky liquid that drips on the foliage and attracts sooty mold fungi.
Hosing down your mesquite tree with a strong stream of water from your garden hose can help wash away mealybugs and beetles. Prune out and destroy foliage and branches infested with pests, but avoid pruning your tree from spring through the summer when the adult borers are active and searching for wounded plant tissue. Removing overwintering sites such as fallen leaves, loose bark and surrounding weeds can also help reduce pest populations.
Insecticides haven't proven to be effective on borers, as of July 2013, but horticultural oil sprays often control scales and mealybugs. Thoroughly spray the tops and undersides of leaves as well as the twigs. Repeat applications every two weeks until you achieve control.