Insect pests can and do injure desert trees both in nature and in the landscape. These pests can damage leaves, twigs, branches, trunks and roots. Insects attack stressed or damaged trees and rarely pose problems for healthy, vigorous trees. With the exception of the Palo Verde Borer, these insects usually do not seriously damage trees. Tree Borers can be divided into three general groups: Flatheaded Borer, Roundheaded Borers and Root Borers.

Flatheaded Borers most commonly invade sunburned or otherwise damaged areas along the trunks and branches of trees. Olive-gray adults lay eggs under the bark of damaged areas. Larvae are cream colored and legless and mature to 1 1/2" long. Maturing larvae feed on dead wood producing small tunnels (galleries) filled with what appears to be sawdust. This damage occurs beneath the bark and can go unnoticed for long periods. Since the insects are hidden within the wood they are well protected from chemical sprays applied to the surface of the tree. Larvae do not attack adjacent healthy, undamaged wood.

Larvae of Roundheaded Borers can also bore into damaged wood. The larvae are cream to white colored and are legless. Like the Flatheaded Borers these insects are limited to damaged wood or weakened trees. An exception is the mesquite twig girdler, which girdles and kills small twigs of mesquite trees. The females then lay eggs in the section that is killed and the larvae develop within the wood. Holes typically observed on trees are the exit hole where the mature adult has chewed its way out of the tree. These exit holes can be occupied by many other non-injurious insects.

Palo Verde Borers are root borers and are rarely seen above ground. Adults are 4" to 6" long with antennae nearly as long as their bodies. Adults are active July through October. Immature larvae feed on roots of Parkinsonia (Mexican Palo Verde) and other non-native trees. Larvae spend up to three years underground feeding on roots. Over a 7 to 10 year period Palo Verde Borers will gradually kill a tree. Most adult borer females lay eggs from spring through summer. With the exception of the Palo Verde Borer most borers complete their life cycle in about a year (from egg to adult). Landscape trees located near areas of mature, undisturbed desert trees are more likely to be attacked by tree borers than are those at greater distances from the desert. Maturing native desert trees can harbor populations of pest insects that can then easily migrate into nearby landscape plantings. White grubs, sometimes encountered when excavating the roots of dead trees, usually are not tree borers. Typically these grub are the larvae of the common June Beetle and are only feeding on decomposing organic matter.

A number of insects attack the foliage of desert trees. These include: Acacia whitefly, aphids and the recently identified Palo Verde Scale. Aphids are a common and wide spread plant pest that can attack desert species. As with other traditional landscape plants, aphids are restricted to the succulent new growth on the tips of twigs of desert trees. Aphids reproduce rapidly and can quickly kill small twigs and deposit honey dew (a clear, sticky material excreted by aphids that blackens leaves and twigs). Acacia White Fly feed by scraping at the undersides of leaves giving the leaves a blotchy, yellowing appearance. Acacia White Fly actually appear dark gray or black because of a large dark spot on the body. These insects also reproduce rapidly and cause significant leaf loss. Palo Verde Scale, a previously unidentified pest of Palo Verdes, has recently been observed in the Phoenix metropolitan area. To date it does not appear to cause injury to the trees. Young trees (3 to 5 years of growth) may require sprays to control foliar insect pests. The damage from these insects can slow growth. On more established trees, chemical control can be reduced or eliminated. Allowing moderate populations of these insects to survive will help support populations of a number of beneficial insects that actually feed on these pests.