Insects that bore holes into trees are usually the larvae of species of moths or beetles, often referred to as "borers." Controlling borers can be difficult because the larvae are resistant to chemicals. Treating borer infestations can be accomplished with spray insecticides, such as chlorpyrifos, carbaryl or endosulfan, or with trunk-injection insecticides, such as pesticides containing acephate, dicrotophos and oxydemeton-methyl.
Also called metallic wood-boring beetles, flat-headed borers (Buprestidae family) are identifiable by their distinctive metallic body colors, usually in greens, blues, coppers and bronzes. The adult beetles are bullet-shaped and hard-shelled, while the larvae are cream-colored and legless with flattened, wide body segments behind their heads. Signs of flat-headed borer infestations in trees include oval or flattened, winding tunnels just beneath the bark or in the sapwood, or cambium. These boring insects most commonly invade recently transplanted, stressed, dying or dead trees, but they are also found in healthier birch, juniper and apple trees.
Round-headed borers (Cerambycidae family), also called long-horned beetles, bore oval or round tunnels into tree wood. The adults have antennae that are usually longer than their bodies, while the larvae are round with three pairs of tiny legs along the first three body segments behind their heads. The tell-tale signs of round-headed borer infestation are the traces of sap and larvae excrement ("frass") that resembles sawdust seen on the bark of the trunks and branches. Attacking a wide range of tree species, this group of borer insects includes the locust, cottonwood, red-headed ash and red oak borers, as well as the twig girdler, and twig and branch pruners.
The most common species of wood-boring caterpillars (Lepidoptera family) are carpenterworms (Prionoxystus robinae) and clear-wing borers, mainly the peach tree borer (Synanthedon exitiosa). Carpenterworms tunnel into the trunks of a wide range of trees, leaving behind piles of frass in the bark crevices. In late spring, you'll see the large adult moths, which have spotted wings. Clear-wing borers bore holes into the tree trunk. The adult moth also resembles a wasp. The larva has a pair of true legs in the middle of its body and a pair of false legs, or "prolegs," beneath its head. Clear-wing borers most commonly attack fruit trees, causing masses of sap to collect around the bored holes and at the base of the trunk.
Bark beetles (Scolytidae family) are small and reddish-brown to black with cream-colored, legless larvae. They tunnel below the bark and into the wood of trees, riddling the bark with small holes that are edged with reddish or white powdery dust. The most common species of bark beetles include the southern pine beetle (Dendroctonus frontalis), black turpentine beetles (D. terebrans), Ips engravers, shothole borers (Scolytus rugulosus) and ambrosia beetles. "Pitch tubes" often form on pine trees infested with these borer insects.
Although weevil (Curculionidae family) larvae are difficult to distinguish from other grubs, as they're cream-colored and legless, adult weevils have their distinct, protruding snouts with chewing mouthparts. Instead of tunneling into the wood or bark, weevil larvae hollow out cavities beneath the bark, usually in the trunks or tree bases. The most common weevils include the deodar weevil and white pine weevil, which attack spruce and pine trees. The poplar and willow borers are also weevil species.
Pinyon borers (Dioryctria spp.) also bore holes into pine trees. The adults are moths, but the larvae cause the damage to the tree trunks and branches. The pinyon pitch mass borer and Zimmerman pine moth are the most common in this borer category. The Zimmerman pine moth larvae usually attack the branch crotches of Austrian pines, while the pinyon pitch mass borer creates large masses of pine pitch to collect around the borer-feeding holes.