Flatheaded Borers (Beetles)
Adult flat-headed borers are generally known as metallic wood borers. They range in size from 1/3 to 1-inch long. Their bodies are flattened, oval, or elongated, and typically they have grooves on their wing covers. The larvae are yellowish-white, 1 to 2 inches long, broad and flat in the front, and tapered toward the rear. Many have segmented bodies.
Females lay their eggs in bark crevices in the spring and early summer. After hatching, the larvae bore shallow tunnels packed with frass beneath the bark and sometimes into the heartwood. Their tunnels are long and winding, forming oval galleries beneath the bark. Dark, dead areas of bark, often with sap, are evidence of larval activity. A wet spot usually forms around the affected bark. Many species complete their life cycle in one year, while others require 2-3 years.
The flatheaded apple tree borer (Chrysobothris femorata) is a common species that feeds on many deciduous shade and fruit trees and shrubs. Favorites include maple, oak, hickory, willow, sycamore, rose, tuliptree, and cotoneaster. Adult beetles are dark metallic brown to dull gray, about 1/2 inch long. The adult female lays eggs in bark crevices. The newly hatched larvae bore into the inner bark, forming tunnels and filling them with frass. If a plant is healthy, the larvae may be killed by sap flow. When nearly mature, the larvae tunnel into the heartwood where they pupate. Newly transplanted trees and shrubs are particularly susceptible.
Two-lined chestnut borer (Agrilus bilineatus) primarily attacks oaks. Some of the more frequently attacked species include white, red, scarlet, northern pin, black, bur, and chestnut oak. The adult beetle is slender, 1/5 to 1/2-inch long, with 2 yellow stripes along its back. Adults emerge in late May to early June, feeding on foliage. After mating, females lay eggs in small clusters in bark cracks and crevices. Eggs hatch in 7 to 14 days. Larvae burrow through the bark, constructing galleries tightly packed with frass. These feeding galleries cut the flow of food and water in the plant’s vascular system, causing the bark to crack and split. Larvae pupate in the outer bark in a pupal cell. The following spring the adult beetle chews a D-shaped emergence hole, similar to bronze birch borer. There is one generation per year. Attacks from two-lined chestnut borer begin in the crown of a tree and proceed downward in each succeeding year of infestation.
Honey locust borer (Agrilus difficilis) attacks large and small honey locust trees, as well as branches more than 2 inches in diameter. Trees under stress are more susceptible. Adults are 1/2 inch elongated beetles, with a metallic, greenish-black abdomen with a distinct yellowish-white band on the sides. Adults emerge in June and feed on honey locust foliage. Females lay eggs that are covered with a frothy substance that hardens. The larvae bore into the trunks or branches causing sap to ooze at the site. Once exposed to air, this sap hardens into a mass of gum. Fully-grown larvae are flat and segmented. Repeated infestations gradually cause decline and dieback of twigs and branches in the crown.
Roundheaded Borers (Beetles)
The adult roundheaded borers are also known as “long-horned” beetles because of their long antennae. They have hard-shelled bodies that are sometimes colored with stripes, bands, or spots. Body length varies from 3/4 inch to over 3 inches. The larvae are white to yellow, have no legs, and are round and fleshy. Females lay eggs under bark scales, in crevices, or in tree wounds. After hatching, the larvae feed beneath the bark for a while before entering the wood. Some species do not tunnel into wood. Life cycles can vary from several months to several years.
The locust borer (Megacyllene robiniae) is a serious pest of the black locust (Robinia pseudoacacia). It does not attack honey locusts (Gleditsia spp.). Larvae tunnel into a tree’s trunk and branches, weakening the tree and making it susceptible to wind breakage. The most obvious sign of attack is a knotty swelling on the trunks, many broken limbs, and irregular holes.
The adult locust borer is a slender, “long-horned” beetle, about 3/4 inch long, with reddish legs and black antennae. Bright yellow bands circle its jet-black body. A distinctive W-shaped band extends across the wing covers. Often the brightly colored adults are found feeding on the pollen of goldenrod (Solidago spp.) during September. Egg-laying occurs in late August through October in bark cracks and scales of host trees. Eggs hatch in seven days and small, white larvae tunnel into the inner bark causing sap to ooze around small holes. Through the spring and early summer, larvae enlarge in their tunnels. By late July into August, they have completed pupation and emerge as mature beetles.
The round-headed apple tree borer (Saperda candida) takes 2-3 years to complete its life cycle. Adult beetles are 1 inch long and brown, with two white longitudinal stripes on their backs. Larvae overwinter in various stages feeding on sapwood and heartwood. Pupation occurs in the late spring of the second year, and emergence begins in early summer.
The presence of piles of frass at the base of trees or on the bark of larger branches is evidence of an attack. Trees become weakened and heavy infestations can kill a tree in one season. Members of the rose family are favorite hosts of the roundheaded appletree borer.
Clearwing Moth Borers
Lilac borer
Podosesia syringae is a clearwing moth with a chestnut, brownish-black body. The forewings are dark brown and the hindwings are clear with brown borders. An overwintering larva is creamy-white with a light brown head. It produces extensive holes and lateral tunnels in the sapwood of lilac, ash, privet, and other members of the olive family. The area around the holes is swollen, the bark is cracked and broken, and oozing sap is mixed with sawdust and frass. Branches are severely weakened at the feeding site and sometimes break. Adults emerge in mid-spring to deposit eggs in cracks and loose bark.
Peachtree borer
Synanthedon exitiosa is a dark blue clearwing moth with yellow or orange markings. Both wings of the male are clear and there are several yellow bands on its body. The female forewings are covered with metallic blue scales and an orange band is on the abdomen. The yellowish-white larvae have brown heads and overwinter at the base of trees and in leaf litter. Pupation begins in the spring and adult emergence takes place from mid-June to late September. Adult flying, egg laying, and hatching of the two peach tree borer species overlap. Purple sand cherry (Prunus x cistena) is highly susceptible to peachtree borer.
Lesser peachtree borer (S. pictipes) adults have clear wings and metallic blue bodies with yellow markings. It is a pest of flowering cherry and almond. Larvae feed on branches, near or in trunk crotches, and at old trunk wound sites found above ground level. An amber-colored gum (gummosis) is frequently found where borers have attacked.