Bark and Wood Borers

Various bark- and wood-boring insects can be pests of trees and shrubs in nurseries. Most of these are sporadic pests and not a problem except when woody species are grown to a relatively large size, such as in boxes. Because the damaging larval stage occurs hidden feeding under bark the symptoms of their feeding as described below under DAMAGE are commonly the first or only observed clue these pests are present.

Erythrina stem and twig borer adults (moths, family Crambidae) have mottled brown and gray forewings, white hind wings, and a wingspan of about 1 inch. At rest the adults hold the rear of their knobby abdomen curved upwards. The moth's coloration makes it difficult to observe on bark.

Adult females lay eggs singly in the axil of leaves near the tips of shoots of coral trees (Erythrina spp.). Eggs are translucent to white, half-dome shapes (convex), and about 1/32 inch (0.8 mm) in diameter. Larvae (caterpillars) are translucent to brown or cream-colored with a black head and dark prothoracic shield on the first segment behind the head. The prothoracic shield becomes lighter colored as a larva matures. Mature larvae are about 1-5/8 inch long and turn pinkish before pupating. The larvae bore into stems and hollow out the terminals of coral trees. Pupae occur in cocoons in protected places of the plant or in litter on the ground. For more information and numerous photographs of this relatively new pest see Erythrina moths Terastia meticulosalis Guenée and Agathodes designalis Guenée and Terastia meticulosalis Guenee: Erythrina Twigborer.

Nantucket pine tip moth adults (moths, family Tortricidae), mature larvae, and pupae are about 2/5 inch long. The adults are reddish brown moths with silver-gray markings. Eggs are spherical, orange or yellow, and occur on needles of pines. Young larvae are whitish with a dark head. Older larvae are yellow to pale brown with a dark head. Larvae chew in and on pine buds, needles, and terminals and cover infested shoot tips with fine silk. It is a pest primarily in Southern California, especially on Monterey pine. It has about four generations per year in Southern California. For more information see Pine Tip Moths—Rhyacionia spp.

Olive bark beetle adult females (subfamily Scolytinae) primarily bore into and lay eggs in olive trees. Other hosts include ash, common lilac, and oleander. Adult females each lay up to 60 eggs. Adults are about 1/12 inch (2 mm) long. They have an hard, oblong, dark brown to black body densely covered with yellowish hairs. The antennae have a clubbed (swollen) end with three distinctive movable, leaf-like segments. Mature larvae and pupae are about 1/8 inch (3 mm) long and occur under bark. Larvae are white and legless with a brown head. The oblong pupae are initially white but as they mature darken and develop distinct appendages folded against the body. Olive bark beetles may have several generations per year.

Be on the lookout for invasive shot hole borers, although these have not been reported infesting nurseries in California. Their appearance, damage, and life stages resemble those of olive bark beetle and other Scolytinae. For more information on olive bark beetle, see Olive bark beetle (Phloeotribus scarabaeoides) (PDF).

Pacific flatheaded borer (family Buprestidae) can infest at least 70 species of trees in 21 plant families. Adults are bullet shaped, hard bodied, and 1/2 to 3/4 inch long. They have a coppery, dark bronze, or gray body and wing covers with pale mottling. This appearance blends with the color of bark making adults difficult to observe. The circular, disklike egg is laid on bark and is white and about 1/25 inch in diameter. The hatching larvae chew and bore into bark. Larva are pale yellow to whitish and legless. They have brown mouthparts and distinct segments and taper towards the rear. The segment immediately behind the head (prothorax) is greatly enlarged. At maturity larvae and pupae are 3/5 to 3/4 inch long. The oval pupa occurs under bark and is initially translucent white, then creamy white, then gradually takes on the color of adults.

Peachtree borer, sometimes called greater peachtree borer (family Sesiidae), this is a different species than the lesser peachtree borer (Synanthedon pictipes), which occurs only in the eastern United States. Peachtree borer primarily infests Prunus species including stone fruits such as apricot, cherry, peach, and plum. Adult peachtree borers are mostly bluish black and have a wasplike appearance. Males have narrow yellow bands on their abdomen. Females have a single orange band on the abdomen. Larvae are pinkish or whitish with a brown head. Pupae are oblong and brown to orange and occur at the base of plants in bark crevices or on the ground. For more information see Pest Notes: Clearwing Moths.

Pitch moth of pines larvae (family Sesiidae) infest Douglas-fir and most pine species, especially Monterey pine. Larvae are pale orange to pinkish with a brown head. Pupae and their empty pupal case commonly protrude from a hole in the pitch masses that form on bark where the larvae fed. Mature larvae and pupae are about 1-1/2 inches long. The adults are day-active moths and have a blackish and yellow head, legs, and thorax. The abdomen is covered with blackish and yellow hairs in alternating bands, resembling a paper wasp or yellowjacket wasp. Females are somewhat larger and plumper than males. The adult's body is about 3/4 inch long with a wingspan of 3/4 to 1-1/4 inches. For more information see Pest Notes: Pitch Moths.

Sycamore borer larvae (family Sesiidae) infest primarilyinfests primarily ceanothus, oaks, and especially sycamores. Adults are mostly yellow with a brownish-black head and black bands on the body, mimicking yellow jacket wasps. The legs are yellowish and black. The mostly clear wings have dark veins and margins. Larvae are pinkish with a brown head. The brown to orangish, oblong pupae commonly occur lodged in bark crevices and at the base of infested hosts. For more information see Pest Notes: Clearwing Moths.

Western poplar clearwing moth (family Sesiidae) is also called the locust clearwing. Its larvae (caterpillars) infest birch, poplar, and willow, especially when trees are stressed. The adult resembles a yellowjacket wasp, but has a thick waist and feathery antennae of a moth, unlike the narrow threadlike waist and filamentous antennae of a wasp. Its forewings range from an opaque pale orange to a brownish color; the hind wings are clear. The thorax is black with a yellow hind border, and the abdomen is yellow with three broad black bands. The entire body of some individuals is pale yellow. Under bark the pale, dark-headed larvae have two hornlike spines on their back. For more information see Pest Notes: Clearwing Moths.

Bleeding, cracked, gnarled, oozing, rough, or wet bark and dieback of limbs, shoots, or entire plants are commonly the first indications that larvae of these boring insects are present. Brown granular excrement from larval tunneling can occur around damaged bark. Because tunneling damages the plant's vascular system scattered limbs or shoot terminals may dieback. Some of these pests can eventually cause the entire plant to die.

Erythrina stem and twig borer hatching larvae bore inside and chew, feed, and tunnel causing die back of terminals. When abundant virtually every terminal on a coral tree can be killed. Infestation greatly reduces seed and flower production and the aesthetic quality of coral trees. Entire trees can be killed by the pest.

Nantucket pine tip moth larvae (caterpillars) boring in shoots cause pitch to exude. Infested terminals discolor and die. Larval damage to the central growing terminal (leader) can significantly alter tree shape, causing regrowth of stems to be bunchy, crooked, or forked.

Olive bark beetle larvae excavate galleries under bark, weakening and often completely girdling killing branches. Severe infestations may cause extensive dieback, stunt the growth of young trees, and reduce the number of flowers and olive fruits.

Bark becomes cracked and roughened where Pacific flatheaded borer larva feeds underneath. Peeling bark back can reveal the larva and its tunnel packed with reddish brown frass (excrement). After the larva matures and pupates under bark the emerging adult leaves a D-shaped to rounded hole in bark about ¼ inch in diameter.

Peachtree borer brown granular excrement and ooze commonly occur on the lower trunk where larvae tunnel and damage cambial tissue. Shaving away bark at the base of infested trunks reveals gum deposits and tunnels packed with brown frass (excrement). Virtually all larval tunneling of peachtree borer occurs within a few inches of the ground near the base of the main trunk, including a short distance above and below the soil line. If infestations persist for several years, the tree may eventually become girdled and die prematurely.

Pitch moth of pines infestations are recognizable by the gray, pink, reddish, or yellowish gummy masses that protrude from infested trunks and limbs where a larva feeds underneath shallowly in bark. People unfamiliar with the damage sometimes confuse pitch moth pitch masses with bark beetle pitch tubes. Bark beetle pitch tubes are usually less than 1/2 inch in diameter, often have a distinct round hole near the center made by an adult beetle, and may resemble the end of a large gummy drinking straw protruding from bark. Pitch moth masses commonly grow to several inches in diameter. Prying a fresh pitch mass off bark commonly reveals a larva feeding underneath. This feeding increases the likelihood of limb breakage during windy conditions and reduces the aesthetic quality of infested trees, but generally does not kill pines.

Sycamore borer larvae tunneling shallowly under bark cause roughened bark. They produce abundant, granular, reddish brown frass (excrement) that can become abundant at the base of trees and in bark crevices. Tunneling generally does not cause limb dieback or kill hosts, but it reduces their aesthetic quality.

Western poplar clearwing larvae feeding under bark can cause gnarled growth and swellings on limbs and trunks. Dark oozing sap may run down bark from the location of an infested. Where a larva tunnels under bark commonly there is a glob of reddish brown frass protruding from the tunnel entrance. Infested poplars can experience dieback, but willows tolerate infestation without apparent serious harm. The aesthetic quality of hosts is reduced by infestations.

Preventive cultural controls and insecticides are the major management methods.

Various parasitic wasps and predatory insects can prey on larvae under bark. But these do not appear to be of importance in providing biological control in nurseries.

Excellent cultural care and providing woody plants optimal growing conditions are critical to avoiding most borer problems. Appropriate irrigation is particularly important as drought stress increases tree susceptibility to bark- and wood-boring insects. Erythrina stem and twig borer apparently is an exception to the situation where most borer problems are caused or aggravated by hosts being stressed.

Provide excellent cultural care to keep plants growing vigorously. Protect trunks and roots from injuries. Whitewash trunks, wrap them with heavy paper, or grow under shade covers or where plants receive afternoon shade to help prevent sunburned bark that attracts the egg laying adult females of many boring insects. If trunks are wrapped, periodically remove wrapping and inspect bark for injuries. If infestations are limited to a few limbs, prune these out and chip or otherwise dispose of cuttings because borers can often complete their development in cut wood then emerge as adults that cause infestation of nearby hosts. If trunks are infested, rough and chip or otherwise destroy the wood or dispose of it off-site.

For peachtree borer pheromone mating disruption has worked well for controlling this pest in crops where it is a regular problem. However, this technique apparently has not been studied for this pest in California nurseries.

Cultural controls, mating disruption for peachtree borer, and white painting or physically covering trunks are organically acceptable management methods.

Regularly inspect plants and their environment and monitor cultural practices especially irrigation to ensure that plants are receiving excellent care and optimal growing conditions. If cultural and environmental conditions have been optimal and borers have still been a problem limbs and trunks can be sprayed with a persistent, broad-spectrum insecticide about twice per season in late winter and early summer to kill adults and hatching larvae before they bore under bark. Neither systemic insecticides or other methods are likely effective for controlling larvae under bark. Erythrina stem and twig borer may be an exception where a systemic neonicotinoid (e.g., imidacloprid, dinotefuran) may potentially be effective.

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