Shothole borer—Scolytus rugulosus
This bark beetle (subfamily Scolytinae) attacks the limbs and trunks of various woody broadleaf plants. Common hosts include apple, hawthorn, mountain ash, pear, and Prunus species, such as English laurel and stone fruit trees.
Clear to brownish sap exuding from holes in bark about 1/16 inch (1.5 mm) in diameter is commonly the first-observed sign that a tree is being attacked (bored into) by adult shothole borers or other Scolytinae. When a tree is successfully attacked and borer larvae and pupae develop under bark, the new generation of adult borers that emerges will leave numerous small holes in bark that are dry without any exuding sap.
Adults are blackish to dark brown, oblong, hard-bodied insects about 1/10 inch (2.5 mm) long, somewhat larger than most species of bark beetle. The tiny eggs, larvae, and pupae are pale yellowish to white. Larvae are legless, whitish grubs with a brown head. Larvae and pupae are about 1/10 inch long or less. They occur hidden in tunnels under bark that are partly packed with the beetle's brown frass (excrement).
Bark beetles develop through four life stages: egg, larva, pupa, and adult. The mated adult female bores through bark into the cambium of plant hosts and lays eggs along a tunnel she chews. The emerging larvae grow through four, increasingly larger instars as they feed and chew winding tunnels under bark. Mature larvae pupate under bark then emerge as a new generation of adults.
Shothole borer overwinters as mature larvae (prepupae) under bark. In spring shothole borers pupate, emerge as adults, then mate and the females seek hosts. Shothole borers can have several generations per year.
The shothole borer attacks and reproduces in trees weakened by insufficient soil moisture, root diseases, sunburn, or other stressful growing conditions. Adults boring into trees cause hosts to exude copious sap. If trees are sufficiently healthy and adequately irrigated, the tree's sap may kill and repel the attacking beetles and the tree may survive. If beetles overcome the tree's defensive response and lay eggs, as the larvae hatch and feed under bark, the vascular system of limbs or trunks become girdled. This causes fading of leaf color and foliage wilting. Limbs or the entire tree can be killed.
Adults also chew and feed at the base of small twigs, which may be killed. But this damage is not commonly noticed unless large numbers of adults are present. Most of the damage is done by the larvae feeding between the bark and wood. Holes in bark made by beetles boring into trees or emerging give the appearance of a BB- or shot-gun blast, leaving "shot holes" for which the species is named.
Prevent sunburn of newly planted or heavily pruned trees. This can be accomplished by whitewashing (painting) trunks with white, interior latex paint diluted with an equal amount of water.
Provide trees with a good growing environment and optimal cultural care to reduce their susceptibility to wood borers. Appropriate irrigation is especially important as drought stress greatly predisposes trees to attack by various boring insects. Providing adequate soil moisture promptly after a drought-stressed tree is first attacked may help the tree to survive.
Prune to eliminate limbs infested with shothole borers if this can be done without removing a large portion of a tree's wood. Remove dying and severely infested trees before the beetle larvae mature and emerge as adults that can move to attack other hosts. Chip all wood or haul it to the landfill before the growing season starts. Do not leave pruned limbs or stumps (e.g., firewood piles) near hosts as shothole borers can emerge from cut wood before it dries and attack nearby hosts. For more detailed prevention and management recommendations including discussion of insecticide treatments, see Pest Notes: Bark Beetles.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Sap oozing from holes of a trunk recently bored into by adult shothole borers.
Emergence holes of adult shothole borers.
Tunnels under bark from shothole borers.
Adult shothole borer, which is 1/10 inch (2.5 mm) long.
Larva of a Scolytus species bark beetle.