Tree Fruit Insect Pest - Shothole Borer

The shothole borer, Scolytus rugulosus, sometimes called the fruit tree bark beetle, is a native of Europe but now occurs throughout the United States.
Tree Fruit Insect Pest - Shothole Borer - Articles

Updated: October 25, 2017

Tree Fruit Insect Pest - Shothole Borer

Source: Natasha Wright, Cook's Pest Control, Bugwood.org

It attacks a wide variety of deciduous fruit trees and other trees, but it usually attacks only trees that have been weakened by some other condition.

Description and life cycle

The adult is a dark brown to black beetle, blunt on both ends, about 1/10 inch long. The tips of the antennae, legs, and wing covers are reddish brown. The wing covers are striated with rows of shallow punctures. Larvae are white with a reddish head, legless, and about 1/8 inch long when fully grown.

Injury

Adult shothole borers drill holes, such as might be made by small shot, in the bark and wood of twigs, branches, and trunks of infested trees. The holes usually occur in clusters and may be either entrance or exit holes. Entrance holes are often near a lenticel and thus can be identified. Adults feed and reproduce beneath the bark, creating 2-inch-long tunnels that usually run parallel with the grain. Larval galleries leave the main tunnel and radiate out across the grain. Galleries are easily visible when the bark of infested trees is removed.

Shothole borer damage usually is limited to weak, declining trees, and borer infestations frequently hasten tree or limb death. Beetles are rarely the primary cause of death. When borers are abundant, they will occasionally attack apparently healthy trees nearby. Attacks on healthy trees may be evident on small twigs where adults bore in or around buds. This injury usually is indicated by small droplets of gum exuding from the tiny, round feeding sites. Buds are often destroyed and twig dieback can result.

Shothole borers overwinter as larvae beneath the bark. They pupate in early spring and adults usually emerge in April to May. Adults can fly considerable distances. Females mate, then locate unhealthy trees and bore through their bark. They excavate tunnels beneath the bark and lay eggs along the sides. Larvae hatch, burrow across the grain, away from the parent gallery, and feed on sapwood for about a month. Larval galleries are generally packed with frass and sawdust, while parent galleries are usually clean. Pupation occurs at the end of the larval gallery, and adults exit directly through the bark. Soon after emerging, the beetles reinfest trees to deposit eggs for the next generation. Two generations or more may develop in a tree after it dies.

Monitoring and management

Good horticultural practices are important in preventing shothole borer infestations. Keep trees healthy and vigorous. Eliminate breeding sites by removing and destroying infested trees or limbs as soon as they are found. Prunings should be removed and destroyed before adults emerge each April. Wild fruit trees and other potential breeding sites near the orchard should also be removed. Painting tree trunks with whitewash or white waterbased latex paint is sometimes helpful in repelling adult beetles, especially on young trees. Infested trees can also be sprayed with a residual insecticide to prevent reinfestation. There are no effective controls for insects already in the trees.

Authors

Insect plant interactions Integrated pest management Biological control Tree fruit insect pests Insects rearing Laboratory and field bioassays Invasive insect pests Pesticide resistance

More by Grzegorz (Greg) Krawczyk, Ph.D.