Shot Hole Borers

Under the bark of your fruit trees, there may be tiny bark beetles, chewing destructive tunnels.

There are over 200 different types of bark beetles in California (600 nationwide). We currently host 20 invasive species of bark beetle, half of which were only discovered since 2002. The shot hole (or shothole) borer has been here long enough that it is considered naturalized.

Female shot hole borer (Dgomezuf) CC BY-SA 4.0

Bark beetle galleries

Bark beetles spend most of their lives in a system of tunnels, called a gallery. They chew these tunnels under the bark, in the sapwood or cambium layer. If you pull the bark away from an infested area, the gallery can clearly be seen. As the larvae feed on sapwood, expanding the gallery, these tunnels criss-cross each other, damaging the xylem and phloem, making it difficult for trees to transport water and nutrients. Parent tunnels tend to be 2 inches long and run parallel to the grain. They are normally clean. Larval tunnels, like a teenage boy’s room, radiate away from the parent tunnels, against the grain, are smaller, and tend to be filled with frass and sawdust.

Shot hole borer host plants

Shot hole borers, also called fruit tree bark beetles, prefer (you guessed it!) fruit trees, especially those in the stone fruit family. This means your almond, apricot, cherry, nectarine, and peach trees are susceptible, along with apple, pear, and avocado. Shot hole borers also feed on English laurel, mountain ash, hawthorn, and elm.

Shot hole borer lifecycle and feeding

Shot hole borers (Scolytus rugulosus) start out as white, round or oval eggs, laid in the gallery. These eggs hatch out into white, legless larvae that are 1/6 of an inch long and slightly larger just behind the head. These larvae overwinter in the gallery, creating more tunnels. As they feed, they poop. To make room for this material, the borers widen their tunnels and push the poop out, leaving signs of frass and sap to drip down the side of your tree. All this chewing causes further damage to the tree’s vascular bundles.

After 6 to 8 weeks of tunneling and pooping in your fruit tree, the larvae are ready to enter a pupal stage. Pupae are white, with tiny hairs and large knobs (tubercles). Pupae are found at the end of tunnels. Finally, adult shot hole borers emerge to chew exit holes and the cycle starts all over again. Adult shot hole borers are only 1/10” long and brown or black.

These new adults first emerge in spring or early summer and will feed on small twigs, buds, and leaf bases before chewing a new entry hole, where they will excavate more tunnels, further weakening your trees. Entry holes are often found near lenticels. Lenticels are lens-shaped openings in the bark used in gas exchange (sort of like a breathing hole, but not really). Female shot hole borers lay up to 50 eggs. There can be 2 or more generations each year.

​Symptoms of shot hole borer infestation

As shot hole borers first start chewing galleries under the bark of your fruit trees, you are unlikely to see their entry holes (unless your vision is a lot better than mine). These holes are only 3/100 of an inch in diameter. What you may see, however, is oozing, gumming, or staining, as the tree tries to defend itself. You may also see crusty white exudates. [An exudate is something that seeps out.] This white crust is sugar from the tree’s sap. You may also see a sawdust-like frass. If you look very closely, you may be able to see a female’s abdomen sticking out from the hole. Twig dieback may also be seen. Much like the shotgun pattern seen on leaves infected with the shot hole disease fungus, shot hole borers leave behind a similar pattern on tree bark, hence the name.

Shot hole borer controls

Healthy trees are generally able to protect themselves against shot hole borers. This means regular irrigation to avoid water stress, proper fertilizing, depending on the age, size, and life stage of the tree, and pruning out dead and diseased wood. It also mean whitewashing trees in late winter to prevent sunburn damage to the bark. If infested wood is found, it should be removed and destroyed, to prevent the borers from spreading to healthy wood. Once shot hole borers are in the wood, there isn’t much else you can do. Insecticides are ineffective. You may be able to thwart some borers by applying sticky barriers around the trunks of your trees.

Bottom line: keeping your trees healthy in the first place is a lot easier than pruning out branches and twigs infested with shot hole borers!