Prune Limb Borers

Prune limb borers can damage stone fruit trees, such as almond, apricot, cherry, nectarine, and peach, as well as oak. Gumming and reddish orange frass are common signs of prune limb borer infestation.

Prune limb borer (David Beadle, Public Domain)

Prune limb borers (Bondia comonana) are not as common as American plum borers, but it is a good idea to know what to look for, just in case.

Prune limb borer description

Prune limb borer moths are not very large. They have a 3/4” wingspan. The forewings are gray with black and brown markings. Like many grubs, prune limb borer larvae are dull white or pinkish with a large, dark head. They are usually 1” long.

Prune limb borer lifecycle

Prune limb borer larvae overwinter inside your trees in cocoons. In spring, adult moths emerge and mate. Female prune limb borers lay their eggs on callus tissue, where narrow crotches between branches create wrinkled bark, near graft unions, and on crown galls. Eggs are also laid in wounds from pruning, tree supports, or poorly aimed weedwackers. There can be as many as four generations each year.

Prune limb borer (Rich Schilk, Public Domain)

Prune limb borer damage

It is prune limb borer larvae that do all the damage. As soon as eggs hatch, larvae begin burrowing into the host tree. Erratic tunnels between the bark and cambium layer interrupt the flow of water and nutrients and weaken the tree structurally. Heavy infestations can weaken scaffold branches, making them likely to break off in strong winds and when supporting heavy crop loads.

Image of prune limb borer damage from page 82 of "Bulletin - United States National Museum" (1877)

Prune limb borer management

Mature, healthy trees can often withstand a prune limb borer or two, but young trees can be killed by heavy infestations. Like other borers, these pests are easier to prevent than control. Inside the tree, they are safe from predators and pesticides. Use these tips to prevent prune limb borers from taking up residence in your trees:

• Keep pruning cuts smooth, so they can heal quickly
• Provide sunburn protection by whitewashing trunks and exposed branches
• Avoid mechanical injury by lawnmowers, weedwackers, and cars

Over-the-counter pesticides and insecticides are not effective against prune borers. If you have a badly infested tree, it  may be worthwhile to hire a professional to apply a residual, contact insecticide.