American Plum Borer

What's eating your trees?

If you see orange frass and bark damage, it's probably the American plum borer.

Orange frass indicates American plum borer infestation (James Solomon) CC BY 3.0

Like flathead borers, clearwing moths, and shothole borers, the American plum borer (Euzophera semifuneralis) feeds on the cambium layer of branch crotches, the lower trunk, and the root crown. This feeding weakens and damages branches, and can even kill a young tree.

Host trees

American plum borers feed on many popular fruit and nut trees, as well as several ornamentals. Here is a partial list of trees favored by plum borers:

• apple
• apricot
• cherry
• elm
• ginkgo

• mountain-ash
• oak
• peach
• pear​
• pecan
• persimmon​

• plum
• sweet gum
• sycamore
• walnut
• white mulberry
• yellow-poplar

Plum borer lifecycle

Each spring, adult plum borers emerge, mate, and eggs are laid in bark wounds. These wounds can be caused by branches rubbing together, a poorly aimed weedwacker, a careless pruning job, or any number of other insect pests. Also, branches that are growing too close together or at a tight angle, and heavily furrowed bark, can provide the necessary shelter. When the eggs hatch, larva enter the bark wound and begin feeding on the cambium, which contains the vascular tissue carrying water and nutrients throughout the plant. The larvae burrow shallow, irregularly-shaped tunnels between the wood and the bark. These larvae go though several stages, or instars, growing progressively larger, over the next 30 to 38 days. Finally, the larvae pupate in a loosely spun cocoon, inside the tree, where they overwinter. There can be up to four generations each year. Pupae that occur during the early part of the season can reach adulthood in as little as 10 days.

Adult American plum borer (xpda)

Plum borer description

Adult plum borer moths are dark gray with black and brown wing markings. The legs and body of the adult moth are a dusky gray with a reflective bronze. The wings have a white fringe. The larva can be white, green, or pink and are usually one inch long. If you use a hand lens, you should be able to see  a circle of tiny hooks on the bottom of a plum borer’s fourth set of feet (B), called prolegs, as opposed to clearwing moth larvae (A), which have two rows of hooks. [Now you know how larva can climb up vertical surfaces!] More often, you will see the damage caused by the American plum borer, long before you actually see the insect.

Adult American plum borer (xpda) CC BY 3.0

Damage caused by plum borers

As plum borers feed, they leave behind a trail of destruction. Reddish orange frass collects wherever they feed. Also webbing and extensive gumming can be seen. Gumming is a gooey discharge plants use to protect themselves. The gum hardens as it dries, but, unlike sap, it is relatively easy to wash off. Eventually, branches are weakened to the point of breaking and the tree’s overall health declines, reducing or eliminating any chance of harvesting those delicious plums.

Controlling plum borers

Plum borers are easier to prevent than control. Once they enter a tree, they are relatively safe from predators and pesticides. Use these tips to prevent a plum borer infestation:

• 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.

If a bark injury is already present, or if your tree has bacterial galls or a fungal canker, and you know that plum borers are nearby, it may be worthwhile to hire a professional to apply a residual, contact insecticide. Over-the-counter products are not effective against the American plum borer.