Identifying and Treating Borer Infestation in Crabapple Trees

Signs of Trouble in a Mature Crabapple

A 20-year-old crabapple tree that has never been particularly vigorous but produced a nice floral display this spring suddenly shows distressing symptoms: leaves curling and drying with rusty-colored spots, fruit shriveling and dropping, and several clean, pencil-sized holes in the trunk base on the south side, some reaching two inches deep. No sticky residue surrounds the openings.

Diagnosis: Borer Activity and Fungal Leaf Spot

The trunk holes are classic evidence of wood-boring insects, likely flatheaded or roundheaded borers that target stressed trees. The "rusty spots" on foliage indicate a fungal leaf spot disease, common after wet spring weather.

Treatment Strategy

For Borers:

Apply a borer-specific insecticide to the trunk and affected areas. Products containing permethrin, bifenthrin, or newer systemic options like dinotefuran (sold under various brand names) can be effective when sprayed on bark where borers are active. Follow label instructions precisely for mixing and timing—typically late spring to early summer when adults are laying eggs. Thoroughly wet the trunk, especially around existing holes.

For Leaf Spot Fungus:

Apply a fungicide labeled for ornamental trees and leaf spot diseases. Chlorothalonil, copper-based fungicides, or myclobutanil are common choices. Begin applications at bud break next spring and repeat per label directions during wet periods. Rake and destroy fallen leaves to reduce inoculum.

Cultural Practices to Support Recovery

• Water deeply during dry spells to reduce stress.
• Mulch 2–3 inches deep (keep away from trunk) to conserve moisture and moderate soil temperature.
• Avoid wounding the trunk with mowers or trimmers.
• Consider a soil test to check pH and nutrient levels; crabapples prefer slightly acidic soil (pH 6.0–6.5).

When to Consider Removal

If the tree is extensively girdled by borer tunnels, shows major dieback, or fails to improve after two seasons of treatment, replacement with a disease-resistant cultivar may be the wisest long-term choice.