Larvae in stem. Jim Baker, North Carolina State University, Bugwood.org
Adult dogwood borer (Lepidoptera). J. Solomon, USFS, Bugwood.org
Series of holes on infested twig. James Solomon, USDA Forest Service, Bugwood.org
Damage at root collar on flowering dogwood (Cornus florida) from dogwood borer (Lepidoptera); note white larvae (caterpillars). J. Solomon, USFS, Bugwood.org
Overview
The dogwood borer, Synanthedon scitula, one of about seven borers that attack dogwood, is the most serious. It also attacks flowering cherry, chestnut, apple, mountain ash, hickory, pecan, willow, birch, bayberry, oak, hazel, myrtle, and loquat. Wounded trees are the most vulnerable since the larvae must gain access to the tree through wounds or scars. The adult is a clearwing moth that looks like a wasp.
Symptoms and Diagnosis
An early symptom is the sloughing of loose bark. More advanced symptoms are crown dieback and adventitious growth. Coarse sawdust-like frass may be present at the entrance holes and pupal skins at the exit holes. Small wet areas on the bark in early summer may indicate young borer activity.
Life Cycle
The adult moth is about 1/2 inch long and resembles a wasp. It has a blue-black body with yellow bands and clear wings. It generally begins to emerge when the last of the flower petals are ready to drop and the weigela begins to bloom at the end of April or early May and continues into September. The females lay eggs on or near wounds or in the crotches of limbs.
The larva is a little more than 1/2 inch long and is white to cream-colored with a light brown head. They feed on phloem and cambium and remain in their tunnels throughout their development. The larvae overwinter in the feeding tunnel and begin feeding again for a short time in the spring before pupating. There is usually only one generation produced per year.
Integrated Pest Management Strategies
1. Prune out and destroy infected branches if possible.
2. Cut out borers with a sharp knife or crush them by inserting a wire into the tunnels.
3. Avoid pruning when moths are active. Prune in late winter.
4. Avoid wounding. Although applying pruning paints is no longer recommended to assist in healing, applying pruning paints may help prevent entry of the borer through wounded areas.
5. Use a chemical treatment if necessary. Endosulfan (Thiodan) or permethrin should be applied to trunks, root collar, and main branches during peak moth flight in early to mid-March and again four weeks later. Sex pheromone traps are useful in monitoring moth activity.
6. Replant with a resistant species. The Chinese dogwood, Cornus kousa, seems to be more resistant.
Organic Strategies
Strategies 1, 2, 3, 4, and 6 are strictly organic approaches.