Borers

Overview

Several species of borers attack apple trees in New England, especially young trees. Dogwood borers are probably most damaging, but roundheaded apple tree borers, apple bark borers, flat-headed apple tree borers, and leopard moths can also be found. Black stem borers, a tiny bark beetle, has recently been attacking stressed apple trees in New England.

Biology

• Dogwood borers and apple bark borers are small wasp-like moths that lay eggs in bark crevices, primarily in burr knots and callus tissue around graft unions. Caterpillars grow up to 3/4 inch long, with an orange tinge. Caterpillars bore in bark, not wood, producing reddish frass on bark surface. Adults fly from mid-June through late August with peak flight in July. Trees with many burr knots (such as M9) are most heavily infested.
• Roundheaded apple tree borers are striped long-horn beetles about 5/8" long that emerge during the month following Petal Fall. Most egglaying occurs from late June to early August, and usually within a couple of hundred yards of where the female emerged. Larvae tunnel through trunks until completing the 2-3 year life cycle.
• Flat-headed apple tree borer is a dark/brown beetle belonging to the family Buprestidae. Adults are about 1/2" long with a metallic luster. They are primarily active in June and July on the sunny sides of trees. Eggs are deposited in bark crevices. Sinuous trails in the bark are visible without cutting into the tree. Eventually, the grubs bore into the wood, leaving tunnels that are oval in cross-section. Grubs are legless with a broad, flattened head end and a cylindrical body. Weakened, stressed or strongly leaning young trees are most frequently attacked.
• Leopard moth (Zeuzera pyrina), lay eggs in bark crevices in July and early August. Larvae bore into the bark and quickly move into the wood. They are usually first noticed because of the moist, fibrous droppings that are pushed out of tunnels. Caterpillars are white or pink with a dark head, and up to 2" long. The life cycle spans 2-3 years.
• Black stem borer is a very small – about 2 millimeters – ambrosia beetle (Xylosandrus germanus) that attacks stressed and apparently healthy trees, and in particular young trees, with trunk diameters of less than 2.5 inches. The insect is rarely seen outside of its galleries and only females emerge from the galleries they create to infest new trees. Signs of infestation include round entrance holes that are approximately 1 millimeter in diameter, toothpick-like strings of compacted boring dust and frass emerging from the holes, and sometimes weeping or oozing of plant sap from the holes.

Monitoring

• Look for signs of black stem borer infestation within 1 meter of the ground and use a simple trap to capture females. Cut two to four windows in the body of a plastic 1- or 2-liter bottle that has a cap. Hang it in the orchard upside down at a height of 1.5 to 3 feet, near wooded areas or in low areas where trees are prone to cold injury and where there are trees with signs of infestation.

  Bait the trap with ethanol using one of the following three methods:
  1. Squirt about a quarter cup of ethanol-based hand sanitizer (unscented) into the cap end (bottom) of your trap.
  2. With the bottle capped, pour in a cup of cheap vodka through one of the holes made in the side of the trap.
  3. Purchase a ready-made ethanol lure to hang inside the trap and fill the bottom of the trap with soapy water.
  If using hand sanitizer, traps must be checked daily because the sanitizer will form a crust on the surface after 24 hours. If using vodka or a purchased lure, traps should be checked at least once per week. Beetles are very tiny and require the use of a microscope and training to identify them correctly to species.

• Leopard moth adults can be monitored with pheromone traps.

Management

• Refer to the spray table for Summer Sprays.  One course spray of Assail to trunk between pink and mid-June. If fresh borer activity found in early July, spray Assail before early August.
• Dogwood borer management can be aided with mating disruption dispensers.
• For black stem borer management, apply insecticide when adult beetles are first caught in traps. Once beetles are inside trees, insecticides are ineffective because larvae do not feed on plant material.