American Plum Borer
Description and life cycle
Adults are night-flying moths with front wings that vary from a reddish brown to a grayish brown with a broad black or dark purple band transversing the outer third of the wing. The hind wings are a light dusky tan color with slightly darker veins. They are about 1 inch long when at rest with the wings folded and have a wingspan of ¾ to 1 inch. Larvae range in color from a dirty white with a greenish tinge along the underside to a deep reddish purple, have a dark brown head capsule, and reach about 1 inch at maturity. Pupae are about ½ inch long and are found in a white silken cocoon under the bark with reddish frass scattered around, but not part of, the cocoon. Larvae of lesser peachtree borer occupy a similar habitat but are white with a yellowish brown head capsule. The pupae of the lesser peachtree borer are similar in size, but the cocoons are dark brown, with frass forming an integral part of their construction. Emerged pupal skins of the plum borer do not extrude from the cocoon or from the bark as do those of the lesser peachtree borer.
American plum borer has two generations per year. Nearly full-grown larvae overwinter within silken hibernaculae near the cambium feeding sites or on the underside of overlying dead bark. Most larvae pupate within the hibernacula as soon as the cherry buds begin to open, and first adult emergence occurs by the white bud stage about 2 weeks later. Peak adult emergence of the first generation occurs just after full bloom. The majority of eggs of the first generation are laid by petal fall, although adult emergence often continues for another 3 weeks. Adult emergence of the second generation begins in June, peaks in mid July, and continues into August or September. Peak emergence and egg laying coincides with the mechanical harvesting of cherries when there is an abundance of fresh cracks and wounds suitable for oviposition and chemical control is impossible because of residue problems on the harvested fruit.
Females are attracted to fresh wounds and lay very small pink eggs singly around the wound. The eggs quickly turn white and upon hatching the larvae immediately enter the wound to feed. In all tree fruits except plum, the larvae feed exclusively on the cambium. Sweet cherries and plums appear to be more tolerant of damage than tart cherries. About 90 percent of larvae are found girdling the trunk and lower scaffold limbs within 4 feet of the ground where the clamps from trunk shakers have caused damage. The use of limb shakers, however, may cause infestations above this height as may heavy top pruning. Populations of over 10 larvae per tree are common in some cherry-growing regions of Michigan and can girdle an 18-inch trunk in 10 years. The decrease in the life of a tart cherry orchard with populations this high has been estimated at 25 percent owing to the entry of disease into these wounds and the eventual complete girdling of the trunk or limb. A gradual decrease in yield would also be expected but has not been investigated. Although larvae may feed on plum in the trunk and on limbs in black knot growths, this host appears more tolerant of damage than tart cherries. This borer may be found feeding in burr knots in apples alongside the dogwood borer, especially where NAA (napthalene acetic acid) has been used to kill the burr knots.
The need for control and timing of sprays for American plum borer management can be predicted by trapping adults using a commercially available sex pheromone to monitor male flight. At least two monitoring traps per ten acres should be placed as close to the orchard center as possible to minimize drawing adults from alternate hosts in adjacent woodlots. If the average catch exceeds six moths per trap per week during the adult flight of either generation, this species is likely causing economic damage.
A more accurate way to measure infestation levels is to check for the white hibernacula and reddish frass around shaker wounds by peeling away any overlying dead bark near the wound. This should be done in the early spring before white bud or in midsummer just before harvest so that new hibernacula can be distinguished from those of previous generations by the presence of live larvae or pupae. The sample trees should be scattered throughout the block, and examination of wounds often requires the use of a long-handled screwdriver or wood chisel to pry away dead bark. A threshold is more than two to three larvae in fresh hibernacula in each of several visibly wounded trees from previous years.
Minimize mechanical harvest shaker injury.
A dilute handgun application of a long residual insecticide is recommended at petal fall if treatment thresholds are exceeded. Specific chemical recommendations for home gardeners are in Fruit Production for the Home Gardener, and recommendations for commercial growers are in the Penn State Tree Fruit Production Guide.