Dogwood Borer

Dogwood Borer, The dogwood borer, a native clearwing moth, can be found from southeastern Canada to Florida, and as far west as the Mississippi. The insect has a wide host range including dogwood, pecan, oak, plum, and apple. The DWB has one generation per year throughout its geographic distribution. On apple, DWB larvae feed primarily in burrknot tissue on clonal rootstocks.

Burrknots are aggregations of root initials which can develop on the above ground portion of the rootstock. All commercial dwarfing and semi-dwarfing rootstocks have a tendency to develop burrknots. The DWB adult has a wing span of 18-22 mm. Both the fore and hind wings are mostly clear.

The thorax and abdomen are deep blue-black with yellow markings. In the female, the entire 4th abdominal segment is yellow, while in the male it is black with a narrow yellow ring. In the Northeast, adult emergence begins in early June and continues into early September, peaking in mid July. Emergence begins earlier further south.

Life Cycle: Dogwood borers overwinter as immature larvae (caterpillars) in tunnels under the bark. Full-grown larvae are 25 mm (1 inch) long and white to cream in color with reddish-brown heads. Larvae change to pupae during spring and adults begin to emerge by early June.

Adults are most abundant in July, though some emerge throughout the remainder of the summer months. Adults are clear-winged moths, active during daylight hours. They have blue-black bodies with a yellow stripe on the second and fourth segments of their abdomen; the legs also have yellow bands. The wings are narrow and transparent. Female moths lay eggs on smooth or rough bark. On older trees, they lay eggs in scars and rough areas of bark on the trunk and larger branches.

Caterpillars hatch in 8 to 10 days and wander around the bark until an opening is found for their entry into the cambium; the larvae are unable to chew through bark. Once inside, they are well protected and difficult to control. Larvae feed in this protected area throughout most of the year. One generation occurs each year.

Damage: Three general feeding types have been identified for the DWB on clonal apple rootstocks. Most frequently, feeding is confined to the burrknot. One or more larvae feed in irregular tunnels beneath the surface of the root initials. At first, feeding is quite shallow, but subsequent feeding may extend as far as 3/4-inch toward the center of the trunk. Feeding confined to the burrknot is believed to be least harmful to the tree.

The second type of feeding may occur as a result of heavy or repeated infestation of a burrknot. As the burrknot tissue is consumed, the larvae move outward and begin to feed on the cambium adjacent to the burrknot. The third feeding type is not associated with a burrknot, but with bark scales and injured bark, and occurs infrequently. Feeding outside the burrknot is thought to be more harmful to the tree because healthy cambium tissue is destroyed. DWB infestations can girdle and kill a tree, but more commonly contribute to a slow decline and yield reduction if they continue over a long period of time.

Management: It is important to maintain the best possible health and vigor of dogwoods by fertilizing, pruning and by watering during dry seasons. Maintaining a mulch layer around the plant aids in keeping the soil cool and moist which helps keep the plant healthy. Do not smother the lower best as an understory plant in a cooler, moist environment.

Avoid wounding the trunks and branches. Lawn mower and line trimmer injuries afford excellent places for borers to enter. Mulching around trunks helps in this regards. Pruning wounds and all other damaged bark areas should be covered with a tree wound paint; pruning is best in later summer during hot, dry weather conditions. Brace newly planted trees in such a way to avoid bark injury and wrap lower bark to improve tree vigor.

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