Biology of dogwood borer
Adults
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 (Fig. 2), whereas in the male it is black with a narrow yellow ring (Fig. 3). In the Northeast, adult emergence begins in early June and continues into early September, peaking in mid July. Emergence begins earlier further south.
Eggs
The eggs are light chestnut brown, oval, 0.6 by 0.4 mm, and are marked with a hexagonal pattern of slightly raised lines. Eggs are laid singly on the trunk, and hatch after an incubation period of 8-9 days.
Larvae
The larvae are off-white to cream colored with a reddish head capsule (Fig. 4). Larvae pass through six instars ranging in length from 1 mm when newly hatched to 15 mm or more in the last instar. Soon after hatching, the larvae begin to burrow into the soft burrknot tissue, or areas under bark scales. As the larvae feed, reddish-brown frass is pushed to the surface, where it collects, held together by silk (Fig. 5, arrow). Larvae overwinter in the feeding tunnel. Feeding resumes whenever the temperature is above 45-50°F (7-10°C).
Larvae of the American plum borer (APB), Euzophera semifuneralis (Walker), have been found on apple in habitats similar to those preferred by the dogwood borer. The larvae of the two species are similar in size. However, the DWB larva is white to cream-colored, and has only one row of crochets on the abdominal prolegs, while the APB larva is dusky purple to gray in color, and has two rows of crochets on the prolegs.
Pupae
Pupation occurs in the feeding tunnel in a tough silken cocoon covered with bits of frass (Fig. 6). The duration of the pupal stage is variable, lasting from 8-20 days depending on the temperature. Prior to emergence, the pupa pushes out of the cocoon and to the surface of the burrknot. The amber-colored pupal case often remains on the burrknot after the adult emerges (Fig. 7, arrow).
Damage of dogwood borer
Three general feeding types have been identified for DWB on clonal apple rootstocks (Fig. 8). Most frequently, feeding is confined to the burrknot (Type I). 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 (Type II). The third feeding type is not associated with a burrknot, but with bark scales and Injured bark, and occurs infrequently (Type III). 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.
Monitoring dogwood borer
Check trunks in the spring, particularly under trunk guards, for active infestations. Pheromone traps are available for DWB to estimate peak adult flight. Traps should be placed 4 to 6 feet above the ground.
