Insect Profiles: Peachtree Borer (Synanthedon exitiosa)
Peachtree borer (Synanthedon exitiosa) , also called peach crown borer, is the most destructive pest of cherry, peach, plum, and other stone fruits. The insect damages trees during its larval stage. The larvae, which resemble grubs, bore into the bark on the lower trunk, as well as into the main roots. As the larvae feed, they disrupt the tree’s vascular system, inhibiting the transportation of water and nutrients throughout the crown. Dense populations of peachtree borer can girdle and kill young trees.
Distribution & Habitat
Peachtree borer is native to the United States, where it is widespread. It also infests vulnerable trees throughout Canada.
Peachtree borer infests cherry, peach, plum, and other stone fruits. Younger trees are typically favored by the insect. Infestations occur frequently in older trees that have incurred bark injuries.
The larvae are dull white to light brown or pink caterpillars, with darkly colored heads. They can reach up to 1 ½ inches in diameter. As the larvae mature, their color deepens. The adults are moths, which appear similar to wasps. The adults generally measure between 15 and 17 mm in length. The females are metallic blue, with a characteristic orange band in the center of the abdomen. The forewings are black. The rear wings are translucent in the center, and have black edges. The males are black, with a metallic blue sheen, and a few pale yellow stripes on the abdomen. The males have transparent wings that are dotted with several black markings.
Peachtree borer produces one generation per year. From spring to early summer – and in warmer locations, into fall – the overwintering adults resume activity, emerging from tunnels they have crafted in the bark of the host, or just beneath the soil surface. The males are the first to vacate the hosts, emerging from April to June. The females become active a few weeks later, appearing from late June to August. The females secrete a pheremone that attracts the males. Once the males have located the females, they proceed to mate. Soon after mating, the males expire. The females then fly to suitable hosts, and lay 200 to 800 eggs on the bark of the lower trunk. They may also deposit eggs in soil cracks located near the tree base.
The eggs generally hatch in 7 to 10 days. The newly hatched larvae tunnel into the tree’s bark, feeding on the woody tissue as they do so. The larvae progress into the cambium, and then burrow into the trunk. As the larvae feed, they distrupt the plant’s vascular system, inhibiting the distribution of water and nutrients throughout the crown. As winter approaches, and colder temperatures prevail, the larvae cease feeding, and overwinter in the bark tunnels. The following spring, as temperatures increase, the larvae resume feeding. They continue to feed until mid-summer, when they have matured.
Once mature, the larvae settle a few inches below the soil surface, or in the bark tunnels, where they construct large pupal chambers composed of silk threads and wood fragments. The chambers resemble gummy cocoons. The larvae envelop themselves in the chambers, and commence pupation. They pupate for 3 to 4 weeks, rapidly morphing into adults. The adults leave the pupal chambers, and seek out bark cracks and soil crevices to use as overwintering sites. The adults overwinter, and the cycle begins anew.
Symptoms of Infestation
The adults will sometimes pull the pupal skins out of the bark tunnels, as they leave their hosts. When this occurs, the pupal skins can be observed scattered around the base of the host’s trunk. The first visible symptom of infestation is often the formation of wet spots on the exterior of the infested bark. As the larvae burrow deeper into the tree, they may cause a jelly-like sap to be expelled from the bark openings. Immature trees may be girdled by the larval feeding, causing them to rapidly decline or expire. Reddish-brown frass may surround the holes in the bark created by the larvae. By mid-to-late summer, the leaves and branches of infested trees may wilt. Older trees can typically withstand peachtree borer infestations, but may experience a reduction in fruit yield and fruit quality.
- Proper detection of peachtree borer may require a close inspection of infested trees. The bark wounds created by the larvae are often apparent around the base of the tree.
- Insecticides can be applied to the lower trunk of vulnerable trees in May, as the eggs hatch. A second application can be performed in August, to control the adults. Once the larvae have entered the hosts, spray applications are ineffective.
- Various sticky traps can be utilized to deter the males. Avoid placing materials directly on the bark, as they can have phytotoxic effects.
- The larvae can be removed from the soil around infested trees using a technique called worming. Remove the soil from around the base of infested trees, and use a pointed instrument to dig the larvae out. Be wary of slicing into the tree’s roots.
- Maintain plant vigor through sound cultural practices. Ensure that trees are sufficiently watered, especially during extended periods of drought. Apply a layer or organic mulch around the base of vulnerable trees to improve the soil quality, moderate the soil temperature, and retain soil moisture.
- Cull severely weakened trees to prevent infestations from occurring.
- Organic neem oil can be sprayed around the base of susceptible trees, and up the first 6 to 12 inches of the trunk. To ensure that the neem oil is effective, thoroughly douse the bark and soil. The neem oil disrupts the insect’s breeding cycle, and smothers any eggs that have been laid. Neem oil should be applied twice a month during the growing season.
- Citrus extract may be sprayed on vulnerable trees to repel the adults, and discourage oviposition. Spray the soil around the base of the tree, as well as on the first 6 to 12 inches of the trunk, just prior to the adults’ emergence.
- Applications of Bacillus thuringiensis, a naturally occurring soil bacteria, will kill the larvae.
- Beneficial nematodes may be employed to eradicate the eggs, larvae, and pupae.
- Woodpeckers and other birds are common natural predators that help to reduce borer populations.
Photo courtesy of Ric Bessin, University of Kentucky College of Agriculture