Peachtree borer—Synanthedon exitiosa
Peachtree borer (a clearwing moth, family Sesiidae) larvae feed and tunnel in the basal trunk and root crown of stone fruit trees. It also feeds in almond if the tree is growing on peach or plum rootstocks. Peachtree borer in California occurs mostly in coastal areas, the Sacramento Valley, and the Northern San Joaquin Valley.
Identification
Damage as described below is generally the first or only obvious clue this pest is present. Look for this damage to almond or stone fruit trees every year in spring when the growing season begins and again in the fall.
Some peachtree borer adults of both genders are entirely blackish to dark blue or are mostly dark with an orange thorax. Some males have narrow yellow bands on the abdomen and some females have a single orange band on the abdomen. Males have mostly clear wings with dark veins. Females have mostly clear hind wings, but the forewings are dark blue. The wingspan is about 1 inch.
Adult behavior and the appearance of some males is that of a yellowjacket wasp. However, clearwing moth adults have a relatively broad waist unlike the narrow constriction between the abdomen and thorax of yellowjackets.
The pale eggs occur on bark of the lower trunk and are oval and about 1/25 inch (1 mm) long. The larvae are pale brown to pinkish with a dark head. They grow to about 1 inch.
Pupae are oblong and tapered toward the rear. They are brown to orangish and about 3/4 inch long. Pupae occur openly in bark crevices or on the ground near the base of the trunk. They can also occur in an earthen cell in topsoil.
Life cycle
Clearwing moths develop through four life stages: egg, larva, pupa, and adult. Adults may be found anytime from May through September. Adult females lay their eggs during summer on bark at the base of tree trunks. Hatching larvae chew and tunnel beneath bark of the trunk base and root crown as they develop through four increasingly larger instars.
Peachtree borer overwinters in the tree as larvae. In the late spring the mature larvae pupate near the entrance of their tunnel. Adult moths emerge 3 or 4 weeks later, mate, and the females lay eggs. The peachtree borer has only 1 generation a year.
Damage
Larvae of peachtree borer feed and tunnel in phloem, the tissue between outer bark and wood. Dark reddish brown frass and oozing on the basal trunk, soil nearby, and the root crown is the most obvious evidence this tunneling pest is present. Larval tunneling can girdle and kill young trees. Older trees are more tolerant of larval tunneling, but can be killed by this pest if there are many larvae feeding for several consecutive years. As trees decline from the borer's injury to their vascular system, foliage becomes pale green then yellowish and wilted.
Solutions
Monitor host trees for peachtree borer in the spring when the hosts flush leaves, and monitor again during the fall. Look for small piles of reddish brown frass (excrement) on the lower tree trunk and nearby soil. Sap may also ooze from the entrance to larval tunnels.
Keep mulch, weeds, and other debris at least several inches back from the basal trunk of host trees. This allows regular inspection for a potential peachtree borer problem. In locations with hot weather, keeping the tree base clear of debris exposes eggs and perhaps other stages to mortality from drying and heat.
Probing tunnels. Where only one or a few trees are infested, it may be possible to kill the larvae by probing tunnels with a stiff wire inserted into the entrance to impale and kill each larva. In addition to probing aboveground tunnels, remove 2 inches of soil from around the base of infested trees to expose tunnels that may be under the surface. When probing, avoid further damage to tree tissue as this injury may be detrimental to tree health. It is difficult to know whether each larva has been reached and killed when tunnel probing. Reinspect trunk bases periodically and probe any tunnels again where fresh frass has been produced.
Nematodes. Peachtree borer larvae can be controlled with applications of an insect-parasitic nematode, Steinernema carpocapsae or S. feltia. These must generally be ordered from online vendors. Nematode applications are most effective when larvae are actively feeding and tunnel openings are largest, in mid- to late summer.
Apply nematodes into borer tunnels with a squeeze bottle applicator or 20-ounce oil can at a concentration of 1,000,000 or more invasive stage nematodes per ounce of distilled water. Agitate the applicator frequently to keep nematodes suspended in the liquid.
First clear the tunnel entrance of frass, then insert the applicator nozzle as far as possible into each gallery. Inject the suspension until the gallery is filled or liquid runs out a hole, then plug the tunnel entrances with rope putty or grafting wax. So you know which tunnels you have treated, spot spray the plug with bright-colored paint.
Nematode-infected larvae continue to feed and push frass from their tunnels for a few days after the application. A second application 1 to 2 weeks after the first, then replugging the tunnel, increases the likelihood that borers will become infected and die. Wait another week and check to see if these plugs are intact. If the gallery opening is no longer covered plugged, the larva has not been killed and retreatment is warranted.
Broad-spectrum insecticide. A broad-spectrum, persistent insecticide such as a pyrethroid (insecticide ending in "thrin") can be sprayed on bark to kill the females when they land to lay eggs or the hatching larvae as they bore into the tree. Before the application, remove about the top 2 inches of soil to expose the root crown. Thoroughly wet the trunk from the bottom of the lowest limb down to soil level. Make one application in early May and a second spray in mid-July.
To improve timing of the first application so the spray is more effective, in April place one or two sticky traps baited with the moth's pheromone (sex attractant). The pheromone lure may be listed as that for either peachtree borer or greater peachtree borer. Do not use the lure for lesser peachtree borer because this is a different species that occurs in the Eastern United States.
Place the trap(s) about 3 feet or less above the ground so they hang freely near the trunk. Make the first application after the first adult peachtree borer is caught. Be sure to properly identify the moths that are trapped. Other clearwing moths such as the strawberry crown borer (Synanthedon bibionipennis), a pest of caneberries and strawberries, may be attracted by the peachtree borer pheromone. Both genders of strawberry crown borer have more orangish or yellow than do adult peachtree borers.
Note that a professional pesticide applicator must be hired to apply the broad-spectrum insecticide because effective products are not available to home fruit growers. For how to get the services you want, consult Pest Notes: Hiring a Pest Control Company.
Adapted from Integrated Pest Management for Stone Fruits, Pest Management Guidelines: Apricot, and Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, University of California Statewide Integrated Pest Management Program (UC IPM). |
