Plant Problem Diagnostic
Peachtree Borer: Identifying Trunk-Base Frass and Controlling the Larvae
The short answer
The peachtree borer (Synanthedon exitiosa) is a clearwing moth whose larvae bore into the lower trunk and root crown of peach, plum, cherry, and other stone fruit at the soil line. The diagnostic sign is a mass of gum mixed with sawdust-like frass at the base of the trunk, from a few inches below the soil to about a foot up; unlike sterile disease gummosis, this ooze contains wood chips and frass.
Adult moths, which resemble wasps, emerge and lay most of their eggs on the lower trunk and nearby soil in July and August. The larvae bore straight through the bark and feed in the cambium at or below the soil line, and over one to a few seasons they can girdle and kill an otherwise healthy young tree.
Control is a mix of tactics. Keep the trunk base free of mulch, weeds, and suckers so it can be inspected and so heat and dryness discourage larvae; probe tunnels with a stiff wire to impale larvae; and where needed apply trunk sprays (pyrethroid class such as permethrin or esfenvalerate) timed to adult emergence and egg-laying per your Extension, not on a fixed calendar. Parasitic nematodes injected into tunnels in late summer are an alternative, and healthy, unstressed trees are attacked less.
Identifying peachtree borer
Look at the very base of the trunk. Penn State describes the key sign as a mass of gum exuding from the trunk base, from about three inches below the soil line to a foot above, containing wood chips, sawdust, and frass produced by the feeding larvae. That frass is the tell: it is what separates borer damage from ordinary gummosis. Stone fruits ooze clear, amber gum for many reasons, drought, mechanical injury, disease, but gummosis alone is sterile, whereas borer gum is dirty with sawdust-like frass and sometimes shows shed pupal skins on the lower bark or in the soil nearby.
The larva itself is white with a brown head and can reach about an inch and a half long, feeding hidden under the bark where you rarely see it. Above ground, an infested young tree may look generally unthrifty, with sparse or wilting foliage and slow decline, because the larvae are quietly cutting off the water and nutrient flow at the trunk. If you scrape back the soil an inch or two at the crown and find gum, frass, and tunnels in the inner bark, that confirms peachtree borer over a disease.
The life cycle that dictates timing
There is normally one generation a year (occasionally two years in cool regions). Larvae overwinter under the bark and in the soil at the crown, resume feeding in spring, and pupate in late spring. The adult clearwing moths, which mimic wasps, emerge and lay most of their eggs in July and August, depositing them on bark at the base of trunks and on nearby soil. Eggs hatch in about ten days, and the young larvae bore through the bark immediately and begin feeding in the inner bark and cambium at or below the soil line.
That summer egg-laying window is what any spray program targets. Because the vulnerable moment is when eggs are being laid and larvae are hatching on the outside of the trunk, before they get under the bark, control is aimed at the lower trunk in mid to late summer, timed by regional Extension guidance or trapping rather than guesswork.
Managing peachtree borer
Start with the tree base. UC IPM advises keeping mulch, weeds, and suckers pulled several inches back from the trunk so the crown stays exposed and can be inspected, which also makes conditions less hospitable to eggs and young larvae. Where you find active tunnels, larvae can be killed mechanically by probing and impaling them with a stiff wire, a low-tech but effective tactic on a few backyard trees. Keeping trees healthy and unstressed matters too, since Penn State and Utah State note stressed, neglected trees are the most susceptible.
For chemical control on home trees, preventive trunk sprays are the main tool, applied to the lower trunk and crown timed to adult emergence and egg-laying. Penn State and Utah State name pyrethroid materials such as esfenvalerate or permethrin for this trunk application; follow your Extension's regional timing (often an early and a mid-to-late-summer application) and the product label rather than any invented dose. As a non-chemical alternative, UC IPM notes parasitic nematodes (Steinernema species) injected into the tunnels in mid to late summer can kill larvae. If you are not certain whether trunk-base gumming is borer damage or disease gummosis, an Extension diagnostic office can confirm before you treat. This pest fits the after-bloom, trunk-directed part of the backyard fruit tree spray schedule.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- Penn State Extension - Peachtree Borer in the Home Fruit Planting — The key sign is a mass of gum exuding from the trunk base from three inches below to a foot above the soil, containing wood chips, sawdust, and frass from feeding larvae; most adults emerge and lay eggs in July and August, with one generation per year; larvae can be killed with a wire, and trunk applications of pyrethroid insecticides such as esfenvalerate or permethrin are used; stressed or neglected trees are most susceptible.
- UC IPM - Peachtree Borer (Home and Landscape) — Peachtree borer produces dark frass and oozing on the basal trunk and root crown; females lay eggs in summer on bark at the base of trunks, and larvae bore beneath the bark and overwinter there; control includes probing tunnels with a wire to kill larvae, keeping mulch and weeds several inches back from the trunk, parasitic nematodes in mid to late summer, and properly timed trunk sprays.
- Utah State University Extension - Greater Peachtree Borer — Infestation signs include sawdust and frass mixed with gummy exudate and oozing sap near the trunk base; larvae bore through the bark and feed in the cambium at or below the soil line; preventive trunk sprays are the primary control for home trees, mating disruption is recommended for commercial orchards, and keeping the tree base free of vegetation reduces egg and larva survival.