Peachtree borer
Synanthedon exitiosa & S. pictipes
Gum and sawdust at the base of a peach or plum trunk, and a caterpillar girdling the tree just below the soil. The commonest killer of young stone fruit.
How to know it
What you find is the damage: a mass of amber gum mixed with coarse sawdust-like frass at the soil line, and if you scrape the bark away, a tunnel with a cream caterpillar up to 30 mm in it. The adult is a clear-winged moth that mimics a wasp so well it is regularly mistaken for one, dark blue with orange bands, flying by day in midsummer.
What you'll see
- Amber gum mixed with brown crumbly frass oozing from the trunk at or just below ground level.
- Loose, dead bark at the base that comes away to reveal a tunnel.
- Young trees declining, leaves pale and small, and eventually dying outright.
- Empty brown pupal cases sticking out of the bark or lying at the base in late summer.
- On the lesser peachtree borer, the same gum and frass higher up at wounds and pruning cuts.
Where it comes from
One generation a year. Moths fly from midsummer into autumn and lay eggs on the lower trunk and the soil beside it. The caterpillars bore in at or below the soil line, feed under the bark through autumn and spring, and pupate in early summer. Because they feed on the trunk, a heavy infestation girdles a young tree and kills it in one or two seasons.
All stone fruit throughout North America, worst on young trees, on trees with mower or strimmer wounds at the base, and where mulch, weeds or long grass keep the trunk base damp and hidden.
Dealing with it
- Scrape the soil away from the trunk base in spring, find the tunnels, and dig the caterpillars out with a stiff wire or a thin knife.
- Clean the gum and frass off so you can see what is happening, and re-inspect through the season.
- Keep the trunk base bare, dry and visible — weeds and mulch against the trunk are what let this build unnoticed.
- Hang pheromone traps to time the flight, and use mating disruption on a group of trees.
- Paint the lower trunk with white latex paint diluted in water, which discourages egg laying and shows up damage.
Keeping it out
- Keep mowers and strimmers well away from the trunk; a single scrape is an invitation.
- Maintain a 30 cm circle of bare soil or gravel around the base of every stone fruit tree.
- Wrap the lower trunk of young trees with window screen or a mesh guard through the flight season.
- Inspect the base of every stone fruit tree twice a year, in spring and autumn.
- Keep the trees vigorous and watered; a strong tree walls off a small attack.
What it goes for
Peach, nectarine, plum, cherry, apricot and almond, plus ornamental Prunus and cherry laurel.
Who hunts it for you
- Beneficial nematodesSteinernema & Heterorhabditis spp. Microscopic worms watered onto damp soil that hunt down soil-dwelling grubs and release a bacterium that kills them within days. Timing is the whole thing: warm, damp soil and a small target grub, which means late summer for most of what they are used against.
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com