Tent caterpillars
Malacosoma americanum & M. disstria
also called Eastern tent caterpillar, Forest tent caterpillar
A silk tent in the fork of a cherry or crab apple in spring, with caterpillars streaming out of it to feed. Alarming, seasonal, and rarely a real threat to the tree.
How to know it
A hairy caterpillar up to 50 mm with a solid white stripe down the back and blue and orange markings along the sides. The tent is the identification and it is in the fork of a branch, in spring — that placement separates it from the fall webworm, which wraps the branch tips in late summer. Forest tent caterpillars make no tent at all, only silk mats, and carry white keyhole shapes down the back rather than a stripe.
What you'll see
- A dense white silk tent in a branch crotch, enlarging through the spring.
- Caterpillars leaving the tent by day to feed and returning to it, following silk trails along the branches.
- Branches stripped bare around the tent while the rest of the tree is in leaf.
- Dark shiny egg bands wrapped round a pencil-thin twig like a piece of styrofoam, visible all winter.
- Caterpillars wandering across paths, walls and drives in early summer when they leave to pupate.
Where it comes from
One generation a year. The egg mass laid last summer hatches exactly at bud break, and the caterpillars build and enlarge their tent for five or six weeks. They then wander off to spin cocoons, and the moths fly in midsummer and lay the bands that will sit on the twigs all winter. Numbers run in cycles of about ten years.
Wild cherry, crab apple and hawthorn in a hedge or a boundary are where it builds, then it spills into the garden. Outbreak years are regional and cyclical, and a healthy tree refoliates in June with no lasting harm.
Dealing with it
- Pull the tent out by hand in the early morning or evening when the caterpillars are inside it, and drop the whole thing in soapy water.
- Do not burn a tent in the tree; the fire does more damage than the caterpillars would.
- Prune out a small tented branch entirely if it is easy to reach.
- Spray Bt onto the foliage around the tent while the caterpillars are small if a young tree is being stripped.
- On a mature tree, leave it. It will refoliate, and the birds and parasitic flies are already working on the problem.
Keeping it out
- Scrape the shiny egg bands off the twigs in winter, when they are easy to see on a bare tree.
- Remove wild cherry and neglected crab apple from the boundary where you can.
- Keep young fruit trees watered and mulched so a defoliation costs them nothing.
- Leave the cuckoos, orioles and tachinid flies alone; they are the reason outbreaks collapse.
- Check the tree every winter; a few minutes with a stick removes next spring problem.
What it goes for
Cherry, crab apple, apple, plum, hawthorn and wild black cherry above all; forest tent caterpillar also takes aspen, maple, oak and birch.
Who hunts it for you
- Garden birdsPasseriformes Both things are true: birds strip your currants and a brood of blue tits takes several thousand caterpillars out of one fruit tree in a fortnight. Net what you must and welcome them everywhere else — a nest box near the apples is worth more than a spray. Grow to recruit it: Echinacea,Elderberry,English holly,Goldenrod,Rudbeckia,Sunflower
- Tachinid flyTachinidae Bristly, house-fly-lookalike parasitoids that lay eggs on caterpillars, beetle grubs, and earwigs — the maggot consumes the host from within. Adults sip nectar from shallow umbels and daisies. Grow to recruit it: Cilantro,Dill,Feverfew,Parsley,Sweet alyssum
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com