Spongy moth
Lymantria dispar
also called Gypsy moth (former name), Lymantria
In an outbreak year it strips oak woods bare and the caterpillars rain frass onto everything below. Between outbreaks you would not know it was there.
How to know it
A hairy caterpillar up to 60 mm with five pairs of blue dots followed by six pairs of red dots down the back — the only caterpillar in North America marked that way. The egg mass is the other identification: a buff, felty, sponge-textured pad the size of a thumb, glued to bark, walls, fences, firewood and the underside of garden furniture.
What you'll see
- Buff felty egg masses on tree trunks, walls, fences and anything left outdoors, present from August to April.
- Caterpillars climbing trunks at dusk and descending at dawn in the later stages.
- Oaks and other broad-leaved trees stripped to bare twigs by June in an outbreak year.
- A continuous rain of small dark frass pellets and the sound of it on a still day.
- Caterpillars everywhere — on walls, washing, garden furniture — in a bad year.
Where it comes from
One generation a year. Eggs hatch at bud break and the tiny caterpillars balloon on silk threads to new trees, which is how it spreads locally. They feed for six to eight weeks, pupate in bark crevices, and the moths fly in midsummer; the female of the European strain cannot fly, so long-distance spread is entirely by egg masses hitchhiking on vehicles and firewood. Populations crash after a few years when a fungal disease and a virus catch up.
Established through the northeast and spreading west and south. Outbreaks run in cycles of eight to twelve years, and are broken by a fungal disease that needs a wet spring — a dry May is what turns a build-up into a plague.
Dealing with it
- Scrape egg masses into soapy water any time from August to April — one mass is up to a thousand caterpillars.
- Band trunks with burlap folded into a skirt; the caterpillars shelter under it by day and can be scraped into a bucket each afternoon.
- Do not use open sticky bands, which trap birds, squirrels and beneficial insects.
- Spray Bt on the foliage of a valued small tree while the caterpillars are under 20 mm.
- Water and mulch defoliated trees; a healthy oak survives one or two defoliations and dies after three.
Keeping it out
- Check vehicles, trailers, firewood and outdoor furniture for egg masses before moving them — this is how it travels between states.
- Walk the property every winter and scrape what you find.
- Encourage the white-footed mice, birds and parasitic flies that hold it down between outbreaks.
- Keep oaks unstressed so they can carry a defoliation year.
- Report a first sighting in an uninfested area; the quarantine programs depend on it.
What it goes for
Oak above all, then apple, birch, aspen, hawthorn, hornbeam, willow and — in a bad year, once the broad-leaves are gone — larch, spruce and pine.
Who hunts it for you
- BatsChiroptera A single bat takes hundreds of moths and mosquitoes in a night, and the moths are the parents of your cutworms, corn earworms and hornworms. Put up a bat box on a warm south wall, leave a night-scented corner unlit, and never spray after dark. Grow to recruit it: Evening Primrose,Honeysuckle,Nicotiana,Summer Jasmine
- 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