Eriophyid mites
Eriophyidae
also called Rust mites, Bud mites, Blister mites
Microscopic four-legged mites that blister, russet and gall a leaf — and carry rose rosette disease from plant to plant on the wind.
How to know it
Far too small to see: 0.1–0.2 mm, cigar-shaped, and with four legs rather than the eight of every other mite. You identify them entirely from what they do — a raised pale or red blister on a leaf, a felty patch of hair on the underside, a russeted skin on a fruit, or a swollen bud that never opens.
What you'll see
- Raised pimple-like galls or blisters scattered over a leaf, green then red then brown.
- A dense felty patch of pale hairs on the leaf underside, matching a bulge on the upper side.
- Fruit skin russeted brown and rough — pears and apples especially.
- Big, swollen, rounded buds on blackcurrant that dry up and never break.
- Bright red, thorny, distorted witches-broom growth on a rose, which means the mite has brought rose rosette with it.
Where it comes from
They live in the buds and the folds of new growth, breed continuously through the season, and overwinter under bud scales and bark. They cannot fly, so they disperse by letting go and blowing on the wind, and by riding on insects. Each species is tied to one host, and most do nothing worse than disfigure it.
Warm dry weather and still air favor them. Almost all of it is cosmetic — the exception, and it is a serious one, is the mite that carries rose rosette across the roses of North America.
Dealing with it
- Ignore leaf galls and blisters on maples, limes and pears; they are disfiguring only and no treatment reaches the mite reliably.
- Pick off and bin big-bud blackcurrant buds in winter, and take out a bush with more than a few.
- Prune out and bag the affected shoots; the mites are in the buds you are removing.
- Space roses so their canes never touch, which is the only practical brake on rose rosette spread.
- Use a horticultural oil at bud swell where a fruit tree is heavily russeted; timing at bud break is everything.
Keeping it out
- Dig out any rose showing bright red bristly broom growth immediately, roots and all, and bag it where it stands.
- Remove wild multiflora rose from the boundary; it is the reservoir for both the mite and the virus.
- Buy clean blackcurrant and rose stock, and inspect buds before planting.
- Do not spray broad-spectrum insecticides, which kill the predatory mites that hold these in check.
- Accept the galls. A maple covered in red bead galls is an odd sight and a healthy tree.
What it goes for
Roses, maples, limes, pears, blackcurrants, tomatoes, grapes and plums — nearly every species has its own.
Trouble it brings with it
Who hunts it for you
- Minute pirate bugOrius spp. A pinhead-sized true bug that is the best natural check on thrips, and happily spears aphids, mites, and insect eggs too. It needs pollen when prey is scarce, so keep something flowering nearby. Grow to recruit it: Cosmos,Fennel,French marigold,Sweet alyssum
- Predatory mitesPhytoseiulus & Amblyseius Fast orange mites that hunt spider mites egg-by-egg, and cousins that clear thrips and fungus-gnat larvae. Sold as a biological control for greenhouses; outdoors they follow humidity — mist and mulch, don’t dust with sulfur.
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com