Thrips
Thysanoptera
also called Thunder flies, Thunderbugs
Minute slender insects that leave silvery flecking and distortion on leaves and flowers in hot, dry spells. Keep plants watered and use blue sticky traps under glass.
How to know it
Slivers of insect barely 1–2 mm long — black, brown or straw-colored, with narrow fringed wings — that scuttle out of a flower when you shake it over white paper. The pale yellow nymphs are wingless and even smaller, and both feed by rasping the surface open and drinking what leaks out.
What you'll see
- Silvery-white flecking or streaking on leaves and petals, with a bronze sheen in strong light.
- Minute black dots of excrement scattered over the silvered areas — the giveaway that separates thrips from mite damage.
- Flowers open distorted, streaked or flecked, and gladioli and roses brown at the petal edges.
- Onion and leek leaves take on a silvered, dry, half-scorched look in a hot summer.
- Growing points distort and stall in badly infested greenhouse crops.
Where it comes from
Eggs are inserted into plant tissue; two feeding nymph stages are followed by non-feeding stages that usually drop to pupate in the soil, and the adults fly back up. A generation takes under three weeks in warm weather, so numbers escalate fast, and adults overwinter in plant debris and soil or breed continuously under glass.
Hot, dry, thundery weather — hence "thunder flies" — and drought-stressed plants. Under glass they are a year-round problem, and they drift in on the wind in vast numbers on close summer afternoons.
Dealing with it
- Water thoroughly and mulch: much of the visible damage is drought stress that thrips have simply exploited.
- Hang blue sticky traps under glass — thrips go to blue in preference to yellow — and use them to catch outbreaks early.
- Introduce Amblyseius predatory mites or minute pirate bugs in a greenhouse; both are excellent on thrips.
- Damp down glasshouse floors and mist foliage to break the dry air they prefer.
- Pick off and bin badly flecked flowers rather than spraying an open bloom that bees are working.
Keeping it out
- Keep the ground mulched and the watering steady through hot spells.
- Clear plant debris and weeds around glasshouses where adults overwinter.
- Grow a nectar strip — alyssum, fennel, cosmos — to keep pirate bugs and lacewings resident.
- Inspect bought plug plants and pot plants, which are how thrips usually arrive under glass.
- Avoid drought-stressing onions and leeks in midsummer, when silvering does the real damage.
What it goes for
Onions and leeks, gladioli, roses, peas, and most greenhouse crops — cucumbers, peppers, chrysanthemums and African violets in particular.
Trouble it brings with it
Who hunts it for you
- Big-eyed bugGeocoris spp. A stubby 3 mm bug with enormous wrap-around eyes, working the soil surface and the low foliage for aphids, mites, thrips and insect eggs. It is the main natural check on chinch bugs in a lawn and it is killed outright by insecticide. Grow to recruit it: Calendula,Clover,Cosmos,Goldenrod,Sweet alyssum
- Damsel bugNabidae A slender, mantis-armed bug that grabs aphids, small caterpillars, and thrips in low crops and meadowy edges. Leave a strip of unmown grasses and flowers for it to overwinter in. Grow to recruit it: Fennel,Sweet alyssum,Yarrow
- Green lacewingChrysoperla carnea Its bristly larvae — "aphid lions" — impale aphids, mealybugs, thrips, and mite colonies, eating hundreds a week. Adults feed on nectar and pollen from small open flowers and overwinter in hollow stems left standing. Grow to recruit it: Cilantro,Cosmos,Dill,Sunflower,Yarrow
- HoverflySyrphidae Bee-mimicking flies whose legless larvae graze aphid colonies clean while the adults pollinate. A ribbon of shallow, nectar-rich flowers kept in bloom all season recruits wave after wave of them. Grow to recruit it: Calendula,Chamomile,Fennel,French marigold,Phacelia,Sweet alyssum
- 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