THRIPS

Thrips are barely visible, extremely mobile, and the only pest in this guide that can hand your plant a virus as well as eat it.

They are insects of the order Thysanoptera, only slightly larger than a spider mite and genuinely hard to see with the naked eye — elongate, usually tan or yellowish, though some species are dark brown or black. The larvae are paler and wingless, and on a leaf they are what you normally spot first: a pale sliver that moves when you breathe on it.

Two species matter indoors, and they are not interchangeable. Frankliniella occidentalis, western flower thrips, lives in flowers and growing points and is the classic greenhouse resistance problem. Echinothrips americanus is the one aroid growers actually get: darker, banded, and it stays out on the leaf blade rather than hiding in a flower. That difference decides which traps work, which predators work, and whether treating the soil is worth the effort — see §05.

They do not pierce single cells the way a mite does. They rasp the surface open and drink what leaks out, so the scar is an irregular silvery patch rather than a field of dots.

The more serious damage is done before you can see it. Thrips feed on young leaves while those leaves are still folded and forming, and the leaf then opens buckled, notched or ruffled — and stays that way for its whole life. The same happens in flowers: infested buds, scarred spathes, sometimes an inflorescence that aborts outright.

Confirmation is the black specks of frass scattered through the silvering. Silver alone can be old mite damage, sun damage or a scald; silver with black dots is thrips.

They are also among the most common insect vectors of plant viruses — the tospoviruses in particular. A virus does not leave when the thrips do, which is the real argument for treating an infestation early rather than tolerating a low one.

At any given moment a large part of a thrips population is somewhere your spray cannot go. That single fact explains every treatment failure on this page.

Eggs are inserted inside plant tissue, where nothing topical reaches them. Two feeding larval stages sit on the plant. Then, in most species, the prepupal and pupal stages drop to the soil or the leaf litter and wait there. Winged adults emerge and start again. Even a perfect application only catches the stages that happen to be exposed on that day.

Echinothrips is the exception that matters: it tends to pupate on the plant rather than in the substrate, so soil drenches and soil-dwelling predators do much less for it. If you have leaf-dwelling thrips and a soil programme is not helping, this is probably why.

Some strains have developed resistance to many products while others are still straightforward to treat, so a product that failed for someone else is not necessarily a product that will fail for you — and vice versa.

Honestly stated: none of these will clear an established thrips population on their own. They lower the number, they buy time, and they tell you what is happening.

1. 01Quarantine incoming plants for three to four weeks. Thrips arrive on plants and in flowers. Nothing else here returns as much.
2. 02Sticky cards, and use blue rather than yellow. Thrips respond to blue more strongly than to yellow. Cards give early warning and, more usefully, a weekly count so you know whether you are winning. They are not a control.
3. 03Remove infested inflorescences. On a flowering aroid the spathe is a breeding chamber — sheltered, humid, full of pollen. Cutting it off an already-infested plant removes a nursery.
4. 04Horticultural or dormant oil. Mechanical, so no resistance is possible, but it only reaches exposed stages: not the eggs inside the tissue and not the pupae in the substrate. Contact only, no residual. Oils burn foliage above roughly 80–85°F and on sensitive leaves — test a couple of leaves first.
5. 05Soap and peroxide, with a cloth. Real soap — Dr. Bronner’s or Safer insecticidal soap — plus 3–4 tablespoons of 3% hydrogen peroxide per cup of water. Spray, wipe with a soft damp cloth, rinse. Contact only, no residual.

Thrips are the pest that taught commercial growers to stop spraying. They resist chemistry quickly and they hide in three places at once — and predators do not care where an insect is hiding, only whether they can walk there.

Because the population is split between the leaf and the substrate, a serious biological programme is usually two agents, not one: something on the foliage and something in the pot. That is a real cost, and it is why this section is longer than the others.

Thrips need repeat applications by definition — the eggs are inside the leaf and hatch after the spray has gone. A single treatment is not a light version of the programme. It is a resistance-selection event.

1. 01Work out which thrips you have. Dark and on the leaf blade points to Echinothrips; pale and in the flowers or growing point points to western flower thrips. It changes §05 completely and it changes whether the soil matters.
2. 02Put blue sticky cards up and leave them up. Count weekly. Without a number you are guessing about whether a programme is working.
3. 03Cut off infested inflorescences and the worst-scarred leaves before you treat. You are removing shelter, not just tidying.
4. 04Treat the foliage and the substrate in the same session unless you have established the thrips are leaf-pupating. Half a programme selects for the half you missed.
5. 05Repeat every five to seven days, at least three rounds. The eggs are inside the leaf; they hatch after your spray has dried and gone.
6. 06Rotate the mode of action between rounds. Thrips are the fastest developers of resistance in this guide after spider mites.
7. 07Choose chemistry or biology and commit. They do not overlap well, and the worst outcome is paying for predators that land on a residue. If you are going biological, do the mechanical work first and skip the spray entirely.
8. 08Re-inspect with a loupe ten to fourteen days after the last round, in the folded new growth rather than on the old leaves.

1. 01Thrips — UC Statewide IPM Program, Pest Notes 7429
2. 02Thrips — Koppert pest profile
3. 03Managing houseplant pests — Colorado State University Extension

1. 01Biological control and natural enemies of invertebrates — UC IPM, Pest Notes 74140
2. 02Biological control of thrips in greenhouses — Michigan State University Extension
3. 03Minute pirate bugs, Orius — UC IPM natural enemies gallery
4. 04Franklinothrips predatory thrips — UC IPM natural enemies gallery
5. 05Western flower thrips Ceranisus parasitoid — UC IPM natural enemies gallery
6. 06Orius insidiosus — Cornell Biological Control
7. 07Neoseiulus cucumeris — Koppert
8. 08Amblyseius swirskii — Koppert
9. 09Amblydromalus limonicus — Koppert
10. 10Orius insidiosus — Koppert
11. 11Stratiolaelaps scimitus (Hypoaspis miles) — Koppert
12. 12Steinernema feltiae — Koppert
13. 13Insect-parasitic nematodes — Cornell Biological Control

1. 01Side effects of pesticides on beneficials — Koppert, searchable database
2. 02Side-effect manual — Biobest
3. 03Pollinator and beneficial insect toxicity — Intermountain Tree Fruit Production
4. 04Tank mixing revisited — GPN magazine PDF · archived
5. 05Mode of action classification — Insecticide Resistance Action Committee (IRAC)
6. 06Insect control, soaps and detergents — Colorado State University Extension