Thrips: How to Spot the Damage and Get Rid of Them for Good

Thrips are tiny, fast-breeding insects that scrape the surface of leaves and leave silvery patches peppered with black specks behind them. They are one of the most common pests in grow tents, glasshouses and on houseplants, and one of the most frustrating, because a single spray almost never finishes them. This guide shows you how to recognise thrips and their damage, why they bounce back, and a simple plan that reaches every stage of their life cycle: monitor, spray thoroughly, treat the surface of the pots and repeat on time.

What thrips are

Thrips are slender insects, usually only 1-2mm long, with two pairs of narrow wings edged with long fringes. Adults range from pale yellow to brown or black depending on the species. The larvae are wingless, pale yellow or cream, and look like tiny moving slivers along the veins on the underside of a leaf. The species most often found indoors and under glass is western flower thrips, but onion thrips and several others turn up too, and the control is the same.

They are weak, drifting fliers but very mobile on the plant, and they jump or run when disturbed. Because they are so small, most growers notice the damage long before they notice the insects.

• Tap test: hold a sheet of white paper under a branch and tap it sharply. Thrips fall onto the paper and show up as tiny moving dashes.
• Not spider mites: mites are rounder, have eight legs, and leave fine pale stippling with webbing in heavy cases. Thrips damage is larger, shinier patches with black specks and no webbing. If it is mites, see our spider mite rescue plan.

What thrips damage looks like

Thrips feed by rasping the surface of the leaf and drinking the sap that leaks from the broken cells. The emptied cells fill with air, which is what gives the damage its typical look.

• Silvery or bronze patches, often starting along the veins and on the underside of the leaf, that spread and merge as numbers grow.
• Tiny black specks scattered over the patches. These are droppings, and they are the clearest sign that it is thrips rather than a nutrient problem.
• Scarred, twisted or stunted new growth, because the larvae like to feed in tight, tender tips where they are sheltered.
• Pale streaks on fruit and petals in fruiting and ornamental crops, where adults feed and lay eggs.

A few patches on older leaves are a warning. Damage spreading onto new growth, or a sticky trap catching more each week, means the population is established. In commercial crops thrips also matter because some species carry plant viruses, such as tomato spotted wilt virus, from plant to plant, which is another reason to act early.

The life cycle: why one spray never works

The female cuts into the leaf and lays her eggs inside the tissue, where no spray can reach them. The larvae hatch after a few days and feed on the leaves for about a week. Then most of them drop off the plant into the growing medium or the debris on the floor, where they pass through two quiet, non-feeding stages before emerging as winged adults. In a warm room the whole cycle takes about two to three weeks, and faster in hot conditions.

So at any moment a large part of the population is out of reach of a leaf spray: eggs inside the leaf, and pupae in the pot. A spray that kills every larva and adult it touches still leaves the next generation waiting. That is the whole reason thrips have a reputation for coming back, and it tells you what to do about it: treat thoroughly, treat the surface of the medium as well, and repeat before the next batch of larvae can mature and lay.

Step 1: monitor, so you know what you are dealing with

Sticky traps catch flying adults and, more usefully, give you a weekly number to track. Thrips are attracted to both yellow and blue. Blue cards are often the more attractive to thrips specifically, while yellow cards catch thrips plus whitefly, aphids and fungus gnats, which makes them the better all-round monitor for a mixed room.

• Hang cards just above the plants, at least one per square metre, plus one near the intake where new pests come in.
• Count on the same day each week and write it down. Thrips on a card look like tiny dark slivers; zoom in with your phone to tell them from gnats.
• Check leaves as well: turn over a few leaves from the middle and top of each plant and do a tap test once a week.
• Watch the room. Thrips build up fastest in warm, dry conditions. A min-max hygrometer shows you whether the room is spending long stretches hot and dry.

Step 2: spray properly, especially underneath

Coverage is everything with thrips. The larvae sit on the underside of leaves and tucked into growing tips, and a spray only works on what it wets.

1. Use a sprayer that mists finely and can be angled upwards. A pump sprayer makes it far easier to reach undersides than a small trigger bottle.
2. Spray the underside of every leaf, then the top, then the growing tips, until the foliage is evenly wet but not dripping.
3. Spray when the lights are off or low and let the leaves dry before they come back on, to avoid scorch. Run the air circulation so the canopy dries rather than staying wet overnight.
4. Treat every plant in the area, not just the ones showing damage. Thrips move freely between plants.
5. Wear gloves, mix only what you need, and follow the label for the dose and the interval.

Which product

LaminAid TH from Crop Guard is a thrips-specific concentrate mixed at 20ml per litre and sprayed over the whole plant, including the leaf undersides and the top of the pots, every 14 days while thrips are present. Spray2Grow Thrip Protection Spray is an organic blend of plant extracts and essential oils aimed at thrips at all stages; 500ml makes 10 litres. Guard'n'Aid for Thrips combines natural pyrethrins with rapeseed oil, with a repeat allowed after 14 days, but it is authorised for ornamental plants under cover only, so do not use it on anything you will eat.

For routine use between outbreaks, neem oil and BioGreen Garlic act more as deterrents. Neem is sprayed onto both sides of the leaf; BioGreen Garlic is taken up by the plant and can be used as a spray or a root drench, at a low rate for prevention and a higher rate when thrips are already present. Rotating between products with different modes of action helps stop a population getting used to one.

Step 3: treat the surface of the pots

Most thrips larvae leave the plant to pupate in the top layer of the medium or in debris on the floor. This is the stage leaf sprays never touch, and treating it shortens an outbreak noticeably.

• Include the medium in your spray. Products such as LaminAid TH are designed to be sprayed over the top of the pots as well as the leaves.
• Use a surface dust. Agrothrin is a pyrethrin dusting powder spread lightly over the surface of the medium, where it treats larvae dropping in to pupate and adults coming back out. Dust every pot in the room at the same time, wear gloves and a dust mask, and reapply after heavy top-watering.
• Clear the floor. Fallen leaves and spilled medium on the tent floor are pupation sites too. Sweep or vacuum them up and bag them, rather than leaving them in the room.
• Keep the surface tidy. Remove dead and badly damaged leaves from the plants and the pots as you go.

Commercial growers also release predatory mites that hunt thrips larvae on the leaves, and soil-dwelling predatory mites that take pupae in the medium. They work well in a stable, not-too-dry room and fit alongside everything above, but they are sensitive to many sprays, so plan one approach or the other rather than mixing them blindly.

Step 4: repeat on time

The eggs that were safe inside the leaf during your first spray hatch over the following days. A second treatment timed before those larvae can reach adulthood and lay again is what actually breaks the cycle.

• Follow the product's repeat interval, which for the thrips sprays above is typically 14 days, and do not exceed the number of treatments the label allows.
• Keep counting your traps between treatments. The count should fall after the second round; if it does not, check your coverage before you blame the product.
• Keep going for at least one more round after the traps go quiet. A few survivors in the medium are enough to restart the population.

A four-week plan that covers two generations

1. Day 1: confirm it is thrips with a tap test and a close look at the damage. Hang sticky traps and note the count. Clear fallen leaves and floor debris.
2. Days 1-2: spray every plant thoroughly, undersides first, plus the top of the pots. Dust the medium surface if numbers are high.
3. Week 1: check humidity and temperature, and bring a hot, dry room back into the normal range for your plants. Recount the traps.
4. Week 2: second full treatment at the label interval. Compare trap counts with day 1.
5. Weeks 3-4: a third round if the label allows and the count has not dropped near zero. Then switch to a deterrent such as neem oil on a routine and keep a trap up permanently.

Keep them out next time

• Screen the intake. Thrips are tiny enough to be pulled straight down the intake duct. A fine mesh or a filter over the inlet stops most of them without restricting airflow.
• Quarantine new plants for a couple of weeks, with a sticky trap beside them, and check the undersides of their leaves before they join the rest. Bought-in plants and cut flowers are a common way in.
• Change clothes after the garden. Thrips ride in on clothing and hair from outdoor plants, especially in summer. Visit the grow room before the garden, not after.
• Avoid long hot, dry spells. Thrips thrive in warm, dry air. Our humidity guide and summer heat guide cover keeping the room in range.
• Clean properly between crops. Pupae in old medium and floor debris carry thrips into the next crop. Remove everything, clean and sterilise, and treat the empty, sealed room with a fumer if you have had a bad outbreak; our guide to cleaning between crops shows the order.

For the bigger picture on keeping every pest out, read our grow room pest prevention guide.

Six mistakes that keep thrips coming back

1. Spraying once. Eggs inside the leaf survive and hatch a few days later. Plan at least two treatments at the label interval.
2. Only wetting the tops of the leaves. The larvae live underneath and in the tips.
3. Ignoring the pots. Most larvae pupate in the medium, where leaf sprays never reach.
4. Treating only the plants with damage. Thrips move between plants; treat everything in the area.
5. Leaving fallen leaves on the floor. They are pupation sites and a source of the next generation.
6. Using a product on a crop it is not authorised for. Read the label, especially for anything you will eat.

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Quick answers

What are thrips?

Thrips are tiny, slender insects, usually 1-2mm long, with narrow fringed wings. Adults are pale yellow, brown or black and the young larvae are pale yellow and wingless. They feed by rasping the surface of leaves and drinking the sap that leaks out.

What does thrips damage look like?

Silvery or bronze patches on the leaves, often with tiny black specks of droppings scattered across them. New growth can come through twisted or scarred. Turn a leaf over and you may see pale larvae moving along the veins.

Why do thrips keep coming back after I spray?

Because a spray only reaches the larvae and adults it wets. Eggs are laid inside the leaf, and most larvae drop into the growing medium or leaf litter to pupate, so a fresh generation emerges a week or two later. Repeat treatments at the label interval and treat the medium surface as well.

How long does the thrips life cycle take?

In a warm room, roughly two to three weeks from egg to adult, and faster in hot conditions. Generations overlap, so plan to keep treating and monitoring for at least three to four weeks.

What colour sticky trap is best for thrips?

Thrips are drawn to blue and to yellow. Blue traps are often more attractive to them, while yellow traps catch thrips and a wider range of other pests such as whitefly, aphids and fungus gnats, so yellow is the better all-round monitoring card.

Do thrips like dry or humid conditions?

Thrips build up fastest in warm, dry rooms. Keeping humidity in the normal range for your plants and avoiding long hot, dry spells slows them down, though it will not clear an established population on its own.

Can I use a thrips spray on vegetables and herbs?

Only if the label allows it. Some products are authorised for ornamental plants only and must not be used on anything you will eat. Always check the label for the crops it covers, the dose and any restrictions on edible crops before you spray.

Is this the same for houseplants, vegetables and glasshouse crops?

Yes. Thrips are a standard pest of glasshouse peppers, cucumbers, tomatoes, strawberries and cut flowers, and of houseplants on a windowsill. Commercial growers use exactly this mix of monitoring, hygiene, sprays and predatory mites.