Thrips are one of the more frustrating houseplant and greenhouse pests in the UK. Tiny, fast-moving and capable of reproducing rapidly in warm conditions, they can establish large populations before the damage becomes obvious. By the time the characteristic silvery streaking and stippling appears on leaves, a significant infestation is usually already in progress. Understanding their life cycle and the range of treatments available makes a real difference to how effectively you can bring them under control – a haphazard approach with the wrong product at the wrong interval repeatedly delays control and allows the population to build further rather than being brought down.

Unlike slower-moving pests such as scale insects, thrips are mobile at almost every life stage and spread quickly between plants both by walking and by flying. A single infested plant brought into a collection can start an outbreak within two to three weeks. Early detection and prompt action are the two most effective tools you have – and because thrips populations can develop resistance to single treatments relatively quickly, rotating between different control methods is more effective than relying on any one product indefinitely. The combination of physical monitoring with sticky traps, cultural controls to reduce plant stress, and targeted treatment with rotated products gives the most reliable and durable long-term results.

Identifying thrips

Adult thrips are tiny – typically 1-2mm long – and range in colour from pale yellow or cream through to dark brown or black depending on species. The most common species affecting UK houseplants and greenhouse plants is Western Flower Thrips (Frankliniella occidentalis), which is pale yellow-orange as a juvenile and dark brown as an adult. Thrips tabaci – onion thrips – is another common species affecting a wider range of plants outdoors and in unheated greenhouses. A third species, Thrips simplex, specifically targets gladioli and is responsible for the severe corm and flower damage seen on these plants.

The adults are difficult to spot with the naked eye but move quickly when disturbed. Hold a sheet of white paper below a suspect plant and tap the leaves sharply – any thrips present will fall onto the paper and be visible as tiny moving specks. Yellow sticky traps hung near plants are another reliable detection tool, catching adults in flight between plants and giving you an early warning before populations build to damaging levels. One or two adults on a trap is a signal to inspect the collection more carefully; a heavily loaded trap means a significant infestation is already established. Nymphs – the juvenile stage – are paler than adults and even smaller. They feed on the underside of leaves and inside flower buds, making them hard to spot until populations are already significant. The pupal stage takes place in the soil, which is why treating the compost surface as well as the plant itself is important for breaking the breeding cycle.

Damage they cause

Thrips damage is distinctive and easier to recognise than the pest itself. Feeding punctures the surface cells of leaves, causing a silvery or bronzed streaking and stippling that runs in the direction of the leaf veins. Heavy feeding turns whole leaves silvery-grey and papery. Flowers develop distorted, streaked petals, and new growth often emerges twisted or scarred where thrips have fed inside the bud before it opened. On fruit crops, feeding causes surface scarring and cosmetic damage that reduces the marketable quality even if yield is not significantly affected. Dark specks of thrips frass are often visible on leaf surfaces alongside the feeding damage, which helps to distinguish thrips damage from other causes of silver discolouration such as spider mite feeding or physical leaf abrasion, both of which produce similar visual symptoms without the frass deposits.

Organic treatments

Several organic approaches are effective against thrips, particularly when started early and applied consistently over several weeks to break the breeding cycle. No single application will eliminate an established infestation – the pupal stage in the soil is protected from surface sprays, and eggs inside buds and leaf tissue are similarly sheltered from contact treatments. Persistence over three to four weeks of treatment is needed to intercept each new generation as it emerges from the soil and the protected plant tissues. Missing a week of treatment during an active infestation allows the population to partially recover, since new adults emerge continuously from the protected soil pupal stage throughout the treatment period.

Chemical treatments

Where organic treatments are not bringing a heavy infestation under control, systemic insecticides provide a more reliable option. Systemic products are absorbed through the roots and transported into plant sap – when thrips feed they ingest the insecticide. This reaches hidden populations inside buds and on the underside of leaves where spray coverage is difficult. Systemic treatments are particularly useful where new growth is repeatedly being attacked before it has a chance to harden off, since the protection is present in the plant tissue itself rather than on the surface.

Provanto Ultimate Bug Killer and Bug Clear Ultra both contain systemic actives effective against thrips and are available from UK garden centres. Follow label instructions regarding rates, application intervals and any restrictions for use on edible plants. Do not use systemic insecticides on plants in flower where pollinators may visit, and avoid applying on warm sunny days which can cause leaf scorch on some species. Systemic insecticides should always be treated as a last resort rather than a first response – they are effective but carry a broader environmental impact than contact or biological controls.

Biological controls

For greenhouse and indoor growing, biological controls are the most sustainable long-term approach to thrips management. They work continuously once established rather than requiring repeated manual treatment, and they do not carry the resistance risk that comes with chemical controls used repeatedly over time. Combining a soil-applied nematode with a foliage-targeting predatory mite addresses multiple life stages simultaneously, which is the most effective overall strategy for persistent infestations in controlled environments.

Amblyseius cucumeris feeds on thrips nymphs and eggs and is widely used in commercial horticulture. It works best at temperatures above 18 degrees C, making it well-suited to heated greenhouses and indoor growing environments where it can establish and reproduce alongside the thrips population. Amblyseius swirskii is a broader-spectrum predatory mite that feeds on thrips as well as whitefly nymphs, making it a useful all-rounder for greenhouse collections where multiple pests are present simultaneously. Both predatory mites are available from specialist biological control suppliers and should be introduced before infestations become severe – they are a preventative and early-intervention tool, not an emergency response to a heavy outbreak. The parasitic nematode Steinernema feltiae targets the soil-dwelling pupal stage of thrips, disrupting the breeding cycle from below – it is an important complement to surface treatments because it addresses a stage that sprays cannot reach.

Thrips on outdoor plants

Outdoor thrips populations are naturally suppressed by predators including lacewings, ladybirds and predatory mites. In a well-balanced garden with diverse planting and minimal insecticide use, thrips rarely reach damaging levels on most ornamentals. Where they do cause problems – typically on gladioli, onions, leeks and greenhouse crops – the same organic treatments used indoors apply outdoors and can be combined with biological control measures without conflict. Timing outdoor treatments for early morning or late evening reduces the impact on visiting pollinators and improves efficacy by avoiding rapid evaporation in mid-day heat, which significantly reduces both the contact time and the coverage achieved by spray applications.

On onions and leeks, thrips often appear alongside other pests on stressed or water-stressed plants. Keeping crops consistently well-watered during dry spells reduces plant stress and makes them less attractive and less vulnerable to thrips feeding. Gladiolus corms that have been infested should be treated before storage – dust corms with an appropriate insecticide and store them in cool, dry conditions to prevent populations persisting and carrying over into the following season. Inspect stored corms in mid-winter and discard any that show signs of damage or decay.

Prevention

Quarantine all new plants for at least two weeks before introducing them to an existing collection. Thrips are one of the most common hitchhiker pests on bought plants and are easily missed on a quick visual check – yellow sticky traps in the quarantine area catch any adults in flight before they spread to the wider collection. Inspect the undersides of leaves carefully under good light before accepting any new plant into a greenhouse or indoor collection. Even plants bought from reputable nurseries can carry thrips, and a plant that looks clean on a brief inspection may have eggs or early-instar nymphs that are not yet visible without close examination.

Inspect your collection regularly, particularly during the warmer months when thrips populations grow fastest. Check new growth, flower buds and the undersides of leaves on every plant at least once a week during spring and summer. Thrips thrive on stressed plants – keeping your collection in good health with appropriate light, watering and feeding is the most effective long-term preventative measure. A plant under moisture stress or nutrient deficiency is significantly more susceptible to thrips infestation than a well-maintained one, and the infestation will escalate faster once established on a weakened plant. Maintaining good air circulation around plants reduces the warm, humid microclimate that favours rapid thrips population growth.