Found silvery spots on the leaves, spotted small insects — diagnosed thrips, bought an insecticide, applied it. Two weeks later the thrips is back and there are more of them. The reason is not that the insecticide was ineffective — it is that most of the population at the time of treatment was in the soil or substrate at a stage where no contact insecticide can reach them. To control thrips, you need to know where it is at every phase of its cycle.
Quick Glossary
- Thrips — a small insect of 1–2 mm in the order Thysanoptera; the most common greenhouse species is Frankliniella occidentalis (western flower thrips); it feeds on cell sap, damages flowers and fruit, and transmits viral diseases
- Thrips development cycle — egg → larva I → larva II → prepupa → pupa → adult; prepupa and pupa develop in the soil or substrate and are inaccessible to contact insecticides
- Resistance — reduced population sensitivity to an insecticide after several generations of treatment with the same product or chemical group
The Development Cycle: Where Thrips Is Vulnerable — and Where It Is Not
Frankliniella — the primary greenhouse species — has six developmental stages. The egg is laid inside plant tissue — leaf or petal — and is inaccessible to contact insecticides. Larvae I and II feed on the plant, and this is the window when treatment is effective. But the larva then descends into the soil or substrate, where it passes through two "pupal" stages — prepupa and pupa — before emerging as an adult.
If you treat only the above-ground parts, you destroy the larvae present on the plant at the moment of treatment. The population in the substrate and eggs inside tissue remain. Within 7–10 days — a new emergence of adults. This looks like "the insecticide doesn't work" — in reality, it works only on a fraction of the population.
The full cycle at +25°C takes approximately 14 days. At lower temperatures — longer. Each generation can lay dozens of eggs. This explains why two-week treatment intervals that ignore the cycle allow the population to recover each time.
Diagnosis: Distinguishing Thrips from Spider Mite and Other Causes of Discolouration
Silvery spots or streaking on leaves is a characteristic sign — but not a unique one. Spider mite produces a similar picture. Key distinctions:
Thrips signs: silvery streaking or discoloured patches without sharp borders, black dots (frass) adjacent to damage, deformation and curling of apical leaves and flowers under heavy infestation. On blue sticky traps — elongated insects 1–2 mm. Damage often begins on flowers and upper leaves.
Spider mite signs: small pale stipples uniformly distributed across the entire leaf surface, webbing under heavy infestation, mites visible under magnification on the underside of leaves. No black frass dots.
Important for treatment: if both are present — thrips and mite — and you treat for only one, the other gets ideal conditions while its competitor is suppressed.
Control: Hierarchy of Methods
Monitoring first. Blue sticky traps are the primary early detection tool. Thrips appear on traps before damage becomes visible on plants. Action threshold: 10–20 individuals per week per trap.
Biological control at low population levels. Amblyseius cucumeris — a predatory mite that feeds on thrips larvae on the plant. Effective when introduced preventively or at low population density. Orius laevigatus — a predatory bug that attacks all thrips stages including adults — more effective at moderate population levels. Beneficials do not eliminate thrips in the substrate — only on the plant.
Chemical control with rotation. When the threshold is exceeded — apply insecticide accounting for the cycle: intervals of 5–7 days between applications to catch each new generation emerging from the substrate. A minimum of 2–3 applications in series is required — a single application is not enough. Group rotation is mandatory. Main IRAC groups active against thrips: group 5 (spintoram/spinosad), group 6 (abamectin), group 28 (chlorantraniliprole), group 4A (neonicotinoids — use with caution due to impact on beneficial insects). Do not use the same group for more than two consecutive applications.
Substrate treatment. Under significant infestation — treat the upper substrate layer with an insecticide or with entomopathogenic nematodes (Steinernema feltiae) that destroy prepupae and pupae in the soil. This is a critical step that most growers skip.
Three Mistakes That Cost the Most
Treating only the above-ground parts and leaving the substrate untreated. Half the cycle takes place in the soil. Without substrate treatment or nematode application, the population recovers with each generation regardless of how effective the plant treatments are.
Not observing the interval between applications. Treated once — 10 days later the population recovers from the substrate and eggs. A series is required: three applications at 5–7 day intervals to cover multiple emergence waves. A single "preventive" treatment provides no sustained control.
Using the same insecticide group season after season. Frankliniella is one of the most extensively documented pests for insecticide resistance. A few generations under pressure from a single product and effectiveness drops to zero. IRAC group rotation is not a recommendation — it is a prerequisite for maintaining control.
Signs That Thrips Is Under Control
- Individual counts on blue sticky traps are stable or declining over two weeks following a treatment series
- No new damage appearing on plants
- On route inspection — fewer than 5% of plants show signs of active feeding
- Beneficial insects are present and active between treatments