Thrips Biological Control in Greenhouses

Why is thrips one of the most persistent greenhouse pests?

Thrips (Thysanoptera) rank among the hardest greenhouse pests to manage: they are tiny, reproduce rapidly, hide inside flowers and leaf margins, and in the case of Frankliniella occidentalis (western flower thrips), readily develop resistance to synthetic insecticides. On top of direct feeding damage, they transmit Tomato Spotted Wilt Virus (TSWV) and Impatiens Necrotic Spot Virus (INSV), both of which can cause economic losses far beyond what the insects alone would produce.

In our greenhouse work we have seen thrips populations that survived successive insecticide applications with minimal impact, while nearby blocks managed under biological programs maintained acceptable damage levels season after season. The critical difference was not a single agent but a multi-layered program addressing the full life cycle: leaf-surface predators for nymphs and adults, a pupal-stage soil treatment, and consistent monitoring. An overview of the principles behind this approach is covered in the biological control section.

Which biological agents are used against thrips?

Three categories of agent target different thrips life stages.

Orius laevigatus and Orius insidiosus

Orius is the strongest tool in the thrips biological-control toolbox. Orius laevigatus suits warmer Mediterranean-style greenhouses; O. insidiosus tolerates a slightly broader temperature window. Both hunt thrips nymphs and adults actively and will also opportunistically prey on aphids, mites, and small whitefly. Their greatest advantage is foraging inside flowers — exactly where western flower thrips shelters and reproduces.

Under good conditions a well-established Orius population can dramatically suppress thrips pressure within weeks. The key requirement is adequate temperature and daylength: as days shorten in winter, Orius enters diapause. Year-round greenhouse operations in short-day conditions need supplementary lighting or a shift in strategy during those months.

Amblyseius cucumeris and Amblyseius swirskii

Amblyseius cucumeris (= Neoseiulus cucumeris) is a predatory mite that primarily targets first-instar thrips larvae. It is effective as a preventive agent but its impact on second-instar larvae and adults is limited. Continuous, regular releases throughout the crop cycle give the best results; reactive releases against established populations rarely catch up.

Amblyseius swirskii covers a broader spectrum: it preys on thrips larvae as well as whitefly eggs and early-instar nymphs. Where mixed pest pressure exists, swirskii offers a practical combined biological option rather than running separate programs for each pest.

Steinernema feltiae

Thrips spend most of their pupal stage in the soil or growing substrate. Steinernema feltiae is an entomopathogenic nematode applied as a soil drench; its infective juveniles seek out and infect pupating thrips and first-instar larvae. Adequate soil moisture and temperature are essential for nematode activity — very dry or cold media reduce effectiveness substantially.

Without addressing the soil stage, even a perfectly executed leaf-surface program will face a continuous re-infestation from the growing medium. S. feltiae closes this gap and is a critical complement to Orius and Amblyseius.

How should monitoring be set up for a thrips biological program?

Blue sticky traps are the monitoring tool of choice for thrips; F. occidentalis responds much more strongly to blue than to yellow. Place traps at canopy level, especially near flowering zones. Count traps weekly, recording trend rather than absolute numbers. A rising count over two consecutive weeks signals the need to review release rates or timing.

Visual plant inspection is equally important. Signs to look for include silvery stippling on leaf surfaces, deformed or scarred petals, and irregular fruit dimpling. These can also signal TSWV/INSV infection, making early detection especially urgent. The sticky trap setup and monitoring protocol page provides additional detail on how to integrate trap data into programme decisions.

What cultural practices reduce thrips pressure?

Biological agents work best when supported by sound cultural management.

Sanitation: Thrips colonise crop debris and spent flowers. Timely removal of old plant material and end-of-season deep cleaning of the greenhouse structure reduce the carryover population entering the next cycle.

Humidity and ventilation: Moderate humidity and good air circulation are important. High humidity accelerates some thrips development stages and also promotes the fungal diseases that often accompany thrips damage on flowers and fruit. Greenhouse humidity and climate management guidance applies here.

Nitrogen balance: Excess nitrogen produces soft, fast-growing tissues that are thrips-preferred feeding sites. Balanced nutrition indirectly reduces host quality for the pest.

Incoming plant material: Thrips enter greenhouses most commonly on purchased transplants. A quarantine zone and inspection protocol for new stock is among the most cost-effective preventive measures.

When is biological control insufficient on its own?

Biological control is a powerful approach but it has operational limits.

High initial pressure: If thrips numbers have already peaked and damaged area is widespread, the agents need more time to establish suppression than the crop can afford. In this case, a targeted application of a compatible biopesticide such as spinosad — known to have low impact on Orius — can knock the population down before biological agents are re-introduced.

Short-day winter conditions: Orius diapause in low-light winter periods means that in greenhouses without supplementary lighting, the program must rely more heavily on Amblyseius and nematode applications during those months.

TSWV/INSV risk situations: Where tospovirus history is present, thrips suppression speed is critical — every new adult represents a potential vector. Physical barriers (insect-proof screening on ventilation openings) and rapid biological establishment are especially important in these cases.

For the broader framework of how thrips management fits into an integrated pest management programme, see the dedicated page.