Here are the key damage symptoms caused by western flower thrips (Frankliniella occidentalis):

Feeding damage: Thrips pierce the outer plant cells and suck out their contents, causing silver-grey or bronzed streaks and patches on leaves and petals. The dead tissue around feeding sites reduces the plant’s ability to photosynthesize and transpire efficiently, lowering overall vigour. Black dots (thrips excrement) often accompany feeding marks.

Distorted growth: Because thrips prefer young, developing tissue such as growing tips and flower buds, their feeding often results in deformed leaves, curled shoots, and twisted or misshapen flowers. In ornamentals, this can severely reduce aesthetic quality, making the crop unmarketable even at low pest densities.

Bud and fruit damage: Infested buds may fail to open, while fruit—especially in cucumbers—can become scarred or deformed (e.g., “pig-tail” fruit). This kind of cosmetic and structural damage impacts both marketability and yield.

Virus transmission: Western flower thrips are the primary vectors of Tomato Spotted Wilt Virus (TSWV) and Impatiens Necrotic Spot Virus (INSV). These viruses infect a wide range of crops, leading to systemic symptoms like leaf necrosis, ring spots, and plant death. Even a small number of thrips can initiate an outbreak of these economically devastating viruses.

The western flower thrips (Frankliniella occidentalis) develops through six distinct stages: egg, two larval instars, prepupa, pupa, and adult. Females insert their eggs into plant tissue—typically in leaves, flower petals, and soft stems—using a saw-like ovipositor.

After hatching, the larvae emerge as nearly transparent to pale yellow or orange-yellow, with prominent red eyes and relatively large heads. These larvae are active feeders, primarily targeting young, tender plant tissues. Following the feeding stages, the insect enters the prepupal and pupal stages, which are non-feeding and largely immobile unless disturbed. These stages are characterized by developing wing buds and changes in body shape, with pupae displaying more pronounced wing structures and antennae curved back over the head.

Pupation usually occurs in the soil, but can also take place on plant surfaces or in sheltered crevices. Once fully developed, the adult emerges with two pairs of delicate, fringed wings. Adult females show a wide range of coloration—from pale to dark shades—which may vary depending on temperature, host plant, and other environmental factors.

Preventing western flower thrips (Frankliniella occidentalis) from entering a growing space begins with strong exclusion practices. These tiny insects can enter greenhouses or indoor grow rooms through small gaps or on infested plant material, making physical barriers essential. Installing insect-proof mesh or screens on all vents and openings can significantly reduce entry points. The mesh size should be able to effectively block adult thrips while still allowing for adequate airflow.

Sanitation is equally critical in reducing the risk of a thrips infestation. All plant material brought into the growing area—whether cuttings, seedlings, or transplants—should be thoroughly inspected for signs of thrips or damage such as silvering, leaf distortion, or black specks of frass. Where possible, quarantine new plant material in a separate area for a few days and monitor it closely before introducing it to the main production space. Remove and dispose of weeds, plant debris, or volunteer plants both inside and directly outside the greenhouse or grow room, as these can act as alternate hosts or hiding places for thrips.

Environmental control and monitoring also play an important role. Install yellow or blue sticky traps throughout the growing space and near entry points to detect early signs of thrips activity. Sticky roller tape can also be installed horizontally just above crop level or vertically along walls to act as a passive barrier and mass trapping system. Consistently monitoring these traps will help catch infestations early before they become widespread.

Lastly, staff hygiene and workflow practices should not be overlooked. Workers can unintentionally carry thrips on clothing, tools, or equipment, especially if they’ve been in other growing areas. Implement protocols such as changing into clean clothing, sanitizing tools, and working from youngest (cleanest) to oldest (more at-risk) plants to minimize the spread of pests. By combining physical barriers, sanitation, monitoring, and good workflow habits, growers can significantly reduce the chances of thrips entering and establishing in a growing space.

Biological control offers an effective and sustainable strategy for managing western flower thrips (Frankliniella occidentalis) in greenhouse crops, indoor grows, and even ornamental or tropical houseplants. One of the most widely used and reliable methods involves the use of predatory mites. Amblyseius swirskii (marketed as Swirski) is a standout performer in and is especially effective for preventive use. It feeds on the first larval stages of thrips as well as whitefly eggs and young nymphs, making it ideal for mixed-pest situations.

Another excellent option is Thripex (Neoseiulus cucumeris), a mite that also targets early thrips larvae. It performs best under cooler or more temperate conditions and is well-suited for vegetables like peppers and cucumbers. For situations with high thrips pressure or in crops with consistent flowering, Limonica (Amblydromalus limonicus) provides aggressive control. It’s particularly effective in greenhouse ornamentals and cucumbers where early-stage thrips must be continuously suppressed. For broader temperature tolerances and more variable climates, Andersoni (Amblyseius andersoni) can provide background suppression and works well in combination with other predators.

In addition to predatory mites, growers can introduce beneficial insects like Thripor (Orius insidiosus), a minute pirate bug that preys on all mobile stages of thrips—including adults. While more expensive and requiring flowers or pollen to establish well, Orius is a powerful curative solution once populations are present. Another valuable generalist is Chrysopa (Chrysoperla carnea), the green lacewing. Its larvae are voracious predators, capable of feeding on thrips larvae, aphids, and other soft-bodied pests. Chrysopa is best used in combination with other controls for a layered IPM (Integrated Pest Management) approach.

To tackle the soil-dwelling pupal stages, which are often overlooked, introduce Entomite-M (Stratiolaelaps scimitus), a predatory mite that lives in the growing medium and preys on thrips pupae, fungus gnat larvae, and other soil-dwelling pests or Entonem (Steinernema feltiae), a beneficial entomopathogenic nematode that parasitizes thrips pupae in the substrate. Once applied via drench or irrigation, it actively seeks out and infects the pupae, helping break the pest's life cycle from below.

Blue sticky cards can be used as a passive monitoring and mass trapping tool specifically targeting adult thrips, as they are more attracted to blue than yellow. Strategically placing these throughout the growing area—especially near vents, doors, and early infestation zones—can help reduce adult populations and provide early detection of outbreaks. When used in concert with biological control agents, blue cards can enhance suppression and slow the spread of thrips throughout the crop. Together, these biocontrol tools form a dynamic and environmentally friendly defense against one of the most persistent greenhouse pests.