Striking back against thrips: Prevention strategies to regain the greenhouse

Figure 1: Frankliniella occidentalis – western flower thrips (Image source: Frank Peairs, Colorado State University,, CC BY 3.0 <>, via Wikimedia Commons)

Thrips is a pest to be reckoned with – its polyphagous appetite, alarming reproduction rate and ability to vector plant viruses presents significant threats to greenhouse growers looking to take their products to market. Tackling these pests is no easy matter but, with 15 years of experience in cut roses, Scarab Solutions experts have found that several intervention actions, when combined, can help growers effectively manage and control the destructive power of thrips and avoid a costly infestation:

1. A fundamental hygiene process

Initial efforts must start in the production area, where cleanliness is a necessity. To prevent introducing a new infestation into the greenhouse, new crops must be clean and free of ‘resident thrips’ that multiply within the crop. ‘Immigrant thrips’ come in from outside and derive from either ‘migratory thrips’, or ‘local flyers’, the latter flying within the hight of the canopy and being guided by visual cues – and these are the type of ‘migrants’ that are most likely to find their way into the greenhouse crops.

2. Identify the hotspots – think ‘the leeward side’ and hunt in the right places

The risk of thrips entering greenhouses from the leeward side than from the windward side is more than double – verified by research on wind vanes that found 75% of thrips caught on the leeward side.[1] We recommend identifying the prevailing wind direction on the farm’s location, with the intent of stopping thrips entering on the leeward side.

Sticky traps, usually placed above the crop canopy, would benefit being nearer to the ground level. Growers will yield more reliable data and support mass capturing – and research confirms that 70% of flying thrips adults are captured at a height lower than 1 meter from the ground.[2]

3. Strengthen your defences and limit infiltration

Nearby grass should be frequently mowed and never allowed to flower, as this will help reduce the build-up of thrips populations in the greenhouse vicinity. We recommended a thorough examination of all outdoor plants on the farm, replacing any plants and trees that are attractive to thrips – particularly those on the leeward side. For the growers who wish to apply repellent measures, we recommend placing these near to the ground to target the local flyers.

The installation of double doors to create a small airlocked room connected to the entrance of a greenhouse can limit thrips entrance – but don’t forget to close the first door before opening the next!

4. Schedule predatory mites and scout accordingly

Predatory mites can be a crucial biological control for thrips population management but be mindful – different predatory mite species are only effective at certain stages in the thrips lifecycle. Neoseiulus cucumeris, Amblyseius swirskii, Amblydromalus limonicus and Transeius montdorensis only predate the larval stages, which means that they will only feed on the ‘resident thrips’ offspring and help keep the resident population low.[3] Soil dwelling predatory mites, Stratiolaelaps scimitus (formerly Hypoaspis mites) and Macrocheles robustulus feed on thrips pupae in the soil and the predatory bug, Orius laevigatus, feeds on both adults and larvae.

A good scouting system can calculate the proportion between thrips adults and thrips larvae, giving growers insight into the type of thrips population (residents or migrants) they have – and helps decide which will be the most effective means to control the thrips population.

5. Enhance entomopathogens effectiveness

Fungi attacking insects (entomopathogens) such as Verticillium lecanii are another biological control for supressing the thrips population. The effectiveness of certain entomopathogens (Paecilomyces and Beauveria bassiana) is amplified when combined with a mixture of insecticides or natural insecticides, such as neem extract.[4]

6. A new thrips generation equals a new Mode of Action (MoA)

For growers who are allowed chemical sprays, a spray at least once a week can control the ‘resident thrips’ but it is important growers change to a new chemical with a different MoA for every thrips life cycle, i.e., about every two weeks, as they can build up a chemical resistance.

Some sprays rely on contact with thrips in order to be effective, so timing is important. Thrips are most actively flying between 8:00AM – 10:00AM and 2:00PM – 4:00PM, so avoid spraying during these periods and instead, concentrate sprays when thrips are settled on the plant.[5] Mixing with liquid sugar additives like those used in beehives (as opposed to dissolved solid sugar) at a dosage between 0.125% to 0.25% has been found to enhance efficacy significantly.[6] Ensure thorough coverage of all plant parts and be careful of spraying during strong midday sunlight as this can increase the risk of scorching.

7. Filter the light to confuse and disorient

As thrips orientate themselves to the sunlight, filtering out some UV light will help to control activity and movement. UV-absorbing plastics will discourage thrips from moving into the perceived dark spaces[7] and highly reflective, metalized ground mulch at the greenhouse circumference can disorientate the thrips, limiting the likeliness of invasion.[8]

It’s time to fight back!

Detecting thrips is a difficult task and can sometimes be too little too late. But by consistently using a combination of prevention and intervention processes, growers can strike back and protect their crops and plants from this destructive pest.

By Lisbeth Riis, CEO, Scarab Solutions


[1]  Ansari, M.A. et al. (2007). Control of western flower thrips (Frankliniella occidentalis) pupae with Metarhizium anisopliae in peat and peat alternative growing media. Biological Control 40, 293–297

[2] Ben-Yakir, D. and Chen, M. (2011). Characteristics of the migratory flight of the onion thrips (Thrips tabaci) and their relevance for pest management. Entomological Society of America Annual Meeting 2011. Dept. Entomol. Inst. of Plant Protection, Agricultural Research Organization, Bet Dagan, Israel

[3] Labbé, R.M. et al. (2019). Comparison of Transeius montdorensis (Acari: Phytoseiidae) to Other Phytoseiid Mites for the Short-Season Suppression of Western Flower Thrips, Frankliniella occidentalis (Thysanoptera: Thripidae). Environmental Entomology, Volume 48, Issue 2, April 2019, Pages 335-342

[4] Wakil, W. et al.(2012). Toxicity of Paecilomyces lilacinus blended with non-conventional agents to control cotton thrips (Thrips tabaci Lind.) (Insecta: Thysanoptera: Thripidae). Journal of Microbiology Research Vol. 6 (3), pp. 526-533; Al Mazraáwi, M. S. (2007) Interaction effects between Beauveria bassiana and imidacloprid against Thrips tabaci (Thysanoptera: Thripidae). Commun Agric Appl Biol Sci. 72 (3):549-55

[5] Liang, X.H. et. al. (2010). The diurnal flight activity and influential factors of Frankliniella occidentalis in the greenhouse. Insect Science 29 November 2010. Zhong‐Ren Lei, Institute of Plant Protection, Chinese Academy of Agricultural Sciences, Beijing 100193, China. Email:


[7] Masami S. and Honda K. (2013). Insect reactions to light and its applications to pest management.  Appl. Entomol. Zool. 48:413–421

[8] Giles, F. (2010). Keep Out Greenhouse Pests. Growing Produce. May 1, 2010