Outsmarting the elusive greenhouse thrips in ten simple steps

In equatorial regions, thrips are a major phytosanitary challenge and can significantly hinder the success of ornamental crop growers across international markets, writes Lisbeth Riis, CEO at Scarab Solutions.

The pest’s direct feeding during the adult and nymph stages can cause leaf deformation or chlorosis on flower petals, leaving growers with a level of cosmetic damage that renders the crop unmarketable. The pest’s polyphagous nature and ability to vector viruses, such as tospovirus, can be an even greater threat to importation hubs and requires workers to constantly monitor crops for the various thrips species. Workers need to monitor greenhouse crops for the various thrips species constantly.¹

The global thrips problem is exacerbating because of the pests’ widespread resistance to chemical pesticides. The continued pressure to reduce chemical pesticide residues, even on non-edible flower crops, and the withdrawal of approvals for certain pesticides, makes this problem even worse. An Integrated Pest Management (IPM) programme is essential to combat this pest.

A good scouting system can collect data on thrips adults and thrips larvae and calculate the difference in proportions – helping to improve a grower’s response time

With these serious market ramifications at play, growers must now significantly improve the management of thrips. This consideration, however, can often be a challenge in itself. Infestations can quickly spread undetected in the greenhouse due to the pest’s small stature (1-1.5mm) and high reproductive rate – but evidence suggests that early action from the beginning of the crop cycle can help growers turn the tables. With over fifteen years of experience in cut roses, Scarab Solutions experts believe that a combination of several intervention methods can help growers address the early signs of thrips before an infestation becomes too costly and unmanageable.


Maintain a clean greenhouse and carry out thorough inspections

In the early stages of crop cycles, good greenhouse hygiene can be a crucial preventative measure against thrips infestations. For instance, growers should use clean growing media in the production area and thoroughly inspect any new planting material to minimise the risk of introducing a new source of infestation. In particular, growers must check any new crops for signs of thrips larvae, as this would confirm the presence of ‘resident thrips’, which are thrips that have multiplied in the crop.


Adjust traps to increase control overpopulation sizes

Sticky traps above the crop canopy are a common way for growers to monitor thrips in the greenhouse, as this is where the tender plant tissue the pest feeds on is located – but an understanding of the pest’s ecology can significantly increase the number of captures.

Research indicates that 70 per cent of flying thrips adults are captured at a height lower than 1 meter from the ground.² Therefore, traps nearer to the ground level can provide growers with more accurate data on the population situation and will better inform decisions concerning intervention measures.

Thrips are attracted to both yellow and blue sticky traps, and can these also be used outside the greenhouse to monitor and trap thrips moving into the farm from neighbouring crops and vegetation.

Growers can also incorporate a formulation of male-aggregation pheromone into the glue layer of sticky traps to double the number of captures and keep the population to a manageable size.


Locate the key points of entry

Unlike ‘resident thrips’, ‘immigrant thrips’ such as ‘local flyers’ enter the greenhouse from the outside, often through doors, holes, and cracks in plastic walls or when lifting wall curtains during ventilation. These thrips favour the leeward side of the greenhouse as they fly upwind during periods of light wind but remain in place during windy weather. Research from traps mounted on wind vanes found that only 25 per cent of the thrips were caught on the windward side.³

Therefore, once growers define the prevailing wind direction on the farm’s location, knowledge of the thrips flight pattern can be used to target measures on the leeward side.
With attention now on the leeward side, growers must start intervention and focus on greenhouse maintenance to minimise the pest’s infiltration. For instance, workers should always report and immediately fix any holes in the plastic walls, especially near the ground level – this is an easy but highly effective strategy to avoid a costly infestation. Growers should limit the movement of thrips with the installation of double doors that can create a small airlocked room connected to the greenhouse entrance.

But don’t forget about the periphery defences. Growers can place repellent substances, such as naphthalene balls, near the ground to repel emerging thrips outside the greenhouse. It is also important that scouts regularly check any outdoor plants on the farm, including bushes and trees, for signs of thrips and take measures to remove or replace plant material that is known to attract thrips. Growers should also frequently mow the grass close to the greenhouse and never allow it to flower to further reduce the build-up of thrips populations in the greenhouse vicinity.


Filter out UV light to disrupt further advancement

However, if thrips manage to elude the early defences, growers can still avoid a costly infestation by manipulating the pest’s affinity for sunlight. For instance, growers can cover certain areas of the greenhouse in UV-absorbing plastics to discourage thrips from moving into the perceived dark spaces and spreading to other plants.

Outside the greenhouse periphery, highly reflective, metallised mulch can be spread on the ground or hung up as a skirt around the greenhouse from 1 meter down to further disorientate the thrips and limit their infiltration into the greenhouse.5


Introduce natural enemies to keep the population low

As growers aim to implement more sustainable pest management practices, biological controls, particularly the use of natural enemies, have become a popular control option. However, it is important to remember that different species are only effective at certain stages in the thrips lifecycle.

For the larval stages, growers can use Neoseiulus cucumeris, Amblyseius swirskii, Amblydromalus limonicus and Transeius montdorensis – but these predatory mites will only feed on the ‘resident thrips’ offspring and help keep the resident population low.6

Growers can target thrips pupae in the soil with the use of soil-dwelling predatory mites, Stratiolaelaps scimitus (formerly Hypoaspis mites) and Macrocheles robustulus.

During the adult and larvae stage, growers can use the predatory bug, Orius laevigatus and fungi attacking insects (entomopathogens) such as Metarhizium anisopliae, Verticillium lecanii, Paecilomyces and Beauvaria bassiana to significantly limit the spread of thrips in the greenhouse.

Entomopathogens can be effectively applied to the canopy despite the short persistence in UV light. Thrips tend to congregate on the upper surface of leaves two hours after sunrise and two hours before sunset, making them easy targets for a topical spray. However, the spray volume and choice of the nozzle must be carefully calibrated to avoid run-off and ensure even coverage of the upper surface of the leaf at this time. At all other times of day, the thrips will continue their cryptic behaviour and avoid the sprays.

At other times of day, the ideal application for thrips will cover all surfaces but this is most likely to be achieved by the use of fogging equipment. Some commercially available entomopathogens have been tested with thermal foggers and have suffered no reduction in viability. Seek advice from your supplier.

The entomopathogens Metarhizium anisopliae is most effective when applied to the soil surface because they are killed by UV light after about 24hrs when applied to the canopy. The soil phase of thrips pupae is an easy target for entomopathogens providing they are applied by a high volume hydraulic sprayer and not applied down the drip irrigation system. Thrips pupae are located very near the soil surface therefore to target them effectively with entomopathogens the application must focus on the soil surface underneath the leaf canopy as well as the edges of beds. Applications down drip irrigation are designed to take the water to the root zone – where there are no thrips. If flower crops are grown on benches, the entomopathogens are generally compatible with soil-dwelling predatory mites.

It is a common practice by rose growers to combine Botrytis and thrips sprays and only focus the sprays on the flowers. This practice allows the thrips population in other parts of the canopy to survive and present a continuous threat. If applied routinely and in sufficient numbers, predatory mites will seek out thrips in all parts of the canopy. They are an essential tool in reducing populations of thrips that are resistant to chemical pesticides.


Rely on a systematic system to guide the releases

However, the time taken to establish biological control is a common concern to growers as it can lead to unnecessary losses. But a good scouting system can collect data on thrips adults and thrips larvae and calculate the difference in proportions – helping to improve a grower’s response time.

With thorough systematic in-crop scouting and data collection measures in place, growers can carry out better spatial targeting of interventions and make data-based decisions on when and where to intervene with either stronger and more toxic means or with milder and more environmentally friendly means to keep the thrips population under control.


Enhance early intervention measures with insecticidal mixtures

As concern mounts from global markets to reduce pesticide use, biocides can be a safer alternative to help limit the spread of thrips infestations in the greenhouse – and to enhance the efficacy of biological controls. For instance, growers can combine a mixture of natural insecticides, such as neem extract, with certain entomopathogens (Metarhizium anisopliae, Verticillium lecanii, Paecilomyces and Beauveria bassiana) to improve the effectiveness of this control.7

Check the label of the entomopathogens used and seek advice from the supplier if they can be tank-mixed with certain chemical pesticides and lower than the label rates. This approach can provide a synergistic effect on thrips control when a chemical action is combined with a biological mode of action. This strategy can not only reduce the amount of chemical applied to the crop but can also help with resistance management of important active ingredients such as that in Tracer (Spinosad). Most labels for Spinosad indicate that there is a high risk of resistance if the product is applied more than once or twice in a season. It is, therefore, recommendable to guard the effectiveness of this active ingredient by tank mixing with an entomopathogen.
While the mode of action is still not clear, longstanding research indicates that growers can also enhance the efficacy of nerve-acting insecticides with liquid sugar additives. The recommended dosage is between 0.125 per cent to 0.25 per cent.8 But note, dissolved solid sugar does not have this effect.


Maximise coverage and limit waste with targeted sprays

Sprays must make direct contact with the pest to improve non-systemic applications’ effectiveness, but this can often be a challenge in itself. Thrips in unopened rosebuds and the underside of developing leaves have far less exposure to insecticide treatment compared to flowers with disc florets. Therefore, we recommend a thorough spray coverage of all plant parts (flowers and leaves) and the use of flat fan nozzles with an inclined spray angle of 30 degrees to reach the underside of the leaves.9

It is also important to target sprays when the pest is less likely to fly away. For instance, we recommend growers concentrate sprays before 8am and between 10am and noon to account for the diurnal flight activity patterns of the adult thrips, which peak between 8am to 10am and again between 2pm to 4pm in greenhouses.10 If growers can intervene in this timely manner, they will be able to maximise the thrips’ contact with the spray before it dries and reduces any unnecessary losses. However, growers must take care to avoid the risk of scorching during strong midday sunlight sprays.


Adjust the action for each life cycle

As thrips can quickly build up resistance to chemical sprays, we advise growers to use a different mode of action for each life cycle. This change can occur approximately every two weeks to ensure the continued effectiveness of sprays against the ‘resident thrips’.¹¹

The compatibility of chemical pesticides with predatory mites and other natural enemies is well known, and information is freely available on the websites of several commercial biocontrol companies. It is critical to the establishment of the routine predatory mite systems that only compatible pesticides are used when natural enemies are deployed. Quantitative scouting systems will indicate if these chemicals are needed and monitor their effectiveness. Due to the high risk of further resistance developing, it is crucial that the effectiveness of chemical pesticides is keenly monitored in this way.


Stay one step ahead with early action and avoid a costly infestation

As thrips remain a constant threat to protected ornamental crops throughout its entire life cycle, there is no one simple management solution that growers can use to take back control and satisfy market demands. But lessons learnt from cut roses demonstrate how growers can successfully manage the pest with a reliable scouting system and a combination of preventive measures and targeted interventions that aim to protect the crop throughout its entire cycle.

1 Royal Entomological Society, The Home of Insect Science www.royensoc.co.uk/entomology/orders/thrips 
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 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
4 Masami, S. and Honda, K. (2013). Insect reactions to light and its applications to pest management. Appl. Entomol. Zool. 48:413–421
5 Giles, F. (2010). Keep Out Greenhouse Pests. Growing Produce. May 1, 2010
6 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, 
7 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
8 www.linkedin.com/posts/dudutech_thrips-management-webinar-activity-6732249360937062400-cLMJ
9 Foqué, D. & Nuyttens, D. (2011). Effect of nozzle type and spray angle on spray deposition in ivy pot plants. Pest Management Science 67 (2):199-208
10 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: leizhr@sina.com
11 Brownbridge, M. & Bruitenhuis, R. (2017). IPM: Prevention and early intervention, www.greenhousecanada.com


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