Cannabis Pests - Thrips

Thrips are a common pest in cannabis cultivation and they can affect many different plants. In the unfortunate event your crop becomes infested, there are a few treatments that can ease your plants back to safety. Getting rid of thrips can be a challenge, but we're here to help. This article can guide you on your way to controlling a breakout of thrips from start to finish.

What Are Thrips?

Thrips (thripidae from the order Thysanoptera) are fast-moving, small insects that measure around 1-2mm in length. They usually become active in early spring and fly onto a plant to lay their eggs inside the plant tissue. Most of them have wings and they can travel from plant to plant extremely easily. 

Thrips go through various stages in their life and live both above and below the soil. Once they evolve past the propupa and pupa stage, they emerge from the soil as adults with wings. From here they make their way up the plant to feed off the leaves and lay their eggs.

Thrips tend to flourish in warmer climates, however there are thousands of different species that appear all across the world on all kinds of plants. When thrips inhabit a cannabis plant, they start to feed off the nutrients stored in the leaves by sucking the sap out from wherever they can.

Young cannabis plants in vegetation are particularly susceptible to thrips as they like to feed off nitrogen and also cannot thrive if the plant has thick foliage and is covered in resin. However, thrips can attack at any moment during the spring and summer, even on flowering cannabis plants. Yields can be hugely affected if the pest is not controlled.

The Causes of Thrips

The arrival of thrips is often caused by other plants nearby that also attract them. They are both a problem indoors and outdoors, but indoor plants tend to have a higher risk due to the way they are protected and isolated from predators, and away from other plants that could 'distract' them.

Thrips can also attach themselves to clothing, meaning you can simply drag them in from outside without knowing. Also, if you have a dog the chance of bringing them in the home is much higher. Another way for thrips to make their way into an indoor grow space is through bad clones that have come from an infected plant.

Thrips like shaded, damp areas where it is warm and there is little air circulation. High temperatures cause them to grow faster.

Identifying Thrips

In order to get a better look at a thrip, it is recommended to place a few yellow adhesive strips around the base of your plants. The thrips will be attracted to the yellow colour and get stuck to the adhesive on landing, allowing you to inspect them more closely and confirm that your garden has a thrip problem.

Symptoms of a thrip infestation:

• Lower areas of the plant often affected first
• Flies around the top of the soil (not to be confused for fungus gnats)
• Dark, silvery/brown spots and stains on leaves and/or stems from bites
• Leaves wilt, turn brown and fall off
• Slow plant growth
• Small black dots on leaves (faeces).

Because thrips move relatively quickly, they are quite easy to spot. If you move your finger close to a thrip, it will likely scutter away before you get a proper look at it.

Thrip Treatment

Due to the fact that thrips develop in the soil as well as the leaves, we must apply treatments to both areas if we want to prevent them from coming back. This tends to involve separate treatments so make sure they are compatible first so you don't end up harming your plant further.

For example, it's best to apply any insecticides before introducing beneficial predators so we do not end up killing them by spraying once they are already on the plant. Try to keep it natural and avoid using chemical pesticides if you can.

Most organic pesticide sprays need to applied at least every 2-3 days to keep populations under control.

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Remove sites with adults - This can be the first step to controlling the speed at which the thrips are reproducing. Even though the eggs are extremely hard to see, you can remove leaves close to the areas where you find adults.

Neem oil foliar spray - Neem oil works to repel most pests associated with cannabis and it also helps to control the thrips, limiting their development by messing with their hormones. Depending on the number of thrips, it is unlikely to eradicate the infestation on it's own but can quickly stop it spreading.

Tip: Avoid spraying heavy concentrations or using during flowering as it can affect the final taste and smell of cannabis buds.

Pyrethrins - An organic insecticide that comes from the chrysanthemum flower and has been used for many years to repel insects. Although natural, it can be toxic for human if inhaled and can be harmful to other insects or aquatic life. However, pyrethrin products are normally safe for humans to use due to their diluted composition and fast degradation.

Pyrethrins paralyze and kill thrips when they come into direct contact, so we suggest using it as a foliar spray and make sure to cover all the affected areas. If you're growing outdoors, spray in the morning before bees begin their activity.

Insecticidal Soap - Potassium soap is used to treat a number of different plant pests and effective against thrips. It can be mixed with water (and neem oil) and applied as a foliar spray every few days to kill off thrips grouping around the leaves and stems. Be sure to spray the whole garden even if only one plant is infected as they may just start populating on another plant.

Natural Predators - If your cannabis plants are already in flowering, it's best not to apply any sprays that might affect the final taste or quality of your buds. Predators can be a be a suitable solution where you have already applied organic insecticides but the thrip infestation continues.

The most effective predators to combat thrips are Orius Laevigatus, Amblyseius Cucumeris and Amblyseius Swirskii. Orius Laevigatus is for the adults above soil and Amblyseius Cucumeris or Amblyseius Swirskii are for destroying the infestation underground.

Tips For Preventing Thrips

Before thrips ever become a severe problem, do what you can to prevent all pests from pestering you and your plants. If you keep a close eye on your cannabis plants, you're more likely to spot the problem before it goes out of control.

Tips for preventing thrips:

• Keep your grow space as clean as possible. Sterilise equipment and change substrate between grows.
• Always remove dead and decaying leaves from your plants.
• Use blue and yellow adhesive strips around the grow room.
• Maintain temperatures below 25°C. Thrips prefer warm, humid conditions.
• Apply preventative neem oil spray once per week during vegetation.
• Provide enough ventilation.
• Avoid taking clones from infected grow spaces.

Thrip larva crawling around in top of grow medium (BillMonroe's diary from GrowDiaries).

Conclusion

Thrips can be a real pain if they are left untreated, so do not wait too long if you've noticed a couple flying around or scuttling around the top soil. The sooner you start applying treatments, the more time you have to get rid of them before flowering starts.

What did and did not work for your when controlling thrips? If this article was helpful, why not leave us a comment down below? We'd love to hear your experience.

External References

Onion thrips Thrips tabaci (Thysanoptera: Thripidae) reduces yields in indoor grown cannabis. - McCune, Frédéric & Morphy, Chad & Eaves, James & Fournier, Valérie. (2020).

A new look at thrips (Thysanoptera) mouthparts, their action and effects of feeding on plant tissue. - Chisholm, I., & Lewis, T. (1984)

Western Flower Thrips In Greenhouses: A Review Of Its Biological Control And Other Methods. - Roy Van Driesche, University Of California. Retrieved 18.12.2020

Homologies and Host Plant Specificity: Recurrent Problems In The Study Of Thrips. - Laurence A. Mound. (2013)

This article was updated December 2020.