What are thrips and how to eliminate them from your cannabis grow?

Thrips are one of the most common threats in cannabis growing and can become a real headache if you let them settle in your grow. These little insects that feed on the sap of the plants are extremely fast, so you also need to act rapidly to prevent serious damage. This post explains everything you need to know to detect and tackle this pest effectively.

Thrips (from the Thripidae family) are small insects with a flat and elongated shape whose body reaches a length of 1 – 1.5 mm. They belong to the Thysanoptera order, and the type that most often appears on cannabis plants is Frankliniella occidentalis (also commonly known as the Western flower thrip). This species is native to North America but has now spread to other continents. It can be found on more than 500 different host plants, with cannabis being one of its favourites.

Thrips can exhibit a wide range of colours, going from grey to yellow, and all the way through to brown. However, the subtype which usually appears on cannabis is normally white, or slightly yellow.

Although they are winged insects, their wings aren’t an outstanding example of evolution: they are narrow, with fine hairs, but just enough to help them fly from one plant to another, infecting the whole grow relatively quickly.

Thrips reproduce rapidly, up to twelve times per year, and especially when they are encouraged by the high temperature and humidity levels of indoor environments, which recreate the ideal conditions for reproduction. Thrips develop more quickly when the temperature is over 25ºC, so it’s vital to tackle this problem before this pest becomes too widespread.

Life cycle of thrips

The life cycle of thrips in cannabis grows consists of several phases: egg, larva, pupa, and adult. Each stage lasts for a specific length of time and, depending on which phase the insects are at, they will be found on different parts of the plant:

• Eggs hatch in 3 – 5 days.
• Larvae develop in the plant tissue, which they feed on (thereby creating small holes). This can make cannabis plants susceptible to bacterial infections.
• The pupa phase occurs in the substrate and lasts between 4 and 5 days. Thrips don’t feed during this stage.
• Once they’ve become adults, thrips emerge once again to feed on the plants, reproduce, and lay their eggs.

The effective life of adult specimens can vary: in males, it’s usually between 30 and 50 days, whereas females usually have a life cycle of 40 – 75 days.

How to prevent the appearance of thrips in your cannabis grow

Prevention is the best tool for any self-respecting grower. Even if it involves some extra work, it is best to maintain certain habits and practices to dodge future problems. In the case of thrips, you can help prevent their appearance by keeping your grow free of weeds and ground debris (like withered leaves).

You should also make sure you check everyone’s shoes before you let them into your grow room. If they’ve been in an area infected by thrips, the eggs could have easily attached to their soles. And given that thrips can reproduce without mating, you only need a few adults to produce a huge population.

Since this insect develops in the substrate during its pupa stage, it’s always a good idea to replace the growing medium after every cycle to prevent risks. You can also introduce some predatory species as a preventive measure: they won’t do any harm to your plants but will certainly get rid of thrips. Look for Amblyseius swirskii, a predatory mite which acts as a Swiss knife in the biological control of cannabis and thoroughly enjoys an order of young thrip larvae for breakfast!

Other mite species, such as Amblyseius cucumeris, or parasitic wasps like Thripobis simulteus or Megaphragma mymaripenne, eat the larvae before they have the chance to become adults. However, although this is a good way to control thrip populations, studies show that this measure alone is unlikely to eliminate all the thrips.

Spraying your plants with an organic insecticide during the vegetative phase can also be a good preventive measure. Avoid nitrogen excess in your fertiliser as well, as this attracts thrips. In fact, the main reason for their rapid growth comes from them infecting commercial crops which have been fertilised with high nitrogen levels, and then spreading further via the transportation of the infected plant material.

How to identify thrips in your cannabis plants

Early identification of thrips is key to control this pest. Therefore, you must regularly check your cannabis plants, paying special attention to the following symptoms:

• If there are thrips in your grow, you’ll find dotted marks on the leaves, like fine silver trails. Depending on the state of development of the pest, these marks will be more or less evident and abundant.
• If the plague is already widespread, you’ll also notice some small brown spots, which are the thrips’ excrements.
• Thrips aren’t easy to spot because their adaptive elongated shape helps them hide in the plants, usually in parallel with the nerves of the leaves. Look for them in the middle and low areas of the plants.
• You can also find them due to deformed leaves, or leaves which have become more brittle.
• Another option to help detect them is by placing yellow or blue adhesive traps in your grow. If there’s an infestation, it is very likely that some of them will get stuck to the traps.

How to eliminate thrips from your cannabis grow

If you’ve already detected that thrips have settled in your grow, it’s imperative that you act immediately to eradicate them as soon as possible. It’s important to identify the stage of the infestation: if it is advanced, thrips will have reproduced and there will be pupae in the substrate, so you’ll need to attack this pest on all fronts.

A good option to eliminate the thrips on the plants is to spray them with biological products like Neem oil, pyrethrins, or potassium soap. You can apply these remedies on the plants providing they still haven’t started flowering.

Remember that everything you spray on your buds will end up in your body once you’ve smoked them. Therefore, it is not advisable to spray any chemical insecticides on the plants. However, if you’re forced to do so, try to spray in the morning or at night, which is when the thrips feed. The higher their activity, the higher their exposure to the chemicals.

You can also use diatomaceous earth, which is made of microscopic algae fossils. This is a 100% natural insecticide which is totally harmless to humans. It kills insects and parasites by dehydration, in a way that prevents them from developing any resistance against this powerful natural insecticide.

While thrips are not one of the deadliest pests of marijuana, they can still cause serious problems if they’re left to roam free. Prevention and early identification are critical to stop them causing havoc in your grow, and so that your plants can reward you with plentiful crops of aromatic buds.