Many organisms are associated with Cannabis crops, some of them are beneficial and some are harmful to the plant. Those organisms considered harmful to the plant are also called pests. Spider mites, thrips, fungus gnats, and aphids are some of the most common insect and arachnid “bug” pests in indoor marijuana crops.

In this article, you’ll learn how to identify bugs that are considered pests and how to get rid of them to have a healthy crop. There’s a list of the environmental conditions (temperature and humidity levels) in which these pests appear and breed most rapidly so you can take preventive measures.

What are spider mites?

Spider Mites are one of the most common pests on cannabis crops. Spider mites are not insects but small arachnids and there are around 1200 species of them. Spider mites are very small so a magnifying glass or 10X hand lens is very helpful to spot them. They are less than 1 mm (0.04 in) in size and may be red, brown, or green. Spider mites live on the underside of plant leaves, where they feed on the plant’s tissue and produce protective silk fabrics.

How to identify spider mites

Early signs of spider mites include small spots or bites on the leaves and small dots on the underside of the leaves and branches. The plant infested with spider mites starts to show white spots and discoloring of the leaves. When the colony grows, spider mites start growing their silky “spider” webs on the underside of the leaves and then on the buds.

It’s very difficult to recover a plant at this stage so act as soon as you spot the first signals. If the pest progresses, leaves might turn yellow, flabby, and eventually die completely. A large spider mite infestation can have a negative effect on a cannabis plant and reduce yield significantly.

Environmental conditions for spider mites

Spider mites breed and multiply quickly with hot temperatures around 27°C/80°F and low humidity levels (below 60%).

How to get rid of spider mites on indoor plants

• Use potassium soap and botanical pyrethrins to kill adult spider mites.
• Apply neem oil to help prevent them from coming back by killing the eggs and larvae.
• Potassium soap and Neem oil work at the first stage of infestation so don’t wait until it gets worse.
• Colonies of spider mites become quickly resistant to chemical insecticides.
• Controlling environmental factors such as temperature and humidity in the growing area is key for prevention.
• Consider using a humidifier if your grow tent or room is too dry.

What are thrips?

Thrips are small sap-sucking insects that live in cannabis plants and feed on them. Thrips have elongated bodies, most of them from 0.6 to 3 mm (0.02 to 0.12 inches) so they can look like small dots moving on a leaf. Use a 10x hand lens to spot adult thrips and larvae on the top and the underside of the leaves.

How to identify thrips

Early signs of a thrip infestation are white, silvery, and shiny spots on the leaves’ surface. After a few days, the leaves show small white dots similar to “rice grains” (larvae) on them and sometimes small black dots (thrips feces). Adult thrips reproduce very fast and can hibernate in the soil mix during the cold months.

Thrips are not the most harmful pest but they breed very rapidly and they can be damaging in great numbers. Leaves start wilting when thrip damage advances and show lots of white and silvery spots, where thrips had eaten the leaf tissue.

Environmental conditions for thrips

Thrips reproduce very fast with high temperatures (25°C to 27°C / 77°F to 80°F) and high humidity levels (60% to 85%) so be careful, especially during the summer months.

How to get rid of thrips

• Apply Potassium soap and Neem oil to repel thrips. Better apply as soon as you spot them.
• Place yellow and blue sticky traps in your tent, they are really useful and catch a lot of thrips. Thrips feel attracted to the color traps and get stuck on them. It’s a good option if you want to avoid pesticides and repellents.
• Spray diatomaceous earth on the topsoil helps to keep the thrips population from growing, as it kills the larvae.
• Thrips are known for developing a quick resistance to pesticides so it’s difficult to find just one formula that works every time. Try combining them and repeating the treatment after a few days.
• If there’s a massive infestation in the vegetative stage of the plant, you can take your plant out and wash the insects off with a stream of water and then spray the plant with the solution.
• Clean and sanitize your environment for preventing thrips from appearing. Keep the growing space clean without dead leaves and water deposits.
• Consider a dehumidifier in your grow tent or room if you can’t lower the humidity levels below 75%.

What are aphids?

Aphids are very small insects but still visible to the naked eye. They may be yellow, brown, or black, the adults have wings and the young don’t. Aphids live on the back of leaves and stems and create large colonies, so check your plant frequently.

How to identify aphids

Aphids can appear in both indoor and outdoor crops and they multiply very rapidly (sexually and asexually).

Aphids suck the leaves’ tissue and then secrete a thick, sticky, sugary liquid that attracts ants. The aphid’s secretion may grow mold fungus on the leaves too. Aphids can also carry diseases, such as viruses, inject toxins into the plant and make leaves curl. Stressed plants are more vulnerable to aphids and take longer to recover.

Environmental conditions for aphids

Most aphid species multiply faster at temperatures from 20°C to 28°C/ 68°F to 82°F and relative humidity from 50 to 80%. 

How to get rid of aphids

• Treat aphid infestations with Neem oil, potassium soap, or homemade tobacco pesticide. When using any of these products, aim only for the leaves and do not apply them to the buds.
• Take your plant out, wash the insects off with a stream of water and then spray the plant with the solution if there’s a massive infestation in the vegetative stage of the plant.
• Prune severely damaged and infested leaves and buds and throw them away.
• Monitor your plants and soil for a few days after applying any product, because aphids may come back soon.
• Use aphid predators as biological control, some of them are parasitic wasps, rove beetles, ladybugs, and lacewing larvae.

What are fungus gnats?

Fungus gnats are small insects with wings. They are visible to the naked eye and they look like small flies. Fungus gnats are attracted to crops with high humidity levels and breed on the soil, filling it with white and very small larvae.

How to identify fungus gnats

Fungus gnats do not eat the plant, they eat decaying matter and fungi, but their larvae may eat some of the root hairs. Plants infected with fungus gnats show yellow leaves and a general wilting of the entire plant.

Environmental conditions for fungus gnats

Fungus gnats thrive in warm and wet conditions with high humidity levels, so do not overwater your plants. Leaving trays with runoff water for a long time in the tent attracts fungus gnats.

How to get rid of fungus gnats

• Treat your plant with Neem Oil to kill fungus gnats. It’s safe to use it in this stage of early flowering. Just be careful and avoid spraying it on the buds.
• Sprinkle Diatomaceous earth on the topsoil. It’s an organic insect killer made of fossilized shells. It’s safe to use at any stage and it’s especially effective for killing the larvae
• Avoid watering until the substrate is dry to a depth of at least 3 cm/1.18 inches. This will prevent the larvae from surviving. 
• Place a fan over the plant to help dry your growing medium and to prevent the insects from flying around your plant and laying more eggs.
• Consider a dehumidifier to balance the environment if humidity levels are extremely high.
• Try placing a yellow or blue sticky trap just above the topsoil. Fungus gnats love the color yellow and blue and they will fly towards the trap and get stuck.

Prevention and Pest management systems

Cannabis plants are intended for human consumption so consider safety in pest management to avoid health issues associated with pesticides. Read all labels carefully and follow the instructions. Always wear protective gloves and face masks to apply any of the mentioned products.

The best way to avoid pests and bugs in a grow tent is by prevention because once they appear, fighting them can be a difficult task. Creating an efficient pest management system is key before pests had already been established.

Sometimes you first spot signs of the presence of insects and bugs before seeing the actual insects, because they are too small or they hide from the naked eye. Check leaves on both sides and look for white spots, dark spots, fuzzy patches, and anything not “normal”. Pest control during flowering is key for a good harvest. Check for bugs and pests before changing the photoperiod to 12/12 as it’s easier to apply product during the vegetative stage.

When finding any sign of pests, act as soon as possible and get rid of them. After that, monitor your plants and environments for the following weeks.

There are many commercial chemical pesticides and products for almost every pest but they get quickly resistant. Also, some of these pesticides have negative environmental and health effects so we give some safer options for treating your plants.

Insecticidal soap has no residual effect against bugs after it has dried, this means it will only kill pests on contact. Apply potassium soap solutions repeatedly to control high populations of pests. On the other hand, neem oil acts as prevention because it has a residual effect, so there’s no need to spray the plant so often.

Reminder: Do not apply these products to the buds during the late flowering stage, always aim at the leaves.

If some of your plants are already infected, isolate them if possible, taking them away from your crop until you deal with the pest. As a means of prevention treat all the plants living in the grow room or tent.

10 tips to avoid pests in your grow room

Read the following article for 10 tips to prevent pests indoors.

Monitor Pest Management with the Grow with Jane App

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Tell us in the comments, which pests are the most common in your area and how do you manage them?

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