Pest Infestations: Know Thy Enemy

By Rich Hamilton
Published: December 18, 2018 | Last updated: May 11, 2021 05:51:22
Key Takeaways

The warm humidity of a growroom is so attractive to pests you will no doubt encounter at least one kind of infestation at some point while growing cannabis. The following are some tips to help you prevent pests, along with solutions to try if you do experience an attack.

To protect valuable crops, most growers are almost obsessive when it comes to keeping pests out of their growroom. While this healthy obsession certainly reduces the likelihood of an infestation, sometimes uncontrollable variables conspire against you and suddenly your precious plants fall victim to a legion of nasty sapsuckers. But don’t get too bummed about it. Sometimes a growroom infestation is like a bout of head lice traveling through a kindergarten class; it just happens due to no fault of your own.


Should an infestation occur (in the growroom, not the classroom), it is much easier to eliminate in the early stages. This can be accomplished by checking leaves at random (top and bottom) every day for patchy discoloration, damage, insects, larvae, eggs, or feces.

How Pests Invade a Cannabis Growroom

Pests are most commonly brought into your growroom by transfer, be it on your clothing or garden tools. Keep a separate set of clothes to wear in your growroom, wear disposable gloves, and thoroughly clean and disinfect any new equipment no matter how clean it looks before it enters your grow environment. This falls under that obsessive category.


Use a bug barrier to cover the intake and outtake ends of your ducting to stop unwanted pests from accessing your growroom via these channels.

For an added layer of defense, try feeding your plants silicon. Silicon helps form a stronger cell structure within the plant, making it harder for pests to feed on, while helping with nutrient uptake.

Neem oil also creates a protective layer over the plant stopping the younger, weaker pests from feeding, thus starving them. Oil also coats the pest, slowly softening their shells, slowing them down, and ultimately killing them. Oils are best used at certain times during plant growth, so always use as instructed or you could do more harm than good to your crop.


For pests that attack plant roots, or thrive in the medium, you can try a top dressing of stones sprinkled over your medium, restricting access for pests while allowing you to keep regular watering schedules.

For flying pests, sticky pads are effective. Placed close to the plant, they catch any insects in flight and provide a good indication of the scale of the problem. If you happen to spot a couple of pests on the pad, it is definitely a good time for that thorough check of the leaves.


In terms of insecticide, I always recommend the use of non-sermetic insecticide, which is sprayed onto the plant, killing the pest on contact but not remaining within the plant's system.

Know Thy Enemy: Common Cannabis Pests

So, who are you are trying to protect your plants from? The usual suspects include spider mites, aphids, thrips, fungus gnats, and whiteflies.

Spider Mites

Spider mites create tiny brown spots on leaves. Less than one millimeter in size, they are red or white in color and rely on transfer to travel. They breed rapidly, laying whitish eggs on the underside of leaves. The lifecycle of the spider mite is two to four weeks, laying up to 20 eggs a day. This rapid reproduction allows the mites to adapt quickly to chemical pesticide measures, making them difficult to eradicate completely. They do prefer a drier climate, so higher humidity in the growroom will help slow them down. Overall, prevention is the best cure for spider mites, along with non-sermetic insecticide.

(If prevention doesn't work, check out How to Treat Spider Mites.)


There are more than 4,400 different species of aphid and they can be found all over the plant. They travel by transfer and are between one to 10 millimeters in size. They have soft bodies and can be black, green, brown, and sometimes a pink translucent tone.

Aphids feed by biting the leaf and sucking out the sap, leaving holes that turn brown while leaving a silvery track. Aphids feed from the roots, in the same way, going unnoticed until it’s too late. These little suckers can spread disease from plant to plant which can damage and kill your crop. They live for 20-40 days while reproducing rapidly, making them very resilient to pesticide preventative measures.

Prevention is, again, the best cure, so practice the precautions mentioned above. Since aphids can attack the roots as well as leaves, try a top dressing on your medium to prevent the insects from gaining access to the roots.


Thrips are very small, slender insects. Some species can fly, while those that can't are good jumpers and are passed on via transfer. Normally one millimeter long or less, they grow to swarm numbers quickly if not remedied. They range from dark brown to translucent yellow in color and can pass disease from plant to plant which can potentially wipe out a crop.

Thrips attack the leaf from the topside, scraping at the leaf (and or stem) while releasing a digestive enzyme that breaks leaf cells down so it can feed on the sap. They leave a silvery/bronze trail that makes the leaf appear scabbed. You may see black specks on the leaves and fruits of your plant, which is the insect’s excrement.

Adult thrips lay 50-300 eggs in their lifetime. They lay them in natural leaf crevices, under the surface of the leaf itself, and on the growing medium under the plant. As with all pests, there is no 100 percent foolproof cure. Prevention is key, so use all methods available to you. If exposed to an attack, however, use sticky pads, preferably in blue or yellow (they are attracted to these colors) around the plant which then catches them as they fly or jump around.

Fungus Gnats

Fungus gnats are small, dark flies with an average 28-day lifespan, in which they lay hundreds of eggs. Three to four millimeters in length, they are good fliers and attack the plant’s root system. They have four different stages—egg, larvae, pupa, and adult. The adult fungus gnat lays eggs in the medium, the larvae hatch and then travel down and eat the roots, thus destroying the plant’s vascular system. The larvae then pupate under the surface and emerge as adults. Fungus gnats can pass on many diseases which, in the worst case, can quickly kill your plants in a matter of days.

Prevention is the best cure, so implement the controls you are now familiar with because once below the medium, fungus gnats are a nightmare to get rid of. If you do end up with an infestation, however, try using a dressing of stones on top of your medium which will have an immediate effect by preventing the eggs from making it into the medium. Use sticky pads to catch flying specimens, placing them around the plant as well as upside down above the medium to catch any emerging adult gnats.

White Flies

White Flies are small, moth-like relatives of the aphid. They are white, one millimeter in size, and have a staged life cycle lasting between one week to almost two months. They are found all over the plant but mostly on the underside of the leaves where they lay their eggs. They are known for spreading diseases to the plants they infest and feed by biting the leaf and consuming the sap, leaving the leaf spotty, droopy, and lifeless.

Whiteflies are airborne pests attracted to yellow so if you do suffer an infestation, use yellow sticky pads around the plant. If you gently shake your plant, the whitefly will be disturbed and briefly fly off the plant, then get caught on the pad’s tacky surface. This only kills adult flies, however. To prevent the flies in earlier stages from maturing, spray the underside of the leaf with neem oil.

Dealing with the threat or the reality of a pest infestation is an inconvenience at best, but it is something all growers will experience at some point. Knowing what to do before it becomes an infestation will help mitigate damage, so hopefully, now you will be a bit more aware and prepared for the inevitable arrival of the usual suspects.

(Looking for an organic solution, check out Controlling Pests Organically: Aphids, Whiteflies, and Thrips)


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Written by Rich Hamilton | Writer, Consultant, Author of The Growers Guide

Profile Picture of Rich Hamilton

Rich Hamilton has been in the hydroponics industry for more than 20 years, working originally as a general manager in a hydroponics retail outlet before becoming an account manager at Century Growsystems. He enjoys working on a daily basis with shop owners, manufacturers, distributors, and end users to develop premium products.

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