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11 Common Pests That Can Destroy Your Cannabis Crop

By Chris Bond | Last updated: May 11, 2021
Presented by Crop King Seeds
Key Takeaways

Many of the same pest species that are the bane of greenhouse and field growers of traditional crops can affect cannabis crops just as easily. The first step in identifying the culprit in your grow room is to determine if you're dealing with sucking insects, or chewing insects.

Just like any other crop cultivated, from the time you plant the seeds, to the time you harvest the flower, cannabis is subject to a wide range of pests and diseases that can distress your plants, even kill them if left untreated.

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Before a grower can make an informed decision about how best to solve a problem, it is necessary to be able to identify the cause of the plant stressor by its symptoms.

Many of the same pest species that are the bane of many greenhouse and field growers of traditional crops can affect cannabis crops just as easily.

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When it comes to identifying pests, there are two main categories of insect pests: sucking insects that feed by pulling nutrients out of a plant, and chewing insects that have moving mouth parts and eat sections or even entire leaves or other plant parts.

Sucking Insects Cannabis Growers Need to Look Out For


Cannabis leaves covered in spider mites and webs.Spider mites can cause wide-scale damage to a marijuana crop.

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Spider mites

These guys are not actually true insects, but are arachnids. Control of mites by way of insecticides will not always work as their biology is different than that of insects.

Spider mites feed on the chlorophyll of leaves, leaving a speckled or mottled appearance on the leaves. They are very difficult to see without magnification, but like spiders, will spin webs and quite often other than the damage to the leaves, this is the best clue that what you have is a mite infestation.

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Tapping a leaf that is suspected to have mites on it onto a white sheet of paper and seeing if anything is scurrying about (that has eight legs) with a hand lens can help to make a positive identification.

Stalk of a plant covered in aphidsAt only an eighth of an inch long, aphids have ovoid bodies and two antennae-like tubes protruding from the rear

Aphids

Aphids are large enough to be seen without magnification. There are several different types of aphids and they may be yellow, green or black in appearance, but each is about the size of the tip of a pencil. As aphids start to feed on cannabis leaves, the leaves will start to pucker and appear distorted.

Before the population of aphids explodes, they are usually found on the undersides of leaves and therefore often go undetected until either their damage is noted or they are seen crawling along stems or even on the upper surface of leaves in search of food.

A fungus gnatA female fungus gnat can lay approximately 200 eggs per day.

Fungus Gnats

These flying insects are easy to spot, but it is not the adults that do so much harm; it is the larvae of fungus gnats that feed on the roots of your plants in the soil. Adult gnats can spread disease in the grow room, hence their name, but they do not feed on the plant.

Damage from fungus gnats on cannabis plants may appear as stunted growth, if at all. They do, however, stress the plant and make it more susceptible to other diseases.

Whiteflies and eggs on a leafThe white fly thrives in a greenhouse or grow room setting where it enjoys the warm weather and ambient humidity.

Whiteflies

Whiteflies quickly make their presence known. Brushing up against a marijuana plant inhabited with whitefly will produce a cluster of them flying about that looks like a puff of smoke. They will feed on the nutrients of the leaf and leave mottled spots much like the mite damage, but are easily seen in large numbers making their identification rather easy.

ThripThrips leave behind sticky silver or white spots on the plant’s surface.

Thrips

Thrips are much a much smaller flying insect than a whitefly and are harder to see. Their damage is usually seen as silver marks on the cannabis leaf, sometimes with “dots”. Leaves that have been attacked by thrips are also often brittle. The damage of thrips will often be seen before the insect can be seen or identified unless using monitoring traps.

Chewing Insects Cannabis Growers Need to Look Out For


Caterpillar eating a cannabis leafWhile some caterpillars prey on other insects or their eggs and larvae, the majority eat the leaves of plants.

Caterpillars

Caterpillars are the larvae of several different moth and butterfly species. Their sizes range from nearly invisible to several inches long and they may be camouflaged to blend in with the leaf or may be ostentatiously colored.

Either way, their damage is quickly noticed either as holes in the leaves or from leaf margins that have been chewed.

Snail climbing a cannabis leafSlugs and snails usually inflict their damage during the night hours.

Snails and Slugs

These are more problematic for outdoor cannabis crops but can be a real threat. They feed on the leaves of cannabis and often leave a slimy, silvery, mucous-like trail making their identification easy.

Leaf miner damage on a cannabis leafTelltale signs of a leaf miner infestation are white or yellow squiggly lines on your leaves.

Leaf Miners

Leaf miners are very tiny maggots that are rarely seen on marijuana plants until the damage is done. They feed on cannabis leaf tissue between the upper and lower portions of the leaf, essentially “mining” tunnels in the leaf. The leaf will appear as though squiggly lines have been drawn on it, but the damage is in fact inside the leaf.

Borer pestBorer insects burrow inside the stems and vines of a plant.

Borers

Borers are a generic term for any number of larvae or maggots that instead of chewing on leaves, chew into and burrow into the trunk or stems of a cannabis plant.

Root maggotsRoot maggots are especially a problem for soil growers with overly humid grow rooms.

Root Maggots

As the name implies, root maggots are fungus gnat larvae that feed on your plant’s roots. You may not notice this pest until you see evidence of its damage which will be a weakening of the plant and it may appear as nutrient deficiencies or too much water.

A little digging in the soil at the base of your cannabis plants could reveal the presence of these damaging pests.

Learning the basic appearance of common pest damage and deficiencies will help cannabis growers make informed and proper decisions when it comes time to take action or prevent the return of unwanted pests in the grow room.

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Presented By

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Written by Chris Bond | Certified Permaculture Designer, Nursery Technician, Nursery Professional

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Chris Bond’s research interests are with sustainable agriculture, biological pest control, and alternative growing methods. He is a certified permaculture designer and certified nursery technician in Ohio and a certified nursery professional in New York, where he got his start in growing.

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