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Part 2 - I Spy a Pest Bug - Pest Identification

This is part two of our three part series on pest identification.

Unfortunately there are many different types of pest bugs that can inflitrate a cannabis grow. Making an accurate identification can mean the difference between being able to control the pest and facing total crop loss. Let's look at the most common cannabis pests and how to best identify them.

Two-spotted Spider mites

Spider mites can be seen with the naked eye, however, they are quite small (less than 0.5mm in length). To be sure, view them with a magnifying glass, hand lens, or even the zoom lens on your cell phone if nothing else is available. Once these tiny bugs are under magnification their signature two black spots on their back should be easily identified. 

Two-spotted spider mites under magnification

The presence of stippling on the plants leaves and webbing (usually near the top of the plant) are signs talked about in the first part of this series that you may have a spider mite infestation. Spider mites have a natural tendency to move upwards, in an infestation they will gather at the top of the plant or other items such as support stakes that are nearby. 

Webbing on cannabis from the two-spotted spider mite

Western Flower Thrips

Thrips are harder to spot, measuring only 1mm in length. They are fast and skilled at hiding. This insect can be spotted with the naked eye, however, some form of magnification will be needed for a positive identification. Coloring can range from yellow to brown and their bodies are long and slender. Eggs cannot be observed as they are inserted under the plant tissue. 

 Western Flower Thrip

Thrips cause scratching damage by jamming their mandible down into the plant tissue like a hammer and then vacuuming out the cell contents. The feeding and egg deposit damage mostly harms the appearance of the final product however, in heavy infestations it can curl and twist leaves ultimately impacting yields. 

Stippling Damage on Cannabis from Thrips

Fungus Gnats

Chances are by the time fungus gnats make their presence known in a grow space, their population is already sizeable. This is because fungus gnats are soil dwellers. Eggs are laid in the soil and the hatched larvae's favorite meal is the tiny root hairs of cannabis plants. As there is not much opportunity to inspect the plant roots, fungus gnats are often not discovered until the relatively harmless adults start appearing in annoyingly large numbers. The 3mm size adults are easily seen by the naked eye and a gentle shake of a plant will reveal their whereabouts. 

 Fungus Gnat adult on leaf

The larvae are the real trouble makers in the fungus gnat life cycle. Their inclination to eat root hairs mostly causes subtle damage in a cannabis plant. The effects of damaged roots will reveal itself in the upper plant parts as nutrient deficiency and/or under watering. These signals of plant health can sometimes be misdiagnosed as issues with the feeding program or watering frequency. 

Fungus Gnat Larvae in soil


The most challenging attribute of this pest is its capacity to adapt. Aphids have the ability to reproduce asexually and when food supply becomes low, winged adults will appear and colonize a new area. Each reproductive aphid can produce up to 300 eggs each over a few week period allow this pest to grow its population at a rapid rate. Aphids can be seen with the naked eye.


Aphids like to gather on the undersides of leaves as well as plant stalks. They uses their long piercing mouth parts to stab the leaves and then vacuum out the contained sap. Generally with a large established plant aphids cannot inflict enough damage to impact outcomes, but smaller indoor plants are much more vulnerable. If the infestation is large they can kill small plants by drinking them dry. They can also spread plant diseases and the sweet, sticky honeydew they leave behind attracts both ants and black mold.

Aphids on Cannabis Leaf

When implementing an integrated pest management program remember that prevention is the best approach. Remain vigilant when scouting and monitoring!

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