Spider mite damage. Photo: Ryan Douglas
Insect damage in many crops, including cannabis, is often detected before the insects themselves. This is because many bugs that feed on cannabis are small and mobile, but the marks they leave are not.
As you teach your cultivation team to scout for pests, emphasize the importance of recognizing insect “fingerprints.”
Here are the telltale signs of damage for five insects that commonly feed on cannabis:
Spider Mites
Spider mites are small, but they’re still visible to the naked eye. When populations are low, they tend to hide under plant leaves close to the veins, so detection can be difficult.
Once their populations explode, heavy webbing on the top of the plant canopy is a telltale sign of their presence. The goal of every scouting program should be to detect their presence long before this webbing occurs.
Since spider mites pierce the vascular system of the leaf and suck out plant sap, their feeding leaves behind tiny white specks on the topside of the leaf. These marks may look like dust particles, but if you blow on them, they don’t disappear. If you flip the leaf over, you’ll likely see small, mobile insects that may appear orange, dark red, or brown. These are spider mites.
Aphids
Larger than spider mites, aphids are easily detectable by the naked eye. However, they come in various colors and easily blend into the plant canopy.
Like spider mites, aphids have piercing-sucking mouthparts that help them directly access the nutrient-rich sap that flows throughout the plant’s vascular system.
Aphids are only after the proteins in this sap, so they pass the sugars through their body as waste. This sugary sap, also known as honeydew, is deposited on the leaf.
This sweet sap attracts ants that actively farm aphids in greenhouse and outdoor environments by protecting them from predatory insects that would otherwise eat the aphids and compromise the ant’s food source.
Another indicator of aphids is the presence of sooty mold, a brown or black fungal disease that grows on the honeydew deposits.
Ants, honeydew, and black or brown patches of leaf mold are sure signs that aphids have made themselves at home in your crop.
Hemp Russet Mites
Hemp russet mites require a microscope for proper identification, so it’s unlikely they’ll be noticed before damage occurs.
Affected plants will appear stunted, and mature flowers will be smaller and less resinous than healthy flowers. Their leaves may tend to roll up, and the flowers and branches may take on a rust-like appearance.
Russet mite damage can resemble another problem for cannabis growers, the dreaded hop latent viroid (HLVd). The only way to know for sure is with a microscope. Look for small, white worm-like insects. If no russet mites are detected, have a lab analyze your plant material for HLVd.
Thrips
Thrips are visible without a hand loupe or microscope, but they jump around when disturbed, and their color helps them blend into the plant canopy.
Like spider mites and aphids, thrips have piercing-sucking mouthparts, but their feeding gives more of a scraped appearance to the leaf than a pinprick. This usually starts on the leaves closest to the substrate since thrips pupate in the soil and begin climbing up the first plant they see. Naturally, the leaves closest to the substrate are the first to be attacked.
Given their small size, talent for hiding, and affinity for hopping, they can be tricky to identify. Fortunately, one of the best methods for flushing out thrips is also one of the easiest.
Hold a white piece of paper under a leaf that you suspect is showing thrip damage and give it a tap. If thrips are present, they will fall onto the paper and start to run around. Thrips are visible to the naked eye and are short, slender, and usually yellow, orange, or brown in appearance.
Fungus Gnats
Fungus gnats are small black flies that feed on plant roots. Larvae can tunnel into young stems compromising plant health, and adults can transmit fungal pathogens such as botrytis.
Just 1/8 to 1/16 of an inch in length, fungus gnats tend to congregate on the substrate surface. They blend in perfectly to the dark color of moist organic soils, peat, and coco coir mixes, allowing them to go undetected.
Perhaps the best clue that your crop has fungus gnats is the presence of their feces on the leaves, nursery pots, and grow benches. Quickly passed over by the unsuspecting, discerning growers will recognize these tiny black dots for what they are: evidence of fungus gnats.
These pests can be a problem for rockwool growers as well since fungus gnats are attracted to algae that grow on top of rockwool blocks.
Poorly designed irrigation systems or floor drains that hold standing water can encourage algae growth and provide ideal conditions for fungus gnat populations to explode.
Train your cultivation team to recognize early signs of insect damage to get a jump start on fixing the issue. Early, visible damage is sometimes the only way that growers know they have a potential pest problem.