Health Library

Spider mites are tiny arachnids — not insects — that feed from the undersides of leaves by piercing individual cells and sucking out the contents. Each puncture destroys one cell's worth of chlorophyll, so the damage shows up on the leaf tops as tiny pale speckles called stippling. They thrive in hot, dry conditions and reproduce explosively — a handful of mites can become thousands in under two weeks. Two-spotted spider mites (Tetranychus urticae) are by far the most common species on cannabis. They're usually introduced on clones, clothing, or pets, and they're so small you won't notice them until the stippling gives them away. Don't confuse stippling with thrips damage — mites leave tiny dots in a scattered pattern, while thrips leave silver streaks along the veins. By the time you see webbing between branches, you're dealing with a serious colony that needs immediate action.

Aphids are small, soft-bodied, pear-shaped insects that cluster on new growth, stems, and leaf undersides — piercing plant tissue to feed on sap. You can identify them by their cornicles, twin tailpipe-like tubes sticking out from the back of the abdomen. They come in various colors (green, yellow, white, brown, pink) and reproduce incredibly fast — females give live birth to already-pregnant offspring, so a small cluster can become a full infestation within days. Unlike whiteflies, most aphids don't fly — they're wingless until the colony gets overcrowded, at which point winged forms appear to colonize new plants. Aphids excrete sticky honeydew that attracts ants and promotes sooty mold growth, so if you see ants marching up your stems, check for aphids immediately.

Thrips are tiny, slender insects that rasp leaf surfaces and suck out cell contents, creating distinctive silvery or bronze streaks and patches. The adults are fast-moving and hard to see without magnification, but the pale, worm-like nymphs are slow and easy to miss — they blend right into the leaf surface. Thrips can fly short distances and spread rapidly through grow rooms. They leave behind tiny black dots of frass (feces) alongside their feeding damage. Here's the critical differentiator: thrips create silver STREAKS and TRAILS — don't confuse this with spider mite stippling, which looks like scattered DOTS. The black frass dots alongside silver streaks are the giveaway. What makes thrips especially frustrating is their life cycle — eggs are laid INSIDE leaf tissue where sprays can't reach them, and pupae develop in the soil rather than on the plant. This means you're fighting them on two fronts at once. Silver scars from feeding damage are permanent, so the goal is always to prevent new damage rather than trying to fix what's already done. Some species also transmit plant viruses, making early intervention critical.

Fungus gnats are tiny dark flies that hover around your soil — annoying, yes, but the adults themselves aren't the real problem. It's their larvae you need to worry about. These translucent white maggots live in the top few inches of wet soil and feed directly on roots and root hairs, creating wounds that let dangerous pathogens like Pythium and Fusarium invade. The larvae are the ones that transmit Pythium — they ingest fungal oospores in the soil and spread them through their excretions. Adults do carry other pathogens (Fusarium, Botrytis, Verticillium) on their bodies as they fly between plants, but they don't vector Pythium themselves. Overwatering is the root cause — wet soil grows the fungi and decaying organic matter that attract egg-laying females in the first place. Fix the moisture problem and you cut off the cycle. Don't confuse these with whiteflies — gnats are dark and hover near the soil, while whiteflies are white and cluster on leaf undersides.

Whiteflies are tiny, white, moth-like insects that cluster on leaf undersides — and the easiest way to confirm them is the shake test: gently shake the plant, and if a white cloud flies up, you've got whiteflies. They feed by piercing leaf tissue and sucking out sap, excreting sticky honeydew that quickly attracts black sooty mold. What makes whiteflies especially hard to eradicate is their life cycle — the adults you see flying around are only part of the problem. The immobile nymph stages are flat, translucent, and scale-like, glued to the undersides of leaves where they're easy to miss and resistant to most contact sprays. You'll often kill the adults only to have a fresh wave emerge from nymphs you didn't notice. Don't confuse them with aphids — whiteflies ALL fly when disturbed and are uniformly white, while aphids are mostly wingless and come in various colors. Whiteflies can also transmit plant viruses (like TYLCV and ToCV) that are incurable once established, so early detection matters. They thrive in warm, still conditions and are a common problem in greenhouses and indoor grows with poor air circulation.

Hemp russet mites (Aculops cannabicola) are among the most devastating cannabis pests you'll encounter — and you won't even see them coming. These microscopic mites are invisible to the naked eye, requiring 60x+ magnification to identify. They colonize new growth from the bottom of the plant upward, causing yellowing, bronzing, and curled leaf edges that are easily mistaken for heat stress, nutrient problems, or even mosaic virus. By the time symptoms become obvious, populations are already massive. One telltale clue is a fine golden-brown dust on leaf surfaces — these are cast skins shed as the mites molt, and it's one of the most reliable visual identifiers before you break out a loupe. Russet mites can destroy entire crops if you don't catch them early, so regular inspection with magnification is essential.

Broad mites (Polyphagotarsonemus latus) are microscopic pests that inject toxic saliva while feeding — and the damage they cause looks nothing like a typical bug problem. New growth comes out twisted, blistered, and weirdly glossy, almost like the leaves are wet or coated in plastic. Because they're invisible without strong magnification, broad mites are one of the most commonly misdiagnosed issues in cannabis — growers often blame pH swings, over-fertilization, or even genetics before realizing it's a mite. An early clue is tiny white dandruff-like specks on leaf surfaces — these are eggs covered in characteristic white tubercles, along with cast skins from molting mites. They strongly prefer new growth and can hitch a ride on your clothing, tools, and hands.

Leaf miners are larvae — usually from small flies or moths — that burrow between the upper and lower surfaces of your leaves, carving out distinctive squiggly white trails or blotch-shaped mines as they feed on internal tissue. Adults lay tiny eggs on leaf surfaces, and once larvae hatch they tunnel inward where sprays can't reach them. The damage itself is mostly cosmetic unless infestations are severe — trails are permanent but don't spread from existing mines. The real risk is secondary infection — entry and exit wounds let in fungal or bacterial pathogens, so keeping things clean matters more than the tunnels themselves.

Mealybugs are soft-bodied scale insects coated in a white waxy armor that makes them look like tiny cotton tufts. They cluster at stem joints, leaf axils, and under leaves — anywhere they can quietly tap into sap flow. Like aphids, they excrete honeydew, which attracts ants and invites sooty mold. Their waxy coating repels many sprays on contact, so repeat treatments are essential. If you touch what looks like mold and it moves, you've got mealybugs — that's the quickest way to tell them apart from powdery mildew or other white residues.