If You Can See the Webs, You're Already Late (Spider Mites)

Watch the episode · 4:13

If you can see the webs, you're already late. Webbing means a heavy infestation. Before that, spider mites are easy to miss: under half a millimetre long, and living on the underside of your leaves. But there is a simple way to catch them first.

Source · Kansas State University · Utah State University

The short answer ▶ 0:21

Twice a week, flip some leaves and look closely. Pale speckles on top and tiny moving dots underneath mean mites. Act then, before any webs.

How fast they build up ▶ 0:34

The reason for checking so often is speed. At 21 °C, a generation takes about two weeks. At 29 °C, about one. One female can lay up to 200 eggs in two weeks.

Even if each mite left just 10 daughters, which is our own illustration and not a measured rate, one mite becomes 1,000 in three generations. That is six weeks somewhere cool, but only three in a warm tent.

Source · Kansas State University

What to look for ▶ 0:55

First come tiny pale speckles on top of the leaf, where mites have sucked out the cells. It is easy to mistake for a nutrient problem.

Then flip the leaf. With a 10× magnifier you will see tiny dots that move, often with two dark spots. Or shake a stem over white paper and watch for dots walking away.

Source · Kansas State University · Utah State University

Where they start ▶ 1:17

Infestations start in patches, not evenly. Nobody has mapped this in cannabis, so the closest study comes from another crop, and it is a 2026 preprint, not yet peer reviewed. In sweet pepper screenhouses, the first patches appeared near the edges, then grew in area by about 4% a day in the early phase. So check your edge plants first.

Source · Gafni et al., preprint, not peer reviewed (2026)

Spider mites love warm, dry air: 20 to 30 °C and 30 to 50% humidity. And thirsty plants are more at risk.

Source · Kansas State University

It doesn't take many of them, either. Nobody has set a limit for cannabis, so again this is a different crop. In strawberries, each extra mite per leaf cost about 1.25% of the yield, and the point to act was just 3 to 4 mites a leaf.

Source · Abdelmaksoud et al., Exp. Appl. Acarol. (2026)

In hemp, heavy infestations slowed cannabinoids in one season, though not clearly in the next.

Source · Pulkoski & Burrack, Environ. Entomol. (2024)

Why not just spray? ▶ 2:07

Because it can backfire. In a 2025 watermelon trial, routine spraying with general insecticides raised spider mites by 45%. The authors' advice: protect the natural enemies that eat them.

Source · Mateos-Fierro et al., Pest Manag. Sci. (2025)

Mites also adapt fast. A 2026 survey in Korea found one population 31 times more resistant to a common mite spray than a lab strain. So if you do treat, check it is legal and safe for your stage, switch between types, and cover the leaf undersides, because that is where they live.

Source · Mostafiz et al., Arch. Insect Biochem. Physiol. (2026)

Sticky cards and other habits ▶ 2:40

A few more habits help:

• Don't rely on yellow sticky cards. They catch flying insects, and spider mites don't fly.
• Check your healthy plants first. Mites hitch rides on hands and clothes.
• Try plain water in veg. A spray can knock mites off the leaves. Keep it to the vegetative stage, because wet buds risk mould.
• Clean the tent between grows. Mites can wait in the cracks for your next crop.

Source · UConn Extension · Kansas State University

What about neem? ▶ 2:59

Plenty of growers rub neem around the stems as a preventative. Its active ingredient, azadirachtin, does put mites off feeding and laying eggs, and a 2025 review rates it moderate to high against spider mites.

Source · Marčić et al., Insects (2025)

But we found no study testing a stem rub. And in one study on roses, neem given only through the soil did nothing to spider mites, so don't count on it working from inside the plant. It is worth trying early on, but keep checking your leaves, and keep it off your flowers.

Source · Bartelsmeier, thesis (2021)

The verdict ▶ 3:28

Twice a week, flip leaves, starting at the edges. Don't let plants go thirsty. Check any new plant before it joins your grow. And act early, with no blanket sprays.

Webs mean a heavy infestation, and mites build up fast. Flip leaves twice a week, edge plants first, and you can catch them before the webs. For another grow habit put up against the evidence, read do mycorrhizal fungi help your plants?, or move on to how feminised seeds are made.