Tiny bugs on my plants: what are they, and what do I do first?

GROWER ANSWER

Most people ask what to spray. Ask what it is first. Fungus gnats, spider mites and thrips each leave their own signs, and the bug you find often points at the condition that invited it. Look, catch, and name it, then pick the gentlest move that fixes what invited it.

Name the bug first.

Look before you do anything

• Turn the leaves over. Most of the trouble lives on the undersides. Check the lower leaves and the newest growth.

• Hang a yellow sticky card at the height of the pots. It catches flying adults, so you can see what you have and whether the count is going up.

• Flick a branch over white paper. Hold a sheet under a branch and tap it. Tiny specks that move are worth a photo.

• Look at the soil surface and the drain tray, especially right after you water.

Then bring us a leaf in a sealed bag, or send a close photo of both sides of the leaf. Naming it costs nothing, and it's the only step that tells a pest from a harmless visitor.

What you're probably looking at

Fungus gnats. Small dark flies that hang around the soil and the pots, and run across the surface when you water. The adults are mostly a nuisance on a grown plant. The larvae live in the top of wet mix and feed there, and on seedlings and fresh cuttings they can damage the young roots. Gnats almost always mean the mix is staying wet too long, and once in a while a wet bag of soil shows up already carrying them. Not overwatering is most of the prevention plan. Everything else is cleanup.

Spider mites. Too small to see well without a magnifier. Look for fine pale speckling on the tops of leaves and tiny moving dots on the undersides. A bad case leaves fine webbing on the leaves and tips. They like warm, dry air and spread fast, so they earn the closest look.

Thrips. Thin, fast insects that hide in the folds of leaves. They leave silvery or bronze streaks on the leaf surface, often with tiny black specks next to the damage. The specks are their droppings.

Something harmless. Small pale critters crawling in the top layer of a living soil or a mulched pot are often just part of the soil's cleanup crew. That's why the photo comes before the move.

For gnats, fix the wet

Gnats breed where the top of the mix stays wet. Lift the pot and water when it's noticeably lighter, then let the top of the mix dry out before you water again. A sticky card counts the adults, but it can't reach the larvae, so the drier top layer is what does the work. Empty standing water from trays, and keep a new bag of soil closed and dry until you use it.

When one spot becomes a move

Use our rule of thumb:

• One spot on one leaf is a watch. Take off the leaf that's hit, and look again in a few days.

• A second leaf, or any new growth, earns a move. Make it targeted and small, aimed at the bug you named, not a whole plant program.

• Scout on a schedule: under the leaves once a week in veg, and twice a week in flower, when the plant is denser and harder to clean up.

What not to do

Don't spray on a guess. A spray picked before the bug is named is a guess with a bottle attached. Some of what you see may be harmless, and a spray can wipe out the evidence you needed.

Don't start a spray schedule on a plant that has no pests. Look on a schedule instead.

Be extra careful once she's flowering. Dense flowers hide bugs and are almost impossible to clean on the plant. Look twice a week, take off what's hit, and talk to us before anything goes near the flowers.

When it's something else

• Gray fuzz or mush inside dense flowers: see Bud Rot Survival Guide

• Yellow leaves with no bugs in sight: see Yellow leaves: is it a deficiency, and is this too much?

• A wet pot that never seems to dry: see How often should I water, and why is she drooping?

Keep going

What's on our shelf, and what we reach for first: see our pest plan shelf guide.

Look first, fix the cause, then one small move. The whole system for a steady grow is written down in the free Stability Playbook. Get the free Stability Playbook

More answers: all grow guides. Every shelf guide and maker feed chart: Grower Resources.