Root Aphids

Root aphids feed on roots, out of sight, and everything they cause looks like something else. The plant goes pale, growth slows, leaves yellow from the bottom, and the obvious conclusion is that it needs feeding.

Feeding it does nothing, because the problem is that it cannot take up what is already there.

What you are seeing#

Above ground, a plant that behaves as though it is underfed and does not respond to being fed. Yellowing lower leaves, pale washed out new growth, stunted development, and drooping that does not track watering. Plants wilt in the afternoon and recover overnight, which looks like a watering problem.

In flower, buds stay small and the plant looks like it ran out of steam early.

Below ground is where the confirmation is. Tip the plant out of its pot, or dig gently at the edge of the medium, and look at the root ball against the inside wall of the container.

You are looking for small pear-shaped insects, roughly 1 to 2mm, pale white, grey or brown, moving slowly on the roots and on the inside of the pot. There may be a waxy white residue on the roots and medium that looks like a fine mold or dusting, which is the thing people most often misread.

Winged adults appear when a colony gets crowded, and those look very like fungus gnats to the naked eye.

How to tell it apart#

From fungus gnats: this is the most common confusion and it matters, because fungus gnats are mostly harmless and root aphids are not. Fungus gnat adults are dark, delicate and fly erratically when disturbed. Root aphid winged adults are stouter and tend to crawl on the medium rather than take off. Decisively: fungus gnat larvae are translucent worms with a black head, in the top inch of medium. Root aphids are pear shaped insects with legs, on the roots.

From nitrogen deficiency: nitrogen deficiency corrects within about a week of feeding. Root aphid symptoms do not improve at all, and the plant may look worse because you have added salt it cannot use.

From root rot: rot produces brown, slimy, foul smelling roots. Root aphid damage leaves roots looking chewed and knotted, with swellings, but not slimy, and the medium smells normal.

From pH lockout: correcting pH produces visible improvement within a week or two. If a careful pH correction has changed nothing, look in the pot.

Why it happens#

They come in with infested medium, on a clone, or from outdoors. Reused soil is a common source, and so is soil stored outside in an open bag.

Outdoor plants brought inside, or a tent in a garage with an open door, both offer an easy route.

How to fix it#

Root aphids are genuinely difficult, because the population is protected inside the medium where sprays do not reach, and eggs survive treatments that kill adults.

1. Isolate the plant immediately. They crawl between adjacent pots and spread along saucers.
2. Decide whether the plant is worth saving. A seedling or early vegetative plant is usually better replaced. A mature plant in flower is worth treating through to harvest, because you only need it to finish.
3. Drench the medium, repeatedly. Beneficial nematodes, specifically Steinernema feltiae, are the most effective option available to home growers and are safe at any stage including flower. Apply to moist medium, in the dark or with lights off, and repeat every 7 to 10 days for a month.
4. Insecticidal soap drenches work on contact for what they reach, which is not everything. Useful alongside nematodes, not instead of them.
5. Yellow sticky traps at medium level catch winged adults and tell you whether the population is still active, which is otherwise hard to judge.
6. Do not transplant an infested plant into fresh medium and assume that fixed it. Eggs and adults travel on the root ball.
7. Keep treating until harvest. There is no point at which you can declare it finished, because you cannot inspect thoroughly without disturbing roots.

Afterward#

This is one of the pests where the cleanup matters more than the treatment.

Dispose of all medium from affected plants, do not reuse it. Wash pots and saucers thoroughly or replace them. Wipe the floor and the tent interior, including the bottom corners where medium collects.

Then treat new medium as suspect: buy from a sealed bag, store it closed and indoors, and consider a preventive nematode application for any plant coming in from outside.