Root aphids

What you are looking at

A plant that flags in the afternoon sun and stands back up overnight. You water it, and for a few days that seems to work. Then one afternoon it flags and does not come back.

The leaves have nothing on them. No aphids, no mites, no spots, no webbing. The compost is damp. Growth has stalled and the colour has gone flat, and by the time the plant collapses there is still nothing above ground to explain it.

Knock it out of the pot, or lift it out of the row with a fork, and look at the roots. Root aphids are grey, cream or yellowish, rounded, mostly wingless, and they cluster on the roots and round the crown. Around them is a white or faintly blue waxy powder, dusted over the roots and often stuck to the inside of the pot.

Why nothing you sprayed made any difference

Aphids on a shoot are a problem people solve. The same insect on a root sits under 20 cm of soil, inside a wax coating that repels water, in a place no spray reaches and no routine inspection covers.

So the pattern is a plant declining for weeks with a diagnosis of drought, overwatering, feeding, light or compost, and a run of treatments that change nothing, because the search never went below the surface.

Lift the plant. Any plant wilting with clean leaves and damp compost gets lifted, and that one habit turns a mystery into a two-minute identification.

The poplar half of the story

Lettuce root aphid has a life cycle that explains why it turns up in the same gardens every year.

The winter is spent in galls on the leaf stalks of poplars, where the aphids hatch in spring and breed inside a swelling on the stalk. In early summer winged forms leave the tree and fly to lettuce, endive, chicory and sow thistle, drop to the soil and start colonies on the roots. Numbers build through July and August, which is when hearts fail to fill and plants collapse in hot weather. In autumn winged forms fly back to the poplars and lay the eggs that overwinter.

Where there are no poplars, or under glass and in a warm climate, colonies skip the tree entirely and carry on underground from one crop to the next. That is also how the pest travels in the compost of container-grown plants, and it is why the rice root aphid has ended up on every continent.

A gardener can predict two things from that cycle. A bed near poplars gets infested in early summer whatever you do, so plant a resistant cultivar. A bed that grew infested lettuce last year starts the season carrying the population itself, so rotate.

What separates it from the things it looks like

Vine weevil grubs leave a plant with hardly any root system at all, and the grubs are fat, white, C-shaped and legless with a tan head. Root aphids leave the roots there and covered in wax.

Root mealybugs live in the same place and are the closest call. A mealybug is oval, slow, and wearing a thick mealy coat over its own body. A root aphid is rounder, moves when you disturb it, and the wax is loose powder in the surrounding compost rather than a jacket.

Root rot browns and softens the roots, and they smell and strip when you pull them. Under an aphid colony the roots stay firm and pale.

Ants running in and out of the soil are worth following up rather than treating. They are usually farming the colony for honeydew, and they will carry it to the next plant along.

What to do about it

Lift any wilting plant with clean leaves and damp compost, and look at the roots.

Wash the root ball under a tap, cut off the worst of the damage and repot into fresh compost, scrubbing the old pot out.

Grow the resistant lettuce cultivars wherever you have had it before, and check the catalogue description before buying.

Rotate lettuce, endive and chicory on a four-year cycle and pull the stumps out at harvest.

Water and feed evenly through hot weather so the plant carries the population instead of collapsing under it.

Sow early enough to cut the crop before numbers peak in August.

Expect it every year where poplars stand near the bed, and plan for it rather than fighting it.

Drench soft soap through the compost of an infested ornamental, twice, a week apart.

Water Steinernema feltiae into pots with warm damp compost.

Move the ants rather than poisoning them.

What it gets mistaken for

Drought wilts a plant whose compost is dry and revives it when you water.

Vine weevil eats the roots away and leaves a plant that lifts out of the pot with nothing holding it.

Root rot turns the roots brown, soft and smelling.

Root mealybugs wear a thick white coat rather than sitting in loose powder.

Verticillium wilt takes one side of the plant at a time and browns the inside of the stem.

Potato cyst and other nematodes stunt without leaving anything visible on the root.