Potato tuber moth

pale, blotchy blister-like mines wandering through the leaves — patches where the caterpillar has eaten the soft green out from between the two leaf skins, so the leaf looks papery and see-through in blotches. Young shoot tips and stems get bored out, then wilt and die back (farmers call these dead shoots "dead hearts").

tiny entry holes near the "eyes" of the tuber, each with a little heap of dark, crumbly droppings (frass — the caterpillar's droppings). Cut the tuber open and you find silk-lined tunnels packed with frass just under the skin, going soft and rotten. In the store you see the same holes and frass, and sometimes fine webbing between the tubers.

the caterpillar is small — only about 1 cm long, the size of your thumbnail — whitish-green to pinkish with a dark brown head. The adult is a narrow greyish-brown moth about 1 cm long, with a little fringe along the back edge of its wings; by day it hides in soil cracks, under leaves and in the store, and it flies at dusk and after dark.

true leaf miners (a different tiny fly grub) make thin, winding, snake-like trails in the leaf and never touch the tubers — the potato tuber moth makes blotchy mines AND kills shoot tips AND tunnels the tubers. Clean, narrow holes in a tuber with no silk and no frass are usually wireworms or a nick from your hoe, not this moth.

start once the tubers begin to form and swell (mid-season) and check hardest as the crop matures and the leaves start to die back (late season) — especially in hot, dry spells, when the soil cracks open, bares the tubers, and the moth moves in. Then keep checking right through harvest and into the store, because the store is where it does its worst.

or two — a small lure that gives off the female moth's scent to pull in the males — and count the catch every morning. Watch the trend, not one day: when the morning counts climb day after day, a flight is building and that is your signal to act. Don't wait for a big number.

so you cover all four corners and the middle, not just the edge by the path. Make about 5 stops, and at each stop check 10 plants: turn up the leaves for fresh mines, feel for wilted dead shoot tips, and scratch the soil at the base for cracks and any exposed, greening tubers — lift one or two near-surface tubers and check for entry holes and frass.

Because this pest only ever multiplies, you act on the first sign, not a big count: 1 mined leaf or 1 dead shoot tip on your walk, a rising trap catch, or — worst of all — 1 tunnelled tuber in the store. Even a single tunnelled tuber indoors means treat the whole store now, before it breeds through the pile.

plant clean, certified seed tubers so you don't carry the moth in from the start. Hill up deeply and re-hill whenever the soil cracks, so every tuber stays buried and out of the moth's reach. Where you can, irrigate to keep the soil from cracking open. Harvest promptly once the crop is mature and never leave lifted tubers lying out in the field — moths lay eggs on exposed tubers within hours. Where you can, rotate potatoes away from last season's plot.

keep tubers in a cool, insect-proof store with screened windows and doors. Cover the pile with a layer of dry sand or wood ash, or lay strong-smelling repellent leaves (dried lantana or eucalyptus) over it, so the moths cannot reach the tubers to lay. Clear away and destroy every cull pile, infested residue and volunteer (self-sown) potato — sun-dry or bury them well away from the crop and store, because each leftover tuber is a breeding site.

Bt (short for Bacillus thuringiensis, a natural soil bacterium that harms only caterpillars and is safe for you, bees and helpful insects) can be sprayed on the leaves to kill the young caterpillars in the mines, and used to dust or dip seed tubers before you store them. Keep the field's tiny parasitic wasps (natural enemies that kill the moth for free) working for you by avoiding broad poisons, and let the pheromone traps also mass-trap the males to cut down mating.

when mines and trap catches are rising you may spray a recommended insecticide on the growing crop, but never spray tubers meant for eating. Protect yourself: cover your skin, eyes, nose and mouth, spray in the cool of the day with the wind at your back, and keep the spray off food, water, children and bees. Respect the pre-harvest interval (the set number of days you must wait between spraying and lifting the crop, so no poison is left on it), wash yourself and the sprayer well afterwards, and rotate to a different chemical group each spray — this moth builds resistance (stops being killed by a chemical it meets too often) very fast.

get the crop out of danger — harvest as soon as the tubers are mature and move them straight into a clean, cool, protected store, rather than leaving a dying, cracked field as a moth factory. In the store, sort the tubers and pull out every tunnelled one at once, re-cover the rest with fresh sand, ash or Bt dust, and sell or eat them quickly before the moth breeds through the pile. Tubers that are heavily tunnelled and rotting cannot be saved — remove and destroy them so they don't seed the store and next season's crop.