African armyworm

whole patches of young cereal and grass stripped astonishingly fast — leaves eaten to ragged shreds or right down to the midrib, and bare ground where a green field stood a day or two before.

young caterpillars scrape one side of the leaf, leaving pale, papery "window" patches you can see through, before they grow big enough to eat right through the leaf.

masses of caterpillars, often in their hundreds or thousands, crawling together in a moving "front" across the ground and up the plants. When they eat one patch bare they march on to the next — seeing them march is the surest sign it is armyworm.

about matchstick length (3–4 cm), and in an outbreak velvety black with thin pale-yellow stripes down the back and along each side and a shiny black head; the belly is greenish and paler. (When few and scattered they can be dull green and easy to miss — the black colour comes with the crowd.)

in hot, bright sun they shelter at the base of the plants, under leaf litter, soil clods and in cracks in the ground — lift and look there. They come out to feed in the cool: at night, at dawn, at dusk and in dull, cloudy weather.

fall armyworm sits one or two deep in the whorl and never marches; the maize stalk borer makes neat rows of shot-holes and tunnels the stem; cutworms hide in the soil and cut single seedlings at night. African armyworm is the one that comes in swarming masses out on the leaves and marches across the field.

from the first rains after a dry spell — especially when a flush of young green grass appears — and right through your crop's seedling and early-growth weeks, its most vulnerable stage. That is when the migrating moths pour in and lay their eggs.

take notice of regional armyworm forecasts, pheromone-trap catches (traps that lure and catch the moths) and word from up-wind neighbours. An outbreak sweeps a district as a wave, so a report from up-wind is your cue to start checking every day.

cross the field in a "W" so you cover all four corners, the middle, and — above all — the edges next to grassland or pasture where the army marches in from. Make about 10 stops.

the young leaves and whorls for scraping and small caterpillars; the grass and weeds in and around the crop; and the soil at the plant base (lift litter and clods) where they hide by day. Scout at dawn or dusk, when they are up feeding.

with armyworm there is no safe "wait and see". Treat immediately if even a few of the plants you check each carry several small caterpillars, if you find a dense patch feeding together, or if you hear of caterpillars marching nearby — presence in numbers is the threshold.

while the caterpillars are still small, spray a caterpillar-specific biological where you can get it — NPV (nucleopolyhedrovirus, a natural virus of armyworm) or Bt (Bacillus thuringiensis, a natural soil bacterium that kills only caterpillars). It is safe for you, bees, birds and the wasps and beetles that also eat armyworm, so avoid blanket poisons that kill those helpers. It only works on young larvae — spray as soon as you find them. For a small patch you can also knock caterpillars into a bucket of soapy water or sweep them into the trap furrow.

if the outbreak is confirmed and numbers are high, a pyrethroid — a fast-acting contact insecticide (it kills on touch, so it need not be taken up inside the plant) — works well because armyworm feeds out in the open, not hidden in the whorl. Target the young larvae and the marching front or infested patches, not a blanket spray. Safety: cover your skin, eyes, nose and mouth; spray in the cool of morning or evening with the wind behind you; keep it off food, drinking water, children, livestock and bees; respect the pre-harvest interval (the set number of days you must wait between spraying and eating or selling the crop); and wash yourself and your clothes afterwards. Resistance: rotate to a different chemical group each time so the armyworm does not stop responding to the spray.

if caterpillars keep marching in despite spraying, keep the trap furrow charged and push neighbours and the extension service to treat together — a migratory army is beaten area-wide, not field by field. If a seedling crop is stripped, don't despair: the army is only passing through and will soon move on, so once it has gone you can often replant and still make a season. If the crop is already mature, harvest what you can rather than spend more on spray.