White grubs

healthy-looking plants suddenly wilt, yellow and die in scattered patches — not the whole field at once — and the patches often spread outward over a few days. Dead and dying plants pull out of the ground easily, because their roots have been chewed away.

dig around a wilting plant and you'll find the roots gnawed short and scarred, and in potato and groundnut the tubers and pods gouged with holes and shallow tunnels. The plant has almost no root left to hold it up or feed it.

in the soil around the roots you'll find a fat, soft grub curled into a "C", white or cream-coloured with a hard brown or orange head and three pairs of little legs near the front. A full-grown one is about as long as the top joint of your thumb (2–4 cm) and curls up when you lift it out.

with the cutworm (a grey-brown caterpillar that cuts young stems off at the soil surface at night and coils into a tight ring — not a fat white C with a brown head), with termites (you'll find their soil tunnels and the insects themselves, not a single fat grub), or with the thin, hard, shiny wireworm. If it is soft, fat, white and C-shaped with a brown head in the root zone, it is a white grub.

check twice — first when preparing the land before planting, and again from emergence right through the young growing stage (the seedling and early-development weeks, when your crop is most at risk). Look hardest after the first rains (when the beetles have just laid their eggs) and wherever you spot a wilting patch.

dig a few test holes about a spade deep (20–30 cm) and a big stride wide (roughly one square metre of soil each), spread out in a "W" across the field. Crumble the soil through your hands and count the C-shaped grubs you find.

walk the same "W" so you cover all four corners and the middle — about 10 stops. At each wilting or yellow plant, dig 20–30 cm around the roots and check for grubs, and gently tug a few seedlings: any that lift out with chewed, rootless bases confirm grubs are feeding.

treat now if you find about 1–2 grubs in a square-metre of dug soil, OR a grub under most of the wilting plants you dig up, OR the plot has a known history of grub damage. Even one grub under most seedlings in a spreading patch means act — waiting only lets more plants die.

where the soil is moist, apply entomopathogenic nematodes (tiny insect-killing roundworms — harmless to you, your crop and bees — that hunt the grubs down in the soil). Water them into moist soil in the cool of the day; they die in hot, dry, sunlit soil, so never put them out at midday or into dust.

where grub pressure is known to be high or scouting hit the threshold, use an insecticide seed treatment (or a soil insecticide placed in the planting furrow). Because grubs live underground, the poison must be on the seed or in the soil at planting — leaf sprays never reach them. Safety: treated seed is poison — never eat it or feed it to people or animals, keep it away from food, water and children, wear gloves and cover your skin, nose and mouth when handling and planting, wash well afterwards, and follow every instruction on the label. Don't lean on the same chemical every season: rotating to a different chemical group and pairing it with the ploughing and crop rotation above stops the grubs becoming resistant (surviving a poison that is used on them too often).

once seedlings have died in a patch, those plants will not recover — the roots are gone. If the season is still young, replant (gap-fill) the dead patches early and water and feed the survivors so they can outgrow light damage. Where a patch is a total loss, cut your losses there and protect the rest — then win the real fight next season: deep-plough, rotate to a non-host, and treat your seed at planting before the grubs ever build up.