Yam beetle

clean, round holes about the width of a finger bored straight into the tuber, often with tunnels running deep into the flesh. Fresh holes are pale and moist inside; older ones turn brown and start to rot. A holed tuber can look fine from the top while it is eaten out below — you usually only find the damage when you dig or lift the tuber.

a big, hard, shiny brown-black beetle about as long as your thumb-tip (2.5–3 cm), with strong, toothed front legs made for digging. It hides in the soil around the plant by day and feeds at night, so dig a hand's depth in the loose soil at the base of the plant to turn it up. Remember it is the adult beetle that bores the tubers — not a soft white grub.

termites also hollow out yam tubers, but they pack their tunnels with soil and mud and coat the tuber with earthen sheeting — the beetle's holes are clean and open. Fat, curled, white C-shaped grubs in the soil are chafer/white-grub larvae, a different pest. Rats and grasscutters leave rough tooth-marks gnawed from the outside, while the yam beetle bores neat round holes inward.

start once the tubers begin to form and bulk (from roughly a couple of months after planting, through mid-season) and keep checking right up to harvest — the longer mature tubers sit in warm, moist soil, the more the beetles bore. Look hardest just after the first heavy rains, when the beetles migrate in from swampy ground (migration = the beetles flying in from their breeding grounds to feed).

cross the field in a big “W” shape so you check all four corners and the middle, not just the edge by the path. Make about 10 stops along the way.

with your hand or a hoe, gently scrape the soil off the top (shoulder) of the tuber and look for fresh bore-holes, then dig a hand's depth in the loose soil around the plant to turn up the beetles themselves. Slice open (cut-test) one or two suspect tubers to see how deep the tunnels run — a cut-test just means cutting a sample tuber open to inspect it inside.

because a single beetle can ruin a whole tuber, the threshold is low — if even 1 in 10 tubers you check has a fresh hole, act now. And if your field (or a neighbour's) had beetle damage last season, or you sit next to swampy ground, act at planting without waiting to see holes — by the time the holes show, the tuber is already spoiled.

every time you turn the soil — at land preparation and again at harvest — pick out the big beetles by hand and kill them (crush them, or drop them in a tin of water and ash); don't just toss them to the field edge. Gather and destroy holed and leftover tubers and any self-sown yams after harvest, so the beetles have nothing left to feed on.

only where beetle pressure is high (a history of damage, or a field beside a swamp) should you reach for chemicals. Dip the setts in a soil-insecticide solution before planting, or work a soil insecticide into the planting hole or mound. (Setts = the cut pieces of seed-yam you plant.) This is a soil treatment, not a leaf spray, because the beetle lives and feeds in the soil.

a soil insecticide is a poison. Cover your skin, eyes, nose and mouth, wear gloves to dip setts, and work in the cool of the day with the wind behind you. Keep it well away from food, drinking water, children, animals and bees — never dip setts in a cooking bowl. Wash yourself, your clothes and your containers afterwards, away from the well or stream. And respect the pre-harvest interval — the number of days the product label says you must wait between treating and harvesting or eating the crop.

using the same insecticide season after season lets the beetles build resistance (resistance = the poison slowly stops killing them). Rotate to a different chemical group between seasons, and keep up the early planting, careful field choice and handpicking so the chemical is only ever a back-up, not your main plan.

treat that field as high-pressure — next season plant early, move your yam away from the swampy ground, dip the setts, and rest the land out of yam for a season or two to starve the beetle population down before you plant yam there again.

if the tubers are already badly holed, harvest now rather than leaving them to be bored further. Sort out the holed tubers and use or sell the lightly-damaged ones quickly — they rot fast in store — and never keep a holed tuber as seed-yam, or you carry the beetle and the rot straight into next year's crop.