Banana weevil

plants that grow weak and stunted, throw small, poorly-filled bunches, and then snap off or topple over at the base — often just as the bunch gets heavy or a strong wind comes through. Slice open the underground stem (the corm) and you will find it riddled with dark tunnels about the width of a pencil, sometimes with a soft, rotten smell where the tunnels have let rot creep in.

down inside those tunnels lives a fat, legless, creamy-white grub (the larva) with a hard reddish-brown head, curled in a C-shape, up to about 1 cm long. This is the only stage that eats the corm — find it and you have found your culprit.

a hard-shelled, dull-black beetle about 1–1.5 cm long — roughly the length of a maize grain, only darker and harder (young ones are reddish-brown before they harden to black). Like all weevils it has a curved snout at the front. It comes out only at night, hides by day in damp leaf litter and crevices at the base of the mat (the clump of banana plants growing together), barely flies, and plays dead — folding its legs and lying stiff and still — the moment you disturb it.

toppling and yellowing can also come from nematodes (tiny root worms) or from Panama wilt disease. The test is to cut the corm open. Banana weevil leaves open tunnels with a soft white grub inside. Nematodes leave reddish-brown flecks and lesions but no tunnels and no grub. Panama wilt shows red-brown streaks running up the inside of the stem, again with no tunnels and no grub.

begin watching from the day you plant and right through the young and growing stages — the first months while a sucker is establishing, then as it builds up towards flowering. This is when a clean plant either stays clean or gets colonised. Keep checking through the whole life of the mat, because the weevil creeps up in numbers quietly over the years.

Cut a piece of pseudostem (the juicy trunk-like false stem) from a felled or harvested plant, split it lengthwise and lay the cut face down on the soil right against a mat — this is a split-pseudostem trap. Or, after cutting a plant, leave a low stump and set a thick disc of pseudostem cut-face-down on top of it — a disc-on-stump trap. The night-active weevils crawl underneath to shelter, and you catch them there in the morning.

so you cover all four corners and the middle, not just the edge by the path, and place a trap at about 10 mats spread along that W. Lift each trap once a week, count the weevils hiding underneath, and collect them.

Lead with that number — more than 5 weevils per trap per week means the population is high enough to be eating into your crop, so start control now. Also act the moment you cut a corm at planting or at harvest and find fresh, moist tunnels, even if the traps have been quiet.

The weevil rides into new fields hidden inside infested suckers, so never plant a sucker straight from an old, infested mat. Use tissue-culture plantlets, or pare your suckers — trim off all the roots and the outer skin of the corm with a knife until the flesh is clean and white, cutting out any tunnels or grubs you uncover. For extra safety, dip the pared suckers in hot water (about 50–55°C — hot but not boiling) for 15–20 minutes to kill any hidden eggs and grubs before you plant. This is your strongest single defence.

After you harvest a plant, dig out and chop up the old corm and pseudostem and spread the pieces to dry in the sun — never leave whole wet stumps and residue lying around the mat, because that is exactly where weevils breed and shelter. You may mulch the mat to hold moisture, but don't pile rotting debris against the stem bases. And feed and water your plants well: a vigorous plant shrugs off far more tunnelling than a starved one.

Turn your scouting traps into a weapon. Set many split-pseudostem and disc-on-stump traps across the field and collect and kill the weevils every week — drop them into soapy water or crush them. Steady trapping over the months pulls the adult numbers right down before they can lay the next generation of grubs.

Dust or spread Beauveria bassiana — an entomopathogen, meaning a natural fungus that infects insects and makes them sicken and die — onto your traps and the base of the mat, so sheltering weevils pick it up and carry the infection to others. Keep the dust off your face: cover your nose and mouth while you apply it. And protect the ants — predatory ants hunt weevil eggs and grubs for free, so don't wipe them out with broad poisons.

If damage stays severe despite clean planting and steady trapping, treat the base of the mat and the planting holes with a registered insecticide. Protect yourself every time: cover your skin, eyes, nose and mouth, apply in the cool of the day with the wind at your back, and keep it well away from food, water, children and bees. Wash yourself and your clothes afterwards, and respect the pre-harvest interval — the set number of days you must wait after treating before the fruit is safe to harvest. Rotate to a different chemical group each time so the weevils don't build up resistance — stop being killed by a product you keep using.

Because the grubs live sealed inside the corm where nothing can reach them, a badly tunnelled old mat cannot be rescued. When toppling and tunnelling keep rising despite trapping, uproot the whole infested mat, dig out and chop up every last piece of corm (that is the weevils' nursery), and let the ground rest or grow a different crop for a season. Then start a fresh planting somewhere else with clean tissue-culture or hot-water-treated material — replanting clean is the only true reset.