Shoot borer

A soft, young shoot tip at the top of the plant suddenly droops, wilts and dries out. Unlike a thirsty plant, it does NOT perk up again in the cool of the evening or after watering — that dead, hanging tip is called a dead-heart.

On the tender shoot you will find a tiny entry hole, often plugged with or oozing frass — the moist, sawdust-like droppings the caterpillar pushes out as it eats. Fresh, wet frass means the pest is still inside and feeding right now.

Slit a wilted shoot lengthways with a knife or your thumbnail: inside is a hollow, tunnelled stem and a small caterpillar — pale pink, cream or brownish with a dark head, up to the length of your little fingernail (1–2 cm). On sweet potato you may instead find a fat, legless white grub (the larva of a small weevil) inside the vine or root.

Once the plant is flowering and fruiting, the same pest bores into flower buds and young fruit, leaving frass-packed holes; the buds drop, and the fruit rots, twists or falls early.

With wilt disease the whole branch or plant wilts and stays down, but there is no bore hole and no frass, and a cut stem shows browned inner 'water-pipes' rather than a hollow tunnel with a grub. With drought or midday heat the plant flags in the sun but recovers by evening or after watering — a borer dead-heart never comes back to life.

start as soon as the plant is in active growth and pushing out tender new shoots (the development stage), and keep it up right through flowering and fruiting (mid-season) — these are the two stages the borer attacks. Scout twice a week, and step it up after spells of warm, humid weather (about 22–33°C), which is when the pest breeds fastest.

cross the field in a big 'W' shape so you cover all four corners and the middle, not just the row nearest the path. Make about 5 stops along the way.

at each stop, look over 10 plants — about 50 plants in all. On each one, run your eye over the tender growing tips for any drooping or wilting, and look just below a wilted tip for a bore hole with fresh, moist frass.

split open two or three wilted shoots — a hollow tunnel with a caterpillar or grub inside proves it is borer, not disease. Once the crop is flowering, also check a few flower buds and young fruit for frass-packed entry holes.

— or the moment you find any fresh, frass-oozing bore hole on a tender shoot. Don't wait: start control the same day, while the caterpillars are still small and near the shoot tips.

As soon as you spot a wilted, bored shoot, clip it off (cut a hand's width down into healthy stem) and burn or bury it away from the field. The caterpillar is still inside, so you kill it before it can move on to the next shoot or into the fruit. Do this every few days from the very first sign — staying on top of it is the most effective thing you can do, and it costs you nothing but your time.

Plant clean, pest-free planting material — on sweet potato, take vine cuttings only from healthy plants, never from an infested field. Rotate your plot so you don't grow the same host crop in the same spot season after season, and after harvest pull out and destroy every old plant, vine and left-over shoot so the borer has nothing to carry over on. Where you can get them, plant tolerant varieties.

Tiny parasitic wasps — 'parasitoids', insects whose young grow inside the borer's own eggs and caterpillars and kill them — plus predators and ants do a lot of this work for free. Don't blanket-spray broad poisons that wipe them out. Where you can get them, hang pheromone traps (lures that give off the scent of the female moth to catch the males) to track the pest and pull down the numbers of egg-laying adults.

Because the caterpillar hides inside the shoot, an ordinary surface (contact) spray cannot reach it. Use a systemic insecticide — one the plant draws up into its own sap, so the poison travels to where the hidden larva is feeding — and aim it at the young larvae on the tender shoot tips before they tunnel deep. Safety: spray in the cool of the morning or evening with the wind at your back; cover your skin, eyes, nose and mouth; keep the spray off your food, water, children and bees; and wash yourself and your clothes straight after. Respect the pre-harvest interval — the set number of days you must wait between your last spray and picking, so no poison is left on the fruit you eat or sell (this matters a lot on crops you pick again and again). And rotate to a different chemical group each spray round — using the same one over and over lets the survivors breed until the poison stops working (that is resistance).

First tighten sanitation: clip and destroy every affected shoot every few days, and make sure any spray goes on as a systemic at the tender-flush stage, not once the larvae are already deep inside. When a plant is riddled with dead-hearts and holed fruit it will not recover — it just becomes a breeding nest, so pull it out whole and destroy it to save the rest of the crop. At the end of the season, destroy every crop residue so nothing carries the borer into your next planting.