Sweet potato weevil
The one pest that decides a sweet potato crop — its grub bores the tuber and a little damage makes the whole tuber bitter and useless. Control is about keeping the tubers covered.
- Pest type
- Tuber- and stem-boring weevil
- Attacks
- Tubers, stem bases, and the planting vines
- Worst in
- Dry-season crops and cracked, un-earthed-up soil
- Main risk
- The whole tuber turned bitter and inedible
Overview
The sweet potato weevil (Cylas formicarius) is by far the most damaging pest of sweet potato in India. The adult is a slender, ant-like weevil about 6 mm long, with a blue-black body and reddish-orange legs and thorax. The female lays eggs in the stems and in the exposed shoulders of the tubers, and the grubs tunnel through the tuber and the stem base.
The real problem is the quality loss. When the grub tunnels into a tuber, the plant produces a foul-tasting, bitter terpene compound throughout the tuber in response — so even a small amount of feeding makes the whole tuber bitter and unfit to eat, not just the damaged part. Losses of 20–50%, and occasionally the whole crop, are common in the dry season and in poorly managed fields.
The weevil reaches the tubers through cracks in dry soil and through tuber shoulders that push above the ridge, and it survives between crops in the soil, in leftover tubers, and in old vines and volunteer plants. So control is almost entirely cultural: keep the tubers covered, start with clean planting material, and do not leave a mature crop in the ground.
How to identify it
- Slender, ant-like weevils about 6 mm long, blue-black with reddish-orange legs and thorax, on the vines and around the crown.
- Tubers with small round exit holes and dark tunnels running through the flesh.
- Flesh that is bitter and smells strongly of a chemical or resin when the tuber is cut.
- Stem bases that are thickened, cracked and tunnelled, with the vine wilting.
The damage it does
- The grub tunnels through the tuber; even light feeding makes the entire tuber bitter and unfit to eat.
- Losses of 20–50% of the crop are common, and a badly infested dry-season crop can be a total loss.
- Damage is invisible from the outside until the tuber is cut, so infested stock can spread through a stored heap or a market lot.
- Stem tunnelling weakens the vine and reduces tuber set.
When to act
- Prevention starts at planting — use only weevil-free cuttings and plant on firm, high ridges.
- Re-ridge (earth up) at about 30 and 60 days, before the soil cracks and the tubers push above the ridge.
- Lift the crop as soon as it matures; every extra week a mature crop sits in dry, cracked soil is more damage.
Organic & cultural control
- Plant clean, weevil-free apical vine cuttings from a healthy nursery — never old vines from last season's field — and dip the cut ends in a recommended insecticide before planting.
- Plant on firm, high ridges and re-ridge at about 30 and 60 days to close soil cracks and cover the swelling tubers, which is the single most effective control.
- Run sex-pheromone traps to mass-trap the male weevils.
- Rotate away from sweet potato for one to two years, remove all volunteer and ratoon plants, and burn or deep-bury every vine and cull tuber after harvest.
- Irrigate at tuber initiation and keep the crop from drying and cracking; harvest on time and cure and sort the tubers strictly.
Chemical control
- A recommended soil or foliar insecticide at planting and at each earthing-up helps where weevil pressure is high, but on its own it does far less than the cultural measures.
- Dipping the basal part of the cuttings in an insecticide solution before planting protects the young crop.
- Observe the pre-harvest interval, since sweet potato tubers are eaten directly.
- Confirm the current recommendation and dose for your zone with your local KVK.
Crops most at risk
Where this problem is called out in our crop guides — open a crop for the full package of practices.
Frequently asked questions
My sweet potatoes look almost undamaged but taste bitter. Why?
That is the sweet potato weevil. When the grub tunnels into a tuber, the plant produces a bitter, foul-tasting compound throughout the whole tuber in response — so even a little feeding makes the entire tuber inedible, not just the tunnelled part. The damage is often invisible from the outside until you cut the tuber.
What is the single most useful thing I can do against the weevil?
Earth up the ridges. As the soil dries it cracks, and the tubers push their shoulders above the ridge — both give the weevil a way in. Re-ridging at about 30 and 60 days keeps the tubers covered and, together with clean planting material and a timely harvest, does more than any spray.
Can I use vine cuttings from my last crop?
No. Old field vines carry the weevil (and the feathery mottle virus) straight into the new field. Always plant apical cuttings — the top 20–30 cm of a healthy shoot — from a nursery raised from certified, weevil-free stock, and dip the cut ends in insecticide first.
Symptoms and control measures follow ICAR, KVK and state agricultural-university package-of-practices; any named chemical is a registered use. Always read the product label and confirm the dose for your crop and zone. editorial policy.