Sweet Potato Weevil
Cylas formicarius
The single most destructive pest of sweet potato worldwide, a small ant-like weevil whose grubs tunnel through the storage roots and vines. Even light feeding triggers the plant to fill the roots with bitter, foul compounds that make them inedible, so a small infestation can spoil a large share of the crop.
🔎 How to spot it
The adult is a slender, striking weevil about a quarter inch long that looks more like an ant than a beetle, with a dark blue head and wing covers, a reddish-orange thorax and legs, and a long curved snout. The grub is a small, legless, white larva with a pale head that feeds hidden inside stems and roots. Tunnels packed with frass inside the roots, and darkened, spongy areas, are the main signs.
🥀 Damage it causes
Adults chew small round holes in leaves, vines, and root surfaces, while the grubs tunnel through the storage roots and the base of the stems. The roots react to even slight feeding by producing bitter terpene compounds, so infested sweet potatoes become inedible and ill-smelling even where tunneling looks minor. Damage continues in storage, and yield losses can be severe, sometimes destroying most of a crop.
🛡️ Prevent it
Plant only clean, weevil-free slips, since infested planting material is a main way the pest spreads, and rotate sweet potatoes well away from last year beds and from any volunteer or wild morning-glory relatives that host the weevil. Hill soil over the roots and keep beds from cracking, because the weevil reaches the roots through soil cracks and exposed crowns; keep the crop irrigated to limit cracking. Harvest promptly and do not leave roots in the ground.
🧯 If it is already here
Because the grubs are hidden inside the roots, control depends on prevention: use clean slips, long rotation, prompt harvest, and destruction of crop residue and cull roots, which removes the weevils breeding sites. Pheromone traps help monitor and, in numbers, trap out males. Where the weevil is established, treat slips before planting and remove all old roots and vines after harvest, since stored and field-left roots carry the infestation forward.
💡 Good to know
A few weevils can ruin a whole harvest because the bitterness they trigger spreads beyond the actual tunnels, so there is little tolerance for this pest. Clean planting stock and strict sanitation matter more than any spray, since nothing reaches the grubs once they are inside the roots. In quarantine regions, moving infested roots or slips is restricted to slow its spread.
🌱 Plants it attacks
2 plants in the library can be attacked by this pest
For educational and informational purposes only. Pest control advice is general guidance drawn from university cooperative extension sources; always identify a pest positively and read and follow the label on any product before use, especially around food crops, children, and pets.