Cylas puncticollis / C. brunneus / C. formicarius
5
Damage Signs
4
Organic Methods
2
Chemical Options
5
ID Tips
2
FAQs
1
Crops Affected
Updated February 2026
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Overview
The most destructive pest of sweet potato in Africa and Asia. Sweet potato weevil larvae bore into storage roots and vines, causing direct damage and making roots bitter and inedible due to terpenoid compounds produced by the plant in response to feeding. Three Cylas species attack sweet potato: C. puncticollis and C. brunneus in Africa, C. formicarius in Asia and the Americas. Yield losses of 60-100% of stored roots are common in badly infested fields.
Field Guide
Adults are small (6-8 mm), ant-like weevils with elongated snout, metallic blue-black elytra, and orange-red thorax and legs (C. formicarius) or uniformly dark (C. puncticollis). Larvae are white, legless grubs (8 mm) found boring inside roots and vine bases. Pupation occurs inside the root.
Cut suspect roots open — tunnels and white grubs inside are diagnostic
Bitter taste when eating raw root tissue indicates weevil damage (ipomeamarone production)
Ant-like weevils with metallic blue wing covers and orange legs (C. formicarius) are distinctive
Pheromone traps confirm presence and species identity
Damage is worst where soil cracks expose roots to adult weevil access
Scouting Guide
Small round holes in sweet potato storage roots (adult entry and exit holes)
Tunneling and frass inside roots when cut open
Roots taste bitter and have unpleasant odor — caused by plant defense compounds (ipomeamarone) produced in response to weevil feeding
Vine damage near soil level — larvae bore in vine-root junction
Cracked and deformed roots
Biology
Egg laid in cavity chewed in root or vine (5-7 days) → larva feeds inside root (14-17 days) → pupa inside root (7-10 days) → adult (3-5 months lifespan). Generation time 30-40 days. Adults are flightless or weak fliers — spread primarily through infested planting material.
Pest Management
Clean planting material
Use vine tip cuttings from above ground (not from root zone or soil level) to avoid transferring weevil eggs and larvae in planting material.
Hilling and mulching
Hill soil over developing roots to maintain at least 10-15 cm soil cover — exposed roots are vulnerable to adult oviposition. Mulch between rows to retain soil moisture and prevent soil cracking.
Early harvest
Harvest roots as soon as they reach usable size — do not leave in ground longer than necessary. Extended field storage increases weevil damage exponentially.
Pheromone trapping
Deploy Cylas sex pheromone traps for monitoring and mass trapping. 20-40 traps per hectare can significantly reduce male populations.
Use as last resort. Follow label instructions. Wear protective equipment.
Bifenthrin soil treatment
Apply bifenthrin granules (Brigade 0.2G) at planting, incorporated into ridges. Provides 6-8 weeks of root protection.
Lambda-cyhalothrin vine dip
Dip vine cuttings in lambda-cyhalothrin (0.5 ml/L) for 10 minutes before planting to eliminate any weevil eggs or larvae on planting material.
Host Range
Sweet Potato Weevil can attack 1 crop species.
Common Questions
When sweet potato tissue is damaged by weevil feeding, the plant produces toxic furanoterpenes (ipomeamarone, ipomeanine) as a chemical defense. These compounds make the root tissue bitter, inedible, and potentially toxic to humans and livestock. Cooking does not fully destroy these compounds. Even a small amount of weevil damage can make an entire root unpalatable.
The most important prevention is field management — hill roots properly, irrigate to prevent soil cracking, and harvest promptly. Once roots are in storage, there is little that can be done because larvae and pupae are already inside the roots. Post-harvest treatments are ineffective. Prevention in the field is the only practical approach.
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