Sweet potato weevil

Cylas formicarius (Fabricius, 1798) (with Cylas puncticollis and C. brunneus in Africa) · formerly or also: Cylas formicarius elegantulus

gorgojo del camote / tetuán · Spanish, Latin America and the Caribbean
charançon de la patate douce · French, Africa and Indian Ocean
甘薯小象甲 (gānshǔ xiǎo xiàng jiǎ) · Chinese, East Asia
fukusi wa viazi vitamu · Swahili, East Africa

Sweet potato weevils are the most damaging insect pests of sweet potato worldwide. Their grubs tunnel through the vine base and the storage roots, and even light damage makes the plant produce bitter, foul-smelling compounds that ruin the whole root for eating or feeding to animals. The weevils reach the roots mainly through cracks in dry soil, so hilling, irrigation and clean planting material are central to control.

Adult sweet potato weevil (Cylas formicarius). Photo: Graham Wise from Brisbane, Australia, CC BY 2.0, via Wikimedia Commons.

Type: Insect · Insect (weevil); larvae tunnel in stems and storage roots
Classification: Animalia › Insecta › Coleoptera › Brentidae
Main crops: Sweet potato
Found in: Asia and the Pacific, Americas and the Caribbean, Sub-Saharan Africa, Oceania
Chemical groups: IRAC 3A, IRAC UNF (mode-of-action groups, not products)
Last reviewed: 2026-10-20

Sweet potato weevil scientific name, order and family

The scientific name of sweet potato weevil is Cylas formicarius (Fabricius, 1798) (with Cylas puncticollis and C. brunneus in Africa), in the order Coleoptera, family Brentidae.

Older or alternative scientific names: Cylas formicarius elegantulus.

Where it occurs

Cylas formicarius occurs throughout the tropics and subtropics of Asia, the Pacific, the Americas and parts of Africa; C. puncticollis and C. brunneus are the main species in sub-Saharan Africa.

Region	Status
Asia and the Pacific	Widespread; C. formicarius
Americas and the Caribbean	Widespread; C. formicarius
Sub-Saharan Africa	Widespread; C. puncticollis and C. brunneus, with C. formicarius in some areas
Oceania	Present

Symptoms and identification of sweet potato weevil

Sweet potato roots cut open to show weevil tunnels. Photo: Scot Nelson, CC0, via Wikimedia Commons.

The organism

Cylas formicarius adults are slender, ant-like weevils about 5-8 mm long, with a metallic blue-black head and wing covers, a reddish-orange thorax and legs, and a long snout; African species are uniformly dark. Larvae are white, legless, curved grubs inside stems and roots.

Signs in the crop, stage by stage

Crop stage	What you see
Vine growth	Swollen, cracked vine bases with tunnels and frass; wilting or yellowing of vines in severe cases.
Storage root formation	Small holes and dark tunnels in roots, often starting near the stem end; frass-filled galleries.
Harvest and storage	Tunnelled roots with a bitter taste and unpleasant smell, even if damage looks slight; infestation continues in store.

Life cycle of sweet potato weevil

Life cycle of Sweet potato weevil — diagram drawn by Laisha from the stages below. Durations depend on temperature.

Females chew small cavities in vines or exposed roots and lay single eggs, sealing them with a plug. Larvae tunnel and feed for two to three weeks, pupate in the tunnel, and adults emerge to repeat the cycle. Adults are active mostly at night and walk more than they fly; they reach roots through soil cracks.

Stage	Duration	Notes
Egg	About 5-7 days	In cavities in vines or roots.
Larva (3 instars)	About 2-3 weeks	Tunnels in roots and stems.
Pupa	About 7-10 days	Inside the tunnel.
Adult	Several months	Nocturnal; females attracted by host odours.

Temperature

A generation takes about one month in warm weather (around 27 °C); dry, hot seasons favour the weevil because soils crack.

Generations

Several overlapping generations a year in the tropics.

Conditions that favour it

Dry seasons with cracking soils, shallowly set storage roots, delayed harvest, infested planting material, and volunteer sweet potato or wild Ipomoea near fields.

Economic threshold level (ETL)

There is no economic threshold in general use: even slight damage makes roots inedible, so management is preventive throughout the crop.

Thresholds are local. They depend on crop price, control cost, variety and natural-enemy activity, so use the figure your local extension service publishes where one exists.

Damage and yield loss from sweet potato weevil

Root losses of 20-100% are reported, highest in dry seasons and where harvest is delayed; terpenoid compounds produced in response to feeding make roots unfit for food and feed. Infested roots also lose value in storage, where adults and larvae continue to develop.

Monitoring

Sex pheromone traps detect males and show when populations rise.
Inspect vine bases and exposed roots for feeding holes and tunnels.
Cut sample roots at harvest to check for damage.
Check stored roots every few weeks and remove any that show holes or tunnels.

How to control sweet potato weevil (integrated management)

Control works best as a sequence: prevent, monitor, then act only when a threshold or a risk forecast says so, using the least disruptive option that works.

Cultural control

Plant clean vine cuttings taken from the tips of healthy plants, and dip them in a biological or registered treatment where available.
Hill up soil around plants and irrigate or mulch to prevent soil cracking that exposes roots.
Harvest promptly at maturity; do not leave roots in the ground into the dry season.
Rotate fields, destroy crop residues and volunteers, and keep new plantings away from old ones.

Mechanical and physical control

Mass trapping with sex pheromone traps reduces male numbers in some programmes.

Biological control

Entomopathogenic fungi (Beauveria bassiana, Metarhizium species) and nematodes infect weevils; ants and other predators eat larvae.

Host resistance

Some varieties with deeper roots or particular root chemistry suffer less damage, but no variety is resistant.

Chemical control of sweet potato weevil — by mode-of-action group only

Laisha names chemical options only by their resistance-committee mode-of-action group (IRAC for insects, mites and nematodes, FRAC for fungi, oomycetes and bacteria, HRAC for weeds), never by active ingredient or product. What is registered differs by country and changes over time; the label and your local plant-protection authority decide what may be used, on which crop, at what rate and with what pre-harvest interval.

Dipping planting material and soil applications are used where registered; sprays have limited effect on larvae inside roots. What is registered differs by country and changes over time; follow the label and your local plant-protection authority.

Scheme	Group	Mode of action	Notes
IRAC	3A	Sodium channel modulators (pyrethroids)	Used as cutting dips or sprays against adults where registered.
IRAC	UNF	Fungal agents (for example Beauveria-based products)	Biological; where registered.

Resistance management

Rely on cultural measures and clean planting material; rotate groups if chemical treatments are repeated.

Look-alikes and how to tell them apart

West Indian sweet potato weevil (Euscepes postfasciatus) — Short, stout, greyish-brown weevil without the blue-and-red colouring; similar damage in the Caribbean and Pacific.
White grubs and wireworms — Surface gouges or narrow holes in roots from soil larvae, without internal tunnels with frass.
Sweet potato virus disease — Stunted plants with distorted, mottled leaves, but roots without tunnels.

Host range

Sweet potato and wild Ipomoea species (morning glories) are the hosts; Cylas species do not breed on other crops.

What is still uncertain

The relative importance of the different Cylas species varies within Africa.
Pheromone mass-trapping effectiveness varies with area and weevil density.

Grow guides for its host crops

How to grow the crop well is the first line of defence. Laisha's free grow guides cover each crop from sowing to harvest:

How to grow Sweet potato

Not sure this is what you are seeing?Ask Laisha: describe the symptoms or send a photo, and get a reasoned first read with what to check next — for your crop, your stage and your region.Ask Laisha

Sources

Cylas formicarius (sweet potato weevil) datasheet — CABI Compendium. https://doi.org/10.1079/cabicompendium.17408
Cylas formicarius (CYLAFO) — EPPO Global Database. https://gd.eppo.int/taxon/CYLAFO
Sweetpotato weevil, Cylas formicarius (EENY-027) — UF/IFAS EDIS (Featured Creatures). https://edis.ifas.ufl.edu/publication/IN154
A review of the biology and control of the sweetpotato weevil Cylas formicarius — Tropical Pest Management (Taylor & Francis). https://doi.org/10.1080/09670878609371084

Written by Laisha from the sources above, in its own words. This page is general reference, not a prescription: confirm identification locally and follow the label and your plant-protection authority.