Maize weevil

Sitophilus zeamais Motschulsky, 1855

gorgojo del maíz · Spanish, Latin America
gorgulho-do-milho · Portuguese, Brazil and Lusophone Africa
charançon du maïs · French, West and Central Africa
dumuzi · Swahili, East Africa
玉米象 (yùmǐ xiàng) · Chinese, East Asia

A small brown weevil with a long snout whose larvae grow hidden inside individual grains of stored maize, rice, sorghum and wheat, hollowing them out. It is one of the most damaging storage pests in the tropics: in a few months of poor storage a smallholder can lose a large part of the harvest. Dry grain, clean stores and airtight (hermetic) storage are the most effective defences.

Maize weevil adult, side view. Photo: Natasha Wright, Florida Department of Agriculture and Consumer Services, Bugwood.org, CC BY 3.0 US, via Wikimedia Commons.

Type: Insect · Insect (weevil); larva develops inside the grain
Classification: Animalia › Insecta › Coleoptera › Curculionidae (Dryophthorinae)
Main crops: Stored grain, Maize, Sorghum, Rice, Wheat
Found in: Sub-Saharan Africa, Latin America, South and Southeast Asia, East Asia, Temperate regions
Chemical groups: IRAC 3A, IRAC 1B, IRAC 5, IRAC 24 (mode-of-action groups, not products)
Last reviewed: 2026-10-19

Maize weevil scientific name, order and family

The scientific name of maize weevil is Sitophilus zeamais Motschulsky, 1855, in the order Coleoptera, family Curculionidae (Dryophthorinae).

Where it occurs

Found throughout the warm, humid tropics and subtropics wherever maize is stored, and carried in grain trade to temperate regions, where it survives mainly in heated stores.

Region	Status
Sub-Saharan Africa	Widespread; major smallholder storage pest
Latin America	Widespread
South and Southeast Asia	Widespread
East Asia	Widespread in the south
Temperate regions	In stored products; limited by cold

Symptoms and identification of maize weevil

Maize cob damaged by maize weevil. Photo: Frank Peairs , Colorado State University, CC BY 3.0 US, via Wikimedia Commons.

The organism

Adults are 2.5-4 mm long, dark reddish-brown to black, with a long narrow snout, elbowed antennae and four faint orange-red spots on the wing covers. The thorax is densely covered with round pits. Larvae are white, legless, C-shaped grubs found only inside grains. Maize weevil and rice weevil (S. oryzae) are almost identical and are separated reliably only by examining genitalia.

Signs in the crop, stage by stage

Crop stage	What you see
Pre-harvest (maize in the field)	Weevils in cobs with poor husk cover, especially when harvest is delayed.
Early storage	Small round exit holes in grains; adults walking on grain and sacks; slight warmth and fine dust in the store.
Late storage	Hollowed, powdery grains; heating and moisture in bulk grain leading to moulds; grain unfit for food or seed.

Life cycle of maize weevil

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

Females chew a small hole in a grain, lay a single egg and seal it with a plug. The larva feeds inside the grain, pupates there and the adult chews its way out. Adults live several months and can fly, which lets them move from stores to fields and back.

Stage	Duration	Notes
Egg (inside grain)	About 4-6 days at 27 °C	One egg per grain; 300-400 eggs per female.
Larva (4 instars, inside grain)	About 2-3 weeks	Hidden; feeds on the kernel.
Pupa (inside grain)	About 5-8 days	Inside the hollowed grain.
Adult	Several months to a year	Flies; emerges through a round exit hole.

Temperature

At about 27 °C and 70% relative humidity a generation takes about 5 weeks; development stops below about 15 °C and in grain drier than about 10-11% moisture. Cooling and drying grain therefore slows or stops population growth.

Generations

Up to 6-7 generations a year in warm stores.

Conditions that favour it

Grain stored with more than about 13% moisture, warm stores, carry-over of infested grain or residues from the last season, mixing new and old grain, delayed harvest of maize in the field, and porous bags or granaries.

Economic threshold level (ETL)

There is no numeric economic threshold for smallholder storage: any live weevils mean the population will grow. Grain trade and food standards commonly specify zero or very low tolerance for live insects and limits on insect-damaged kernels, which are set nationally. Action is triggered by detection of live insects during regular inspection.

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 maize weevil

Smallholder maize stored in traditional granaries or woven bags without protection commonly loses 10-30% of its weight over six to nine months in the tropics, with higher losses in humid lowlands. Damaged grain also loses germination, nutritional value and price, and moulds on infested grain can produce mycotoxins.

Monitoring

Inspect stored grain every two to four weeks: sieve a sample from several depths and count live insects.
Use probe traps or pitfall traps in bulk grain.
Measure grain moisture and temperature; a warm spot in a bulk often marks an infestation.

How to control maize 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

Harvest maize promptly at maturity and dry grain to 13% moisture or less before storage (12% or less for long storage).
Clean stores, bags and equipment thoroughly before the new harvest and remove all old grain.
Sort out damaged and insect-infested cobs and grains before storage; do not mix new and old grain.
Grow varieties with good husk cover.

Mechanical and physical control

Hermetic storage: airtight triple-layer plastic bags, sealed metal silos or drums cut off oxygen so insects die; this has reduced losses to very low levels in many African and Asian studies.
Sieving and solar heating of grain kill or remove adults in small lots.
Inert mineral dusts mixed with grain, where registered for this use, damage insect cuticles.

Biological control

Parasitoid wasps (Anisopteromalus calandrae and others) attack larvae inside grains and can suppress populations in stores.
Some traditional botanical protectants (for example ash or plant powders mixed with grain) give partial protection; their effect varies.

Host resistance

Maize varieties with hard kernels, good husk cover and grain chemistry that reduces weevil development have been bred by public programmes; they reduce but do not prevent losses.

Chemical control of maize 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.

Grain protectants and fumigants must be used only as registered for stored food grain, with strict waiting periods; fumigation is dangerous and should be done only by trained, licensed operators in gas-tight conditions. What is registered differs by country and changes over time; follow the label and your local plant-protection or food-safety authority.

Scheme	Group	Mode of action	Notes
IRAC	3A	Sodium channel modulators (pyrethroids)	Used as grain protectants where registered.
IRAC	1B	Acetylcholinesterase inhibitors (organophosphates)	Used as grain protectants where registered.
IRAC	5	Nicotinic acetylcholine receptor allosteric modulators, site I (spinosyns)	Grain protectant where registered.
IRAC	24	Mitochondrial complex IV electron transport inhibitors (phosphides)	Fumigant; licensed operators only; resistance widespread in stored-product insects.

Resistance management

Resistance to Group 24 fumigants and Group 1B protectants is documented in stored-grain insects including Sitophilus species in many regions, often because fumigation is done in stores that are not gas-tight.
Fumigate only in sealed conditions for the full exposure time, and use hermetic storage and drying to reduce chemical dependence.

Look-alikes and how to tell them apart

Rice weevil (Sitophilus oryzae) — Almost identical and with the same biology; slightly smaller on average; separated by genitalia.
Larger grain borer (Prostephanus truncatus) — Cylindrical beetle with the head hidden under the thorax and no snout; bores through grain and cobs, leaving large amounts of dust; very destructive in African maize stores.
Angoumois grain moth (Sitotroga cerealella) — Small moths in the store; grains with a round 'window' where the moth emerged; no weevils.

Host range

Cereal grains in store: maize, sorghum, rice, wheat, barley and millets; also dried cassava chips, pasta and processed cereal products. Adults can infest maize cobs in the field shortly before harvest.

What is still uncertain

Storage loss estimates vary widely between surveys and methods.
The value of many traditional protectants has not been tested rigorously.

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 Maize How to grow Sorghum How to grow Rice How to grow Wheat

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

Sitophilus zeamais (greater grain weevil) datasheet — CABI Compendium. https://doi.org/10.1079/cabicompendium.10926
Sitophilus zeamais (CALAZM) — EPPO Global Database. https://gd.eppo.int/taxon/CALAZM
Chemical bases for maize grain resistance to infestation and damage by the maize weevil — Journal of Stored Products Research (Elsevier). https://doi.org/10.1016/j.jspr.2016.06.001
The metal silo: an effective grain storage technology for reducing post-harvest insect and pathogen losses in maize — Crop Protection (Elsevier). https://doi.org/10.1016/j.cropro.2010.11.015

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.