Larger grain borer

Prostephanus truncatus (Horn, 1878)

scania · Swahili, East Africa
grand capucin du maïs · French, West Africa
barrenador mayor de los granos · Spanish, Central America

The larger grain borer is a small cylindrical beetle that tunnels through stored maize and dried cassava, turning them into dust far faster than ordinary weevils. Native to Central America, it was accidentally introduced to East and West Africa around 1980 and spread across much of the continent, where it became the most destructive pest of farm-stored maize on the cob. A predatory beetle from its home range, hermetic storage and shelling grain help control it.

Larger grain borer beetle on maize grain. Photo: division, CSIRO, CC BY 3.0, via Wikimedia Commons.

Type: Insect · Insect (wood-boring beetle); adults and larvae bore into stored grain
Classification: Animalia › Insecta › Coleoptera › Bostrichidae
Main crops: Stored grain, Maize, Cassava
Found in: Central America and Mexico, East and southern Africa, West and Central Africa, Other regions
Chemical groups: IRAC 3A, IRAC 1B, IRAC 24 (mode-of-action groups, not products)
Last reviewed: 2026-10-20

Larger grain borer scientific name, order and family

The scientific name of larger grain borer is Prostephanus truncatus (Horn, 1878), in the order Coleoptera, family Bostrichidae.

Where it occurs

Native to Mexico and Central America. Introduced to Tanzania (reported in 1981) and Togo (1984), it has spread through much of East, West, Central and southern Africa. It is a quarantine pest in many other regions.

Region	Status
Central America and Mexico	Native
East and southern Africa	Invasive since about 1980; widespread
West and Central Africa	Invasive since the mid-1980s; widespread
Other regions	Absent; regulated

Symptoms and identification of larger grain borer

Larger grain borer adult, side view (specimen). Photo: Sarah McCaffrey, Museum Victoria, CC BY 3.0 AU, via Wikimedia Commons.

The organism

Adults are cylindrical, dark brown, about 3-4.5 mm long, with the head bent down under the hood-like thorax so it is not visible from above. The rear end of the wing covers is flattened and steep, edged with small bumps - a key feature. Larvae are C-shaped and white, living inside the tunnels.

Signs in the crop, stage by stage

Crop stage	What you see
Field (shortly before harvest)	Adults may enter maize cobs in the field, especially if harvest is delayed.
Early storage	Round holes bored through husks and grain; fine, flour-like dust accumulating under stored cobs.
Later storage	Grain and cassava chips reduced to dust within months; beetles visible in large numbers.

Life cycle of larger grain borer

Life cycle of Larger grain borer — diagram drawn by Laisha from the stages below. Durations depend on temperature.

Adults bore into grain and lay eggs in chambers at the end of tunnels. Larvae develop in the dust and grain, pupate there, and the new adults continue boring. Males release an aggregation pheromone that attracts both sexes, which is used in monitoring traps. Adults fly between stores and to and from wooded areas.

Stage	Duration	Notes
Egg	About 4-7 days	In chambers bored into grain.
Larva	About 3-4 weeks	Feeds in grain and flour dust.
Pupa	About 1 week	Inside grain or dust.
Adult	Several months	Bores continuously; flies.

Temperature

Development is fastest at about 30-32 °C and moderate humidity; it thrives in hot, relatively dry stores where maize weevils do less well.

Generations

Several generations a year in warm stores (a generation in roughly 4-6 weeks).

Conditions that favour it

Storing maize on the cob with husks, long storage periods, warm conditions, stores near woodland where wild populations breed, and moving infested grain.

Economic threshold level (ETL)

There is no numeric economic threshold for farm stores; any infestation spreads quickly, so stores are inspected regularly and protected from the start. Pheromone trap catches are used to monitor seasonal flights and spread.

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 larger grain borer

Weight losses of 30-40% of farm-stored maize on the cob within three to six months were reported after its arrival in Africa, far higher than losses from native storage pests; dried cassava chips can be destroyed.

Monitoring

Pheromone traps around stores and in the landscape track flights and detect new infestations.
Inspect stores every two to four weeks for holes, dust and live beetles.

How to control larger grain borer (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 promptly, shell maize and store grain rather than cobs where possible.
Dry grain well and clean stores before each harvest; do not mix old and new grain.
Sort out infested cobs before storage.

Mechanical and physical control

Hermetic triple-layer bags and sealed metal silos stop larger grain borer damage when grain is properly dry and bags are not punctured.

Biological control

The predatory beetle Teretrius nigrescens, brought from Central America and released in several African countries from the early 1990s, has established and reduced larger grain borer numbers, particularly in the wild habitat.

Host resistance

Maize varieties with hard kernels and good husk cover suffer less, but none are resistant.

Chemical control of larger grain borer — 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 registered for stored food grain are applied to shelled grain at storage; some groups that work well against weevils are weak against this beetle. Fumigation must be done only by trained, licensed operators. 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)	Effective against this species as grain protectants.
IRAC	1B	Acetylcholinesterase inhibitors (organophosphates)	Weak alone; used in combination products against weevils.
IRAC	24	Mitochondrial complex IV electron transport inhibitors (phosphides)	Fumigant; licensed operators only.

Resistance management

Use protectants and fumigation correctly (full dose and exposure in sealed conditions) and rely on drying, hermetic storage and biological control.

Look-alikes and how to tell them apart

Lesser grain borer (Rhyzopertha dominica) — Smaller (about 3 mm), with a rounded, not flattened, rear end.
Maize weevil — Has a long snout; larvae develop inside individual kernels; produces less dust.
Angoumois grain moth — Small moths and kernels with round exit windows, no dust or beetles.

Host range

Stored maize (especially on the cob) and dried cassava chips are the main hosts; it also attacks other stored products and wood, and breeds in dead wood of forest trees, which allows it to survive outside stores.

What is still uncertain

The long-term impact of Teretrius nigrescens differs between regions.
Its full current distribution in Africa is not completely mapped.

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 Cassava

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

Prostephanus truncatus (larger grain borer) datasheet — CABI Compendium. https://doi.org/10.1079/cabicompendium.44524
Prostephanus truncatus (PROETR) — EPPO Global Database. https://gd.eppo.int/taxon/PROETR
Teretrius nigrescens against larger grain borer Prostephanus truncatus in African maize stores — Journal of Applied Ecology (Wiley). https://doi.org/10.1046/j.1365-2664.2003.00805.x
Biological control of the larger grain borer Prostephanus truncatus — Bulletin of Entomological Research (Cambridge). https://doi.org/10.1079/BER2003247

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.