Insect of the Month: Lesser Grain Borer
The lesser grain borer is a primary pest of whole grains, especially corn and cassava root.
The adults chew small holes in the kernel and then lay several eggs in the grain or in the grain mass.
Damage is seldom noticed until exit holes appear and adults are seen roaming the grain mass.
What do lesser grain borers look like?
- Lesser Grain Borer Adults: These small 3 mm (0.125 inch) beetles have a uniform dark brown to black color. They have distinctive pits on the elytra and small teeth on the front of the thorax. The ends of the elytra appear jagged. These beetles are excellent fliers and are attracted to insect light traps.
- Lesser Grain Borer Eggs: Eggs are white when first laid, turning reddish-brown before hatching. The egg is ovoid in shape and 0.6 mm (0.02 inches) long.
- Lesser Grain Borer Larvae: The immature stage is approximately 3-4 mm long, white to yellowish in color. It has a grub like in appearance and stubby thoracic legs.
- Lesser Grain Borer Pupae: Pupae are reddish brown and 4 mm (0.15 inches) long.
What is the lesser grain borer life cycle?
Female beetles will deposit 200 – 500 eggs in grains over a 4 -6-month life span. Eggs hatch in about three days, and the larvae can mature within 30 days or up to 58 days depending on temperature. Adults will feed on grains continuously.
Damage & Detection
The Lesser Grain Borer (Rhyzopertha dominica) is one of the most destructive pests of stored grain worldwide. Despite its small size (around 2–3 mm long), this dark brown to black beetle can cause extensive damage in a short amount of time.
Damage
Both adults and larvae feed inside whole grains, making infestations difficult to spot until damage is advanced.
Feeding leaves behind hollowed kernels filled with frass (powdery grain dust and waste).
Infested grain can lose weight, quality, and market value, and heavy infestations may even produce a distinctive sweet, musty odour.
The borer can attack a wide variety of cereal grains and processed products, including wheat, barley, oats, rice, maize, and stored grain-based stockfeed.
Detection
- Hidden feeding: Because larvae develop inside kernels, infestations often go unnoticed until damaged grains are cracked open.
- Frass and dust: Accumulation of fine grain dust near storage bins or in handling equipment is an early warning sign.
- Exit holes: Small, round holes in grain indicate adult emergence.
- Traps: Pheromone or probe traps are effective tools for monitoring populations in bulk storage.
- Visual inspection: Checking samples with a hand lens or sieve can help identify live adults.
Early detection is critical, as infestations can spread rapidly under warm conditions. Effective management relies on a combination of good hygiene in storage facilities, regular monitoring, and when required, the use of registered control products.
Recommended monitoring systems for the Lesser Grain Border are:
- Hanging NoSurvivor Trap with PTL Multi-Species Bullet Lure (IL-408)
- All Beetle Trap with RTU Gel (IL-2750)
