Rhyzopertha dominica (Fabricius, 1792)

Lesser grain borer

A tiny cylindrical borer that drills straight into whole wheat kernels, and carries some of the strongest pesticide resistance in the insect world.

Order: Coleoptera
Family: Bostrichidae
Group: Stored-product beetles
Ontario status: Established, a grain-elevator and farm-storage pest in Ontario; less common in homes

Why it matters

A primary pest of stored wheat, it attacks the kernel itself, not just the flour. Its claim to fame in science is extraordinary phosphine resistance, among the strongest known in any insect.

Origin

Tropical origins; now a global grain pest moved by the wheat trade.

Behavior

Strong fliers that disperse between grain storages. Adults chew into kernels and lay eggs inside; larvae develop entirely within the grain.

Living conditions

Whole stored grain, wheat, barley, corn, in elevators, farm bins, and mills. Unlike flour beetles, both adults and larvae bore directly into intact kernels.

Signs of presence

Tiny (2-3 mm) dark beetles in stored grain
Grain with round bore holes and internal feeding damage
Heating and mustiness in grain bins from heavy infestation

Weaknesses

Needs whole grain, absent from milled flour and home pantries
Killed by freezing and by properly aerated cool grain
Populations are monitored and managed at the elevator level, not the household level

Myths and facts

Myth Fumigation always works on grain pests.
Reality This species evolved some of the strongest fumigant resistance known, grain managers now test for resistance before fumigating.

Research notes

Morphology

2-3 mm, cylindrical, dark brown to black. Pronotum rasp-like with tubercles, covering the head from above. Elytra with rows of pits. Strong fliers.

Diagnostics

Cylindrical body + hooded rough pronotum + boring in whole kernels = lesser grain borer. Distinct from weevils (no snout).

Easily confused with

Granary weevil, Sitophilus granarius Weevils have a long snout (rostrum); the lesser grain borer has no snout and a rough hooded pronotum.

Life cycle

Adult: Bore out, feed, fly, and reinfest; live several months
Egg: Laid on or in kernels; hatch in ~1 week at 30 °C
Egg to adult: 4-8 weeks at 30 °C
Generations per year: 3-5 in warm storage
Nymph or larva: Larvae bore and feed inside the kernel for 3-5 weeks

Development

Complete metamorphosis: egg → larva → pupa → adult, largely inside the kernel.

Reproduction

Females lay ~200-400 eggs on grain surfaces and in cracks.

Distribution

Global: Cosmopolitan in the grain trade; dominant in warm regions
Native range: Tropics (likely South Asia)
Ontario: Established in grain handling; farm bins and elevators rather than homes

Ecology

Primary pest, attacks intact grain. Flight disperses populations between storages; aggregation mediated by male pheromones (dominicalure).

Resistance

Among the strongest phosphine resistance documented in any insect, highly resistant strains survive doses hundreds of times the normal lethal concentration, mediated by rph1/rph2 genes (dihydrolipoamide dehydrogenase). A textbook case of fumigant resistance evolution.

IPM implications

Resistance testing before phosphine fumigation is standard practice where this species occurs, fumigating a resistant population selects harder. Non-chemical levers (grain cooling/aeration, sanitation of bins) carry more weight precisely because the chemistry is compromised.

Chemical ecology

Male aggregation pheromone dominicalure; responds to grain volatiles.

Genetics and DNA

No reference genome published as of 2026 (resistance-gene genetics well studied at the locus level).

Taxonomy

Order Coleoptera, family Bostrichidae (auger/horned powderpost beetles). The head is tucked under a rough, hood-like pronotum, the family signature.

Open questions

Fitness costs of extreme resistance alleles; spread dynamics of resistant genotypes through global grain movement.

References

Hagstrum, D.W. & Subramanyam, B. (2006). Fundamentals of Stored-Product Entomology. AACC International.
Phillips, T.W. & Throne, J.E. (2010). Biorational approaches to managing stored-product insects. Annual Review of Entomology, 55:375-397.