Stored grain insects

Seed stored for more than six weeks must be protected against insect damage. Seed should only be stored when 'dry', as grain of high moisture causes temperatures to rise and mould to develop. High temperatures reduce the efficacy of grain protectants, allowing insects to multiply. Cooling of grain (below 15 degrees C) helps to suppress insect activity.

Insecticides applied to uninfested grain will protect against pests developing and damaging the grain (for a specified duration). Application of seed treatment insecticides after infestation and their over-use is discouraged. Such management facilitates the development of resistance to the chemicals within the insect population. Additionally if an infestation is current, seed treatment insecticides will assert little control.

Some Hannaford seed treatment products contain cypermethrin or triflumuron insecticide. Both insecticides protect against a range of insect pests of stored wheat, barley, oats and other small grains. Existing adult insects will die after ingesting cypermethrin; while triflumuron, an insect growth regulator, will kill any emerging larvae. Triflumuron will not destroy adult insects. Infested grain will therefore suffer insect damage before triflumuron acts upon the larvae as the insect life cycle proceeds.

Granary weevil (Sitophilus granarius)

Adults are dark brown and range from 2.0-3.5 mm in length. They are easily distinguishable from the borers and beetles by their long snout, characteristic of the true weevils.

Females produce from 200-300 eggs in their 2-3 month lifespan. Adults cannot fly and feign death if disturbed. Eggs are deposited in whole kernels of grain; emerging larvae feed on whole grain.

Adults feed in and on whole and broken grain.

Rice weevil (Sitophilus oryzae)

As for the granary weevil, although distinguished by the four orange-reddish patches on its body.

Indian meal moth (Plodia interpunctella)

Attacks oilseeds and cereals.

Adults have a wingspan of 20 mm; the outer portion of the forewing is reddish-brown and the inner portion creamy-white. Eggs are laid on the surface of the grain.

Larvae spin webs on the surface of the grain and consume kernels within the webbing.

Lesser grain borer (Rhyzopertha dominica)

This pest is capable of infesting all small grain and develops more rapidly in damaged than in whole grain.

Adults are dark brown and range from 2.5-3.0 mm in length. Females produce between 200 and 400 eggs in their 2-3 month lifespan. Eggs are laid on the surface of the grain and larvae burrow into the kernels. Adults can fly, and also feed on the grain.

Rust-red flour beetle (Tribolium castaneum)

Capable of infesting both whole grain and oilseeds, but a more serious problem in stock feed and processed grain (i.e. flour).

Adults are reddish brown and range from 2.3-4.4 mm in length.

Females produce from 10-20 eggs per day, and over a lifetime of 200 days – 2 years, produce more than 1000 eggs.

Larvae and adults feed on broken grain and flour.

Sawtoothed grain beetle (Oryzaephilus surinamensis)

Infests a wide range of commodities, but does not readily breed in whole grain. However, most samples of grain contain enough broken fragments for the beetle to become established.

Adults are brown and 2.5-3.5 mm long with characteristic tooth-like projections on either side of the thorax.

Adults may live for several years and females produce an average of 375 eggs.

Larvae and adults feed on broken grain and flour.

Tropical warehouse moth (Ephestia cautella)

Attacks cereal grain and oilseeds.

Adults are grey with a dark band across the middle of the forewing. They have a wingspan of 15 mm.

The moths live for only two weeks, but during this period females may lay up to 250 eggs on the surface of the grain.

Larvae spin webs on the grain surface and consume kernels within the webbing.

Seed stored for more than six weeks must be protected against insect damage.


Control of these insects by seed treatments is reliant on either the adults, in the case of cypermethrin, or larvae, when triflumuron is used, feeding on the treated grain and hence the insecticide. When feeding on treated grain, insecticide is ingested and the insect is killed. Eggs and larvae feeding exclusively within the seed are not controlled by this method, as there is no intake of chemical. However, when they emerge, insecticide is ingested from the surface of the seed and control is achieved.

Only the larval stages of the moths consume grain. Therefore, adult moths may be present in treated grain, which is protected from larval damage by the insecticide. Further, the life cycle of the insect is interrupted and multiplication does not occur, as all larvae are killed by the protectant.

In order to be effective, grain protectants are developed and formulated to maintain their toxic properties for an extended period.

By contrast, a major stored grain insect of field peas, the pea weevil (Bruchus pisorum) is not controlled by seed treatments. Pea weevils infest the seed before harvest and do not ingest the seed during storage. Thus, multiplication can occur, uninhibited.

Stored grain insects may also be controlled by:

  • Cooling stored grain through aeration
  • Treating grain with an insecticide powder such as Dryacide® or liquid residual chemicals
  • Treating infested grain with dichlorvos
  • Fumigating grain with phosphine
  • Controlled atmosphere such as carbon dioxide in a gas-tight silo
  • Treating storage areas and equipment with residual chemicals

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