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Extension > Agriculture > Crops > Small Grains Production > Harvest > Preventing stored-grain insect infestation

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Preventing stored-grain insect infestation

Phil Harein and Bh. Subramanyam

Stored-grain insects

Minnesota grain is generally safe from infestation by stored-grain insects before harvest. This is not true for many of the southern states. The single exception in Minnesota may be where grain is cut and swathed adjacent to storage bins being treated with an insecticide (including fumigants) in preparation for the new crop. Stored-grain insects may migrate out of treated bins for short periods of time to escape from the insecticides.

Accumulations of postharvest grain are preferred targets for insect infestations, especially if stored with or adjacent to grain from previous harvest. The initial site where this postharvest infestation can begin is within the harvesting machinery. Unfortunately, farm harvesting equipment is not designed to remove old grain from last year's harvest. The same applies to grain augers and elevators and the sub-floors or aeration ducts in storage bins.

Stored-grain insects can be killed by fumigants, although thorough sanitation, stored grain aeration, and residual insecticides are the most effective procedures for preventing insect infestation. For details on fumigating stored grain, refer to the University of Minnesota Extension, Fumigating Stored Grain, FS-1034.

Conditions encouraging these insects


Figure 1. Disassembled plastic probe trap. A = perforated region of the trap; B = funnel-shaped teflon cap that fits on top of the vial (C). The vial with the teflon cap is placed inside the bottom unperforated portion of the trap. The vial is held in place by a snap cap (D) that fits at the bottom of the trap. The top of the trap has two large holes for rope attachment, to facilitate easy removal from grain. Actual size of the probe is 15 inches.

High grain temperatures and moisture, along with excessive grain dockage or broken kernels, interact, providing the necessary conditions for stored-grain insect reproduction and survival. The most favorable grain temperature for these insects is about 80°F. At temperatures above 90°F or below 60°F, reproduction is nil, developmental time is extended, and survival time is reduced.

The most favorable grain moisture range for stored-grain insects is 12% to 18%. Insects that feed on mold prefer the higher moisture levels. These insects also prefer grain with the highest dockage. This is the primary reason insects often accumulate in spout line areas.

Over-filling bins is a common practice in Minnesota. Unfortunately, this results in inadequate space to inspect or treat the grain. Uneven grain surfaces also contribute to nonuniform air flow during aeration. Level the grain, after binning is complete, at least 6 inches below the top of the bin wall. Or, immediately following harvest, level the grain to the proper height by removing sufficient amounts from the spout line area. This grain will contain a relatively high percentage of broken kernels and foreign matter. Feed it to livestock or screen and treat before rebinning.

Inspect grain at 7 to 14 day intervals, depending on the potential for insect infestations. Grain with temperatures exceeding 50°F should be checked every 7 days. Check for insects by screening them from the grain, examining kernels for damage, looking for webbing, detecting off-odors, or monitoring grain temperatures. The temperature could be as high as 110°F just due to the insect activity. During the summer and fall, insect infestations are usually near the surface of the grain. Cylindrical plastic insect traps are now available to collect insects in the grain thus providing an effective means of detecting damaging populations early. The trap is shown dismantled in Figure 1. During cold weather stored grain insects will congregate at the center and lower portions of the grain mass and may escape detection until extensive heating has developed.

Preventing infestation of insects

Sanitation - Be sure that the storage structure as well as any grain handling equipment (conveyors, wagons, trucks, elevators) are free of leftover grain. The cleanup is most effective if completed in early spring. Bins should be cleaned immediately after they are emptied. Repairs such as sealing cracks and/or holes can be completed simultaneously. Old grain being moved to different storage should be screened and, if infested, treated.

Bin wall, ceiling and floor treatments. As soon as the bin is thoroughly cleaned, it can be treated with protective insecticides. It is better to treat during the warmer months when insects are active. If treatments were applied more than three months earlier, an additional treatment should be applied two to three weeks before new grain is placed in the bin. The treatment will kill insects emerging from their hiding places (cracks, crevices, under floors, and in aeration systems). Also, insects crawling or flying in from the outside will be killed.

Four formulations are labeled and registered for this type of application: methoxychlor (50% W.P. and 25% E.C.), malathion (50 to 57% E.C.), for small grain (wheat, oats, barley) storage only, chlorpyrifos-methyl (Reldan 43.2% E) and methoprene (1 oz. Diacon to 3 gals. water). The label will give the rate of application, safety precautions, and directions for use (E=emulsifiable, E.C.=emulsifiable concentrate, and W.P.=wettable powder).

Apply these residual insecticides to as many surfaces as possible, especially joints, seams, cracks, ledges, and corners. Spray the ceilings, walls, and floors to the point of runoff. Use a coarse spray at a pressure of 30 pounds per square inch (psi) and aim for the cracks and crevices.

Spray the area beneath the bin, and spray the bin supports; apply spray to a six-foot border around the outside foundation. Treat the outside surface, especially cracks and ledges near the door and fans. In addition, treat pertinent areas in your cleaned harvesting equipment, elevators, augers, trucks, or wagons.

The increased use of metal bins with perforated floors for grain drying and aeration has helped produce a serious insect problem in farm-stored grain. Grain dockage (broken kernels, grain dust, and chaff) sifts through the floor perforations and collects in the subfloor plenum creating a favorable environment for insect development. If possible, remove the perforated floors to clean the plenum area and spray it with an approved insecticide. When the floor or the screen over aeration ducts cannot be removed, the area could be fumigated.

Grain protectants

Insect infestation is prevented by treating grain as it is moved into storage with one of three approved insecticides (see labels for application rates, permitted target commodities, and other essential information):

  1. Chlorpyrifos-methyl (E.C. or dust) is registered for wheat, oats, barley, sorghum, and rice (not registered for application to corn). Reldan works relatively well even on high-moisture grains and is effective against insects resistant to malathion.
  2. Malathion (E.C. or dust) is labeled for use on corn and small grains. Water is required as a diluent for malathion E.C.
  3. Pirimiphos-methyl (E.C.) is registered for application to corn and sorghum and is effective against most insects that are resistant to malathion.


Liquid formulations. Any low-pressure sprayer that can be calibrated to deliver a known volume of liquid is suitable for application of protectants. The garden-type compression sprayer may be used for treating small lots of grain. The power sprayers and the metering-type sprayers are generally used when large lots of grain are to be treated.

A simple gravity or "drip-on" applicator that does not use any moving parts may be purchased or constructed. An application system may be built by fitting two brass valves and polyethylene tubing in sequence to an opening in the bottom of a plastic jug. The upper shut-off cock on the jug serves as the on-off valve, while the lower needle valve regulates the amount of insecticide flowing through the plastic tubing.


The effectiveness of treating bulk grain depends on at least five factors.

  1. Proper mixing. One disadvantage of emulsifiable formulations is that most of them must be agitated to avoid settling. Consequently, gravity-flow or "drip-on" applicators and pressure-type sprayers must be shaken periodically to ensure that the formulation is mixed evenly. Power sprayers do not have this problem because the formulation is agitated continuously.
  2. A fresh spray mixture. Mix only enough insecticide for one day's use. In addition, the concentrated insecticide, mixed spray, or insecticide dust should be kept cool and not stored in direct sunlight.
  3. Point of application. Protectants should be applied to the grain just before it reaches final storage. Grain that is treated and then transferred long distances through numerous grain handling systems (such as pneumatic systems, belt augers, conveyors, spouts, legs, etc.) before storage will have less insecticide residue when the grain is finally dropped into the bin.
  4. Application pressure. If you use other than a gravity-flow system, the spray pressure should be as low as possible, preferably 10 to 20 psi. With low spray pressures, larger spray droplets are produced. The larger droplets fall on the grain and are less likely to drift off into the air.
  5. Moisture and temperature of the grain. Most failures with protectants occur because of excessive grain moisture content (mc) and/or temperatures. Grain should not be treated if it is above 13% (16% for corn) and its temperature is above 90°F.


Liquid formulations applied under pressure. After the turning rate of the grain passing on the belt, auger, or conveyor is known (i.e., the amount of grain passing a point in a given time), choose a nozzle that will deliver 10 gallons per hour at a pressure of about 15 psi. Install this nozzle on the sprayer and, using plain water, operate the sprayer at the pressure required. Catch all the water delivered in 10 minutes. Weigh or measure the amount collected, and multiply this by six to determine the amount delivered per hour. The output of the nozzle will vary slightly from the rated output. If the amount collected was more than a gallon, decrease the pressure slightly and rerun the test. If the amount collected was less than a gallon, increase the pressure slightly and rerun the test. Do this until the sprayer is delivering the correct amount.

Dust formulations. For small lots of grains, a special dust applicator may be used. Several different models, available from various manufacturing companies, utilize the 1 or 2% dust formulations. A large dust applicator using 6% formulation is more suitable for treating large quantities of grain, as in a grain elevator. These dust formulations are sometimes applied by spreading them evenly over the grain surface while the grain is in a truck prior to binning. It is then mixed in with a shovel and is mixed further as it falls into the auger hopper.

Surface grain treatment

Immediately after the bin is filled and the grain leveled, apply a surface treatment ("top dressing") of an approved grain protectant. The surface treatment will help control insects that enter the grain through roof openings.

Surface treatments alone generally will not keep the grain insect-free, but they can reduce insect populations during the storage period. Surface treatments are effective if the following limitations are understood.

  1. Surface treatment will not control an established insect infestation already in stored grain.
  2. The surface treatment should not be disturbed, since it provides the protective barrier against insect infestations.

Keep in mind that the malathion surface treatment will probably not control or prevent an infestation of the Indianmeal moth because these insects are resistant to malathion. Chlorpyrifos-methyl is effective against insects resistant to malathion, and it has a label approval as a dust application for surface treatment.

The same equipment used to spray the bin wall can be used to apply the surface spray. Surface treatments should be applied no more than one time during a storage year.


Insecticides are poisonous. They should be used only when needed, and should be handled with extreme care. Always refer to current pesticide labels before selecting or using pesticides. Follow the directions and precautions carefully.

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