Maintaining the quality of stored grain so it doesn’t deteriorate over time requires various techniques, including insect pest management.
More than 60 species of insects can infest stored grains, with Indian meal moths being the most common. Damage by insects can go unnoticed until the grain is removed from the storage facility. Regular monitoring can help ensure that grain quality will be maintained at the highest level possible.
A report by D. Ames Herbert, extension entomologist, Tidewater Agricultural Research and Extension Center (AREC), Virginia Tech University, Suffolk, Virginia, U.S., reviewed methods for preventing and controlling pests in stored grain.
Primary grain insects are those that attach whole, undamaged grains. Immature stages of these insects occur on the inside of the grain, where detection is more difficult, according to the AREC report. Examples of primary insects include rice weevils, bean weevils and lesser grain borers.
Secondary grain insects are those that feed on fragments of grains and cereals. They are also referred to as bran bugs. These include grain moths, mites, psocids and various beetles. Specific examples include flour beetles, sawtoothed grain beetles, rusty grain beetles and Indian meal moths.
Generally, treatment is suggested for wheat, rye or triticale if one live insect is found per quart sample and for corn, sorghum, barley, oats or soybeans if one live weevil or five other insects are found per quarter sample, the AREC report said.
If those thresholds are exceeded, fumigation is recommended. But if the weather is cool, fumigation should be delayed because effectiveness is greatly reduced. For conditions that do not favor fumigation, the grain can be cooled to below 55 degrees F. At temperatures below that level, insects are for the most part inactive, the AREC report said.
Pest management begins with preventive measures. Bins should be weather tight, steel, rodent proof and on a moisture-proof concrete base. They should be equipped with a perforated-floor aeration system and weather-proof vents.
Bins should be inspected on a regular basis to prevent leaks, condensation and deterioration of any kind, the AREC report said. Once a bin is filled, the bottom and sides should be sealed so insects and rodents can’t enter. Roof aeration exhaust or inlet vents should not be sealed, except during fumigation, so the top of the bin can easily be sampled and top dressings applied, if necessary.
Prior to adding grain, the storage facility should be clean and free of old grain, trash and insects. The walls, ceiling, sills, ledges, floors, loading/unloading equipment and the ventilation system should be cleaned.
The area outside the bin should also be free of insects, weeds, and grain products, the AREC report said. Insects can breed and persist in these areas and infest new grain when it is placed in the bin. It is best to clean and treat storage facilities at least two weeks prior to adding new grain.
Many infestations begin in the immediate area of the storage facility so area sanitation is important. Many stored-grain pests can move from one facility to another. Practices that limit the pests’ access to food and shelter will help reduce the potential for future infestations, the report said.
Grains harvested and stored in the hottest part of the year stand a greater chance of becoming infested, since insects reproduce rapidly at temperatures in the range of 60 degrees F to 90 degrees F. Bin aeration during times of low outside temperature and humidity will aid in reducing the temperature of the grain, the AREC report said.
An aeration controller is helpful when managing aeration for insect control. Controllers start aeration fans when outside temperatures reach a preset point. Small reductions in grain temperature have proven to significantly reduce the reproductive rates of stored-grain insects and reduce insect damage.
When filling bins, there are several practices that ensure even aeration of the grain mass. The upper surface of the grain should be level or slightly inverted. The use of a grain spreader will help prevent the accumulation of fines such as broken grain, weed seed, dust and debris, in the bin’s center.
If not spread evenly, this material will accumulate in the center, preventing even aeration and providing an environment for insects and fungi to develop, the AREC report said. Fines accumulation can be reduced by removing a portion of the grain mass after the bin is filled. Removing the core from the bottom will remove the column of fines and invert the peaked grain in the top of the tank.
For grain stored through the winter, aeration in the fall can deter moisture migration in the bin. Moisture migration is caused by differential temperatures in the grain mass resulting in convective flow of air through the grain, the report said. The convective flow of air can result in accumulating moisture from condensation in the upper center of the grain mass, which will contribute to the development of molds and insects.
Several pesticide options are also available to manage pests in stored grain, including empty-bin sprays, chemical grain protectants, top dressing and air/head space treatments and fumigation.
Sprays. Empty-bin sprays help prevent the infestation of new grain by existing insect populations. It is recommended when grain is stored in the summer, if there are areas that are difficult to clean, or if there is a history of insect problems, the report said.
After bins have been cleaned, spray to run-off the inside surface and the outside, including nearby ground surfaces, aeration ducts and grain handling equipment. Concentrate on cracks, crevices and areas that are difficult to clean. Applications should be made at least two weeks prior to adding new grain. Allow 24 hours for the spray to dry.
Protectants. Chemical grain protectants can be added when the bin is being filled to guard against insect damage, the AREC report said. They can also be added to the upper surface of the grain to guard against insects entering the top of the storage facility.
However, protectants will not eliminate existing infestations. They are recommended if grain is going to be stored for extended periods, in flat structures, under circumstances that favor pest development or in facilities with a history of insect damage.
High grain moisture and temperatures will shorten the residual life of grain protectants. They should be applied after high-temperature drying is complete and the grain has cooled.
Top-dressing and air/head space treatments. It may be necessary to apply an insecticide to the top few inches of the grain mass (top-dressing) to prevent the introduction of insects, the AREC report said.
Resin strips can also be hung in the air/head space in the top of the bin to help control adult moths. One strip should be used per 1,000 cubic feet of air/head space and replaced after three months. To be effective, the top of the bin must be temporarily sealed, including the roof vent.
Fumigation. Fumigation should only be done by trained, experienced, registered applicators, the report said. The goal of fumigation is to maintain a toxic concentration of gas long enough to kill the target pest population. The gases penetrate into cracks, crevices, the commodity and the facility.
Fumigants come in several forms and formulations. All label instructions and precautions should be read and carefully followed. The report said fumigant selection should be based on: pest susceptibility, volatility, penetrability, corrosiveness, safety, flammability, residues, odors, application method, required equipment and economics.
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