Controlling Pests in Stored Grains
Managing grain in storage to prevent insect damage is just as important, or more so, than managing the crop while it is growing in the field. When insect infestations develop, they can reduce grain weight by up to 10 percent. A farmer with a grain bin containing 30,000 bushels of corn valued at $3.00 per bushel would lose $9,000 if an insect infestation developed and consumed 10 percent of the grain mass. This loss does not include dockage or the cost of eliminating the insects from the grain.
To prevent losses from insect feeding, growers should implement a sound integrated pest management (IPM) program in their grain bins. An integrated pest management program involves: (1) grain bin and harvest equipment sanitation, (2) proper harvest equipment adjustment, (3) grain cleaning, (4) uniform distribution of the grain during uploading into the bin, (5) moisture management, (6) temperature management, (7) periodic grain monitoring, (8) residual insecticide treatment for long-term storage, and (9) fumigation as a last resort. If the first eight tactics are used effectively, fumigation should be needed only rarely.
Preparing the grain bin to receive grain is one of the most important steps in an IPM program. Before placing new grain in a bin, remove all old grain, which can act as a potential source of insect populations that will infest the new grain rapidly. Once you have removed the old grain, clean the bin. Sweep or wash off the walls and floor, and be sure to remove grain from any ledges above the door or between the ceiling and walls. Use a shop vacuum to remove debris from under metal flashing at the floor/wall interface and other tight areas that cannot be reached with a broom or hose. It is a good idea to remove the false floor and grain auger once every 5 years or so and clean out any grain residue and dirt.
Remember that the insects that infest grain can be very small, ranging in size from 1⁄16 to about 1⁄2 inch in length. Large numbers of these insects can be harbored in very small amounts of debris. If half of an initial population of 1,000 insects is female, and if each female produces 200 offspring, the population can produce approximately 10 million insects in two generations. At a grain temperature of 90°F, many species produce one generation per month. If a grain handler detects live insects in your grain, you may be docked or sent home with the grain to eliminate the infestation. A clean grain bin is a good start toward preventing insect injury.
After cleaning the interior of the grain bin, remove old grain that has collected around the outside of the bin. Bury or discard this grain to eliminate any inhabiting insects. Also remove or mow any weeds that have grown up around the outside of the bin. Because many grain-infesting insects are seed feeders, they can develop on the seed of these weeds and then move into the grain. Weedy areas also can harbor rodents that can enter grain storage structures, especially wooden ones. After cleaning, inspect all areas of the bin for leaks or gaps where insects could enter. Repair any cracks and continue to monitor the facility for damage.
Once you have prepared the grain bin, make sure that your harvest equipment is cleaned and properly adjusted. A corn picker or grain combine that was used last year and then parked in the equipment shed without cleaning can harbor insect infestations at the lower ends of augers or on screens inside the combine. Remove plant residue from all augers and other areas within the combine before using the equipment to harvest grain. It also is important that a combine be adjusted properly to prevent shattering of the grain. Small pieces of grain make it easy for grain-infesting insects to become established and develop; most insect species have a difficult time feeding on whole kernels of grain.
Grain harvested with a clean and properly adjusted combine also should be run through a grain cleaner to further remove fine materials on which insects can feed. Grain with many broken kernels also can be run through a cleaner, but this will lead to additional weight loss. Clean, whole grain helps with insect management, but also contributes to proper aeration of the grain mass during drying and storage. If fine materials are absent, air drawn into the grain from outside the bin will move more uniformly through the mass. A grain distributor is extremely helpful in preventing aggregations of fine material by spreading it uniformly across the grain mass. When a distributor is not used, the fine material will concentrate in a column at the center of the grain mass. Because this column is more dense, air will move around it through the grain mass and allow moisture to build up in the center of the bin. Moisture buildup is the first step toward deteriorating grain quality.
Lowering the grain temperature and levels of moisture will help inhibit fungi, bacteria, and insect feeding and reproduction. Thus, harvesting and storing grains during cool, dry periods will help prevent pest infestation. Reduce the grain moisture to 13 percent or lower while loading or after the grain has been loaded into the bin. Level the grain surface and remove any peaks. As with a fine material buildup, the peak in the grain does not dry properly because air moves through the grain mass below it. Once you have loaded the clean, dry grain into the bin and leveled off the top, the next step is to cool the grain after outside temperatures fall below 40°F in late fall. Lower the grain mass temperature by running the fan during periods when the temperature is below 40°F. If possible, do not run the fan if the outside air is moist; wait until the air is dry. The amount of time necessary to cool the entire grain mass varies depending on the fan capacity, the initial bin temperature, and the height of the bin. Do not cool the grain below 32°F. An airflow rate of 1⁄10 cubic foot of air per minute per bushel usually is ample to cool dry grain.
After the grain has been prepared properly for storage, its quality still can decline if not properly monitored. During the winter, check grain for moisture and temperature buildup once every 2 months. During the summer, check grain once a month. Changes in grain moisture content and temperature are the first signs that grain quality is declining. Moisture migration can occur in a bin when outside temperatures are greater or lower than the grain mass temperature. When the outside temperature is cooler than the grain temperature, air in the grain mass drops along the bin walls and then rises in the center of the warmer grain mass. As the air rises, it picks up moisture and drops it in the center at the top of the grain. When the outside temperature is warmer than the grain mass, the air circulates in the opposite direction and deposits moisture along the bin wall at the top of the grain mass.
As the moisture level increases, fungi and bacteria begin to grow as they feed on the grain. Through respiration, these organisms give off heat. This cycle then continues to drive up the grain temperature, which in turn increases the moisture levels. Once the temperature rises above 40 to 50°F, insects can begin to develop and reproduce. The higher the temperature, the faster the insects complete each generation and increase in numbers. This cycle will continue until the grain is dried and the temperature is brought back down using the fan.
Once grain crusting occurs, drying efficiency is reduced because the air will not move through this dense area in the grain mass. It may be necessary to break up the crusted area. Prevent this cycle from getting out of control by monitoring the grain periodically and using the fan as your primary management tool.
If you will be storing the grain for only 6 to 9 months, these measures should be sufficient to prevent insect infestations. If you will be storing the grain for more than 9 months, however, you may need a residual insecticide treatment (Table 1.3-2). Cooled grain will maintain temperatures below 60°F well into the summer, but by late summer temperatures may rise to levels at which active insect populations will colonize the grain and build up population levels. The longer grain is stored, the greater the chances are that insect infestations will develop. When possible, do not store grain for more than one year. The best residual insecticide treatments provide only 18 months of protection.
Be sure to keep grain storage facilities animal proof. Rodents, birds, cats, and other animals can quickly contaminate clean grain.