Over 60 species of insects infest stored grains and can greatly affect its quality and subsequent value. Damage can go unnoticed until the grain is removed from the storage facility. Managing stored grain insects requires the use of various techniques including sanitation; storing sound, dry grain; managing temperature and aeration; using chemical protectants; regular sampling; and fumigation. Bin facilities also play an important role in determining whether grain quality is maintained. Bin facilities should be inspected regularly for deterioration of any type.
Stored grain insects can be categorized as follows:
1. Weevils and other insects that feed inside kernels. These insects are considered primary pests because their eggs are laid on or in whole, undamaged kernels. The immature stages of these insects occur on the inside of the grain where detection is more difficult. The most common pests include rice, maize, and granary weevils, and lesser grain borers. They are small (0.0625 to 0.125 inch long), and the weevils recognized by the prolonged snout on the head. These insects can be found in any portion of the grain mass.
2. A complex of beetles, mites, and book lice called bran bugs that develop and feed outside the kernels. These insects are considered secondary pests because they attack only cracked kernels (or other fine material such as flour) or fungus growth on moist grain. Species that are often collected in grain bins include the sawtoothed grain beetle, flat grain beetle, rusty grain beetle, foreign grain beetle, hairy fungus beetle, larger black flour beetle, red flour beetle, book lice, and grain mites. The adults of most beetle species are 0.0625 to 0.5 inch long with reddish-brown to black bodies. Larvae are usually cream-colored and cylindrical. Like weevils, these beetles are not confined to the top layer but are found anywhere in the grain mass. Concentrations of secondary beetles produce an increase in grain temperature and moisture, resulting in fungus growth and more favorable beetle population growth.
3. Surface-feeding caterpillars. The Indianmeal moth is the most common pest in this group. These cream-colored caterpillars (about 0.75 inch long) inhabit primarily the outer portions of the grain mass, usually the top layer but also the bottom just above perforated drying floors or aeration ducts. They produce fine, silken webbing as they feed and move about. Mature larvae pupate within silken cocoons. The reddish-brown adults (about 0.75-inch wingspan) fly and mate in the bin headspace where they can be seen resting on the bin walls and roof.
For insect prevention, as well as other reasons, bin facilities should be weathertight, rodent proof, steel, and on a moisture-proof concrete base. They should be separated from areas where hay, straw, feed, and animals are housed. Bins should be equipped with a perforated-floor aeration system and weatherproof roof vent. All bins should be inspected on a regular basis to guard against leaks and deterioration of any kind. Once filled, attempt to seal the bottom and sides of the bin so insects and rodents can only enter the top of the facility. Do not seal roof aeration exhaust or inlet vents except during fumigation so the top of the bin can be easily sampled and topdressings applied if necessary.
For many reasons, stored grain should be clean, not cracked, and have a moisture content of 12 percent or lower. Insects have a more difficult time multiplying to serious levels if grain moisture is 12 percent or lower. Mold activity is greatly reduced at moisture contents below 14 percent.
Insect infestations in stored grain generally begin as the result of poor management practices and from insects already established in old grain and grain debris that was not removed or fumigated before storage of clean grain. Pests in stored grain rarely infest grain in the field; therefore, do not store new grain on top of old grain unless absolutely necessary. However, limited storage space forces many growers to violate this rule of maintaining harvests separately. If long-term storage is planned, and live, damaging insects are detected in stored grain, having the old grain treated with a registered fumigant should reduce the risk of serious losses. (Refer to the section on fumigants.)
The following suggestions to prepare bins should be completed 2 or more weeks before storing grain: (1) Brush, sweep, or vacuum the combine, truck beds, transport wagons, grain dumps, augers, and elevator buckets to remove insect-infested grain and debris. Do not forget to burn, feed, or otherwise destroy the first few bushels of grain that leave the combine auger. (2) If possible, thoroughly sweep or brush down walls, ceiling, ledges, rafters, braces, and handling equipment and remove debris from bin. (3) Remove all debris from fan, exhaust, and aeration ducts (also from beneath the slotted floor when possible). (4) Feed, burn, bury, or otherwise destroy the removed debris because it usually contains insect eggs, larvae, pupae, or adults ready to infest the new grain. (5) Remove all debris and vegetation growing within 3 to 10 feet of the bins.
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 to 90°F. Farm-stored wheat, rye, barley, or oats are more likely to have insect problems than corn or beans, which are harvested during the cooler months of the year. Bin aeration during times of low outside temperature and humidity will aid in reducing the temperature of the grain. In the southern United States, it is recommended to maintain warm temperatures and low humidity until cooler temperatures arrive, and then to cool the grain to 55 to 65°F or lower as soon as possible.
Several practices should be followed when filling bins to permit even aeration of the grain mass. The upper surface of the grain mass should be level or slightly inverted to permit even aeration. The use of a grain spreader will help prevent the accumulation of fines (broken grain, weed seed, dust, and debris) in the center when filling bins. If not spread evenly, this material will accumulate in the center of the bin, preventing even aeration and providing an excellent environment for insects and fungi to develop. The accumulation of fines in the center of the bin can be greatly reduced by removing a portion of the grain mass after the facility is filled several times during the loading process. Removing the core from the bottom with the centrally located unloading augur or conveyer will remove the column of fines and invert the peaked grain in the top of the tank. After this process is completed the grain can be left alone or leveled. This process, called “coring,” will increase aeration efficiency and reduce problems with insects, fungi, and hotspots. 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 convective flow of air can result in accumulating moisture condensation in the upper center of the grain mass. These factors will contribute to the development of molds and insects.
After empty bins are cleaned, spray all surfaces inside and, if possible, outside the bin with a residual surface insecticide. (See accompanying table.) Inside bins, spray walls, ceiling, ledges, braces, rafters, and floor. Outside bins, spray the base and the walls up to 15 feet above the base and the soil around the bins.
Surface insecticides should be applied 2 to 5 weeks before the grain goes into the structure to provide the best kill of existing infestations. A lengthy time period after treatment and before binning the grain is suggested to allow eggs to hatch and hidden insects to cross the toxic barrier and die. Use a garden or power sprayer at a pressure of at least 30 psi and apply spray solution to “run off” (2.5 gallons of mixed spray per 1,250 square feet of surface or as directed on the label). Do not mix more than 1 day’s supply of diluted spray at a time. Spraying the outside of a structure just before the grain inside is fumigated is often worthwhile. Insects temporarily leaving the grain to escape the fumigant are killed before they can reenter the structure.
