Insect Pests of Stored Grains and Seed

R. T. Cotton, Wallace Ashby.

The inauguration of the Government grain-loan program under the Agricultural Adjustment Act of 1938 brought a large increase in the stocks of grain carried as reserve supplies and in the length of time such stocks were held. On October 1, 1941, for example, the carry-over of corn was 646 million bushels nearly four times more than the 1928-32 average of 163 million bushels. Much of the extra corn was shelled and stored in farm-type bins; a method of storage previously unused in the United States. Although the reserve supplies of corn, wheat, and sorghum grain since 1940 fell at times as the result of increased world requirements for grain as human food or animal feeds, the years since 1948 again brought a marked increase in the quantity of grain in reserve. Every conceivable type of storage structure was pressed into service.

To meet the many problems that arose in the storage of these large reserves, State and Federal agencies started a research program to determine the best type of storage structures to use and the most efficient insect-control programs to follow. Studies were made of the changes in the condition of the grain during extended periods of storage, the insects involved, their origin, the factors favoring or deterring their abundance, temperature changes in the grain and their effect on insect abundance, methods of drying and cleaning grain, and the effectiveness of various fumigants in controlling insect infestations. Observations were made in hundreds of bins of corn, wheat, and sorghum grain in various parts of the country, in special storage sites, and under laboratory conditions.

The scope of the work is constantly changing to meet the need for information on special types of storage, to evaluate the usefulness of new fumigants, sprays, drying, cleaning and grain-handling equipment, and storage structures, and to make adjustments for shortages of insecticides or structural material.

INSECTS are one of the most important hazards to the safe storage of grain but the investigations have demonstrated time and again the basic truth that the factors favorable for preserving the keeping quality of grain are unfavorable for the development of insects.

The insects that attack stored grain are rather general feeders, but some of them definitely prefer certain grains. In the commercial corn area Illinois Iowa, Nebraska, Minnesota, and South Dakota the six species most commonly found in stored shelled corn and constituting more than 98 percent of the insect population were the saw-toothed grain beetle, flat grain beetle, red flour beetle, foreign grain beetle, larger black flour beetle, and hairy fungus beetle. The first three comprised the greater portion of the insect population. In the South, where field infestation is common, the rice weevil is by far the most abundant species and constitutes the largest proportion of the insect population of stored corn.

In the Great Plains hard winter wheat region, seven species constitute more than go percent of the insect population of wheat in farm storage the flat grain beetle, saw-toothed grain beetle, lesser grain borer, red flour beetle, long-headed flour beetle, cadelle, and rice weevil. Their abundance varies with climatic conditions. In the northern parts of the region, the hardier species, the flat grain beetle and the saw-toothed grain beetle, predominate. In the southern part, the lesser grain borer and the rice weevil become increasingly abundant. Along the eastern seaboard the Angoumois grain moth is occasionally one of the common pests of stored wheat, although ordinarily the flat grain beetle and the rice weevil are the main species there. Found in greatest abundance in rough rice in storage are the Angoumois grain moth, rice weevil, flat grain beetle, lesser grain borer, and red flour beetle.

The moths that attack grain and seed are not among the most abundant species, but they occasionally appear in tremendous numbers wherever grain is stored. They confine their activities largely to the surface grain, where the caterpillars spin silken threads that mat the kernels together and form a silken web over the tops of bins. The Indian-meal moth, the chief offender, attacks all types of grain. It and the almond moth are also troublesome in seed stores and hybrid seed-corn establishments.

INSECTS DESTROY at least 5 percent of the world production of cereal grains. A survey in 1947 by the Food and Agriculture Organization of the United Nations indicated that in 29 countries the total loss of cereals was 25,750,000 tons, of which 50 percent could be attributed to insects.

Weevils, flour beetles, and many other bran beetles devour at least their own weight of food each week. Their larvae destroy many times their own weight of food during the 3 or 4 weeks they are developing. Loss of stored wheat in the Great Plains region may be as high as 10 percent in a season. Corn in storage in the deep South may be destroyed at the rate of 9 percent a month.

Insects may do other kinds of damage. Many species feed almost entirely on the germ of the grain, so that its viability is reduced and much larger amounts of infested than uninfested grain must be used for seed. They frequently cause grain to heat. A musty odor may result; deterioration and rotting of surface grain may be caused by the translocation of water vapor from the heated area to the cooler surface grain. A lowering of the grade may be caused by off-odors or insect damage. Finally, the milling quality of infested grain is reduced by the presence in the kernels of immature stages of weevils, which are hard to detect and cannot easily be removed during the milling process.

MANY PEOPLE still believe that insects are generated spontaneously in grain probably because most of the insect pests of stored grain are so small that they remain unobserved until they have multiplied to such large numbers that the grain may suddenly seem alive with them.

The sources of insect infestations vary with crop and region.

In all grain crops in the South infestation begins in the field. The farther north the crop is grown, the less the degree of field infestation, until in the Great Plains and northward field infestation is almost negligible. Corn grown in the Corn Belt is comparatively free from field infestation except in the southern parts of Ohio, Indiana, Illinois, Missouri, and Kansas. Along the eastern seaboard, wheat may be infested to some extent in the field by the Angoumois grain moth, but in the main wheat-growing regions small grains are seldom infested in the field by this moth or by other stored-product pests. Leguminous seed beans, peas, cowpeas, chickpeas, and others are invariably infested with weevils in the field, so that the seed grower always must be prepared to prevent further damage in storage.

Besides field infestation, infestation in stored grain and seed originates in storage facilities or from nearby stores or accumulations of feed, grain, or other infested dry food products.

In the South, to minimize field infestation, early harvest is imperative. In the Gulf States, when harvest is delayed until October or November, 60 to 90 percent of the ears of corn in the field may be infested by the rice weevil and many ears almost completely destroyed. We know of no practical method of destroying insect infestation in grains and seeds in the field; hence harvest must be prompt so that control measures can be applied in storage before serious damage occurs. Prompt harvesting of small grains is desirable wherever the Angoumois grain moth occurs. It helps to prevent infestation by the moth in the field and in the bin, because the soft-bodied moths cannot make their way far below the surface of binned grain to lay their eggs. The use of combine harvesters reduces damage to grain from the insect because the unthreshed grain does not stand in shocks or lie in the mow, where the moth can continue to breed in the entire mass.

THE INSECT PESTS of stored grains and seed depend on their food supply for water. Seed or grain that is low in moisture is unfavorable for their development. The true grain weevils cannot breed in grain that has a moisture content below 9 percent, and their breeding is greatly restricted in grain unless the moisture content is above 11 percent. The bran beetles, of which the red flour beetle and the saw-toothed grain beetle are examples, do not breed in clean seed unless the moisture content is 11 percent or above or the temperature is above 80 F. If the seed contains floury dust or broken kernels, however, the beetles can breed in it regardless of the moisture in it. Moisture and temperature requirements of stored-grain insects are closely related. Up to certain levels their rate of development rises with the increase in temperature and the moisture content of the food.

Molding in bins causes considerable loss because the grain was not properly dry when stored or because water or snow leaked into the bin during the storage period. The percentage of loss is heaviest when temperatures are high.

Under storage conditions similar to those in Nebraska, wheat, oats, shelled corn, and like grains not intended for planting can be stored for a year with very little loss if the grain moisture content does not exceed 13 percent. Soybeans, which contain a high percentage of oil, are more difficult to store, and their moisture content should not exceed 11 percent for long-time storage. If the grain or seed is to be planted, the moisture content should be 1 or 2 percent less than we have indicated. The warmer the climate, the lower the moisture content at which it is safe to store grain or seeds.

Ear corn, since it is harvested in cool weather, can be stored safely in well-ventilated cribs in the Northern States with as much as 20 percent moisture and usually will dry to safe moisture levels by late spring.

The moisture-content requirements for long-time farm storage of grains and seeds are lower than those set by the United States official grain standards. The official standards are used primarily in marketing grain after it leaves the farm and enters commercial channels, where much of it is processed in a few weeks. Also, commercial handlers have better facilities than farmers for caring for grain. The bins in most large elevators have electric thermometers that enable the operator to read on a central instrument board the temperature of the grain at several places in each bin. The elevators have driers, conveyors, and elevator legs that make it easy to move the grain from one bin to another to break up hot spots or to mix damp grain from one bin with dry grain from another and thus reduce the average moisture content to a safe level for short-time storage.

Farmers hesitate to dry grain to safe moisture contents because in many markets they lose money if they have fewer pounds to sell even if part of that weight is water. Removing moisture from grain does not result in any loss of the dry matter, which is the valuable part. In fact, after a few months of storage, dry grain retains more of its original dry matter than moist, since the moister the grain the more dry matter is burned up by respiration of the kernel and by mold activity.