Stored-product Insects and Biological Control Agents

Vera Krischik
Department of Entomology, University of Minnesota
St. Paul, MN

Wendell Burkholder
USDA ARS and the University of Wisconsin, Madison (retired)

Reproduced with permission of authors from Stored Product Management, Chapter 13 Stored-product Insects and Biological Control Agents. Publication E-912, Oklahoma Cooperative Extension Service, Oklahoma State University, Stillwater, OK 74078

Stored grain is subject to insect infestation and deterioration from molds and bacteria. High grain temperatures and moisture, along with dockage and broken kernels, provide conditions that accelerate mold and insect development. Many grain insects are good fliers and move to newly stored grain from fields and from infested grain bins. Insects can reach a high population size in unchecked grain bins, in subfloors or aeration ducts in bins, in equipment used to move grain, or in discarded refuse grain. These areas must be kept free of insects to reduce migration to newly harvested grain.

Grain insects move within the grain mass at a rate that is determined by the and grain temperature. During the summer and fall, insect infestations are usually on the surface of the grain. In cold weather, insects congregate at the center and lower portions of the grain and may escape detection until high population numbers are reached.

The most favorable grain moisture range for stored grain insects is from 12 to 18 percent. In many instances, insect infestation amplifies mold problems in grain by exposing otherwise hidden endosperm surfaces to molds, transporting mold spores to new areas in the grain, and encouraging mold germination in microhabitats made moist by insect metabolic activity. Indeed, insect and mold metabolic activity can raise grain temperatures to 110°F (43°C).

It is important to control insect population size before grain is irrevocably damaged by insect boring, feeding, and mold germination. Grain should be inspected every 21 days when grain temperature exceeds 60°F (15°C). Plastic pitfall traps should be checked for the species and numbers of insects, and grain temperatures should be monitored. The number of insects found in a trap should be recorded and charts constructed so that changes in population size can be easily noticed. Increasing numbers of insects indicate that management tactics need to be changed to prevent levels of infestation that damage the grain. Also, grain can be inspected by screening or sieving and searching in the screenings for insects, examining kernels for damage, checking grain for webbing, and investigating off odors.

Some insects damage grain by developing inside kernels (egg, larvae, pupae), feeding on the inner endosperm, and producing holes in the kernel through which the adult insects exist. The cycle is repeated when the female lays eggs inside the kernels. The maze weevil, rice weevil, granary weevil, lesser grain borer, and Angoumois moth all develop inside the kernels. Other insect species do not develop within the kernels, although they may hide inside cracked grain, making detection very difficult.

Species such as the flat grain beetle, rusty grain beetle, and the foreign grain beetle feed primarily on mold. Other species such as the sawtoothed grain beetle, the red and confused flour beetles, the Indianmeal moth, and the larger black flour beetle feed on damaged grain or fines. Pest species vary in different parts of the U.S., although all stored grain insects are capable of decreasing grain quality.

Insects damage grain by boring into the kernels and reducing grain quality through weight, nutritional, or quality loss; spreading and encouraging mold germination; adding to the fatty acid content of the grain; and leaving quantities of uric acid that cause grain rancidity. Insects also create fines and broken kernels when feeding that reduce air flow through grain and prevent proper aeration when fans are used. In addition, the presence of insects in a grain sample can cause cash discounts for the grain.

Two insects of any kind in 1,000g of wheat, rye, or triticale cause the grain to be graded as U.S. Sample grade, the lowest possible grade. In corn, soybeans, and sorghum, the tolerances for insect infestation are different. Grain may be designated as Sample grade if two weevils, one weevil and five other live insect harmful to stored grain (OLI), or 10 OLls are found in 1,000g of corn or sorghum. Insect tolerances in finished commodities such as flour or cornmeal are stricter.

It is important to distinguish between species of stored-grain pests since the insects have different damage potentials, biologies, growing temperatures, moisture requirements, and reproductive potentials. Insect species create different types of damage and have different activity periods.

The following colored drawings are part of the USDA Federal Grain Inspection Service (FGIS) Interpretative Line Slide Series for insects. Both the slides and caption cards are available through Seedburo Equipment Co., Chicago, Illinois. There are three categories in which an insect can be placed according to the FGIS insect tolerances for a grain: LW is a weevil or borer; OLI is an insect injurious to stored grain; and NOLI indicates that the insect is not counted toward the tolerance.

These pictures and caption cards provide a way of identifying the insect pests and include a description of their basic biology. Identifying insect pests is the first step in understanding and controlling insect problems in grain bins and commodity storage warehouses. Insect traps are useful in either grain storage bins or commodity storage warehouses for collecting insects for proper identification. A knowledge of insect biology and appropriate control strategies is necessary for Integrated Pest Management programs in both grain bins and commodity storage warehouses.

In the 1990s, policies were amended by FGIS to permit greater inclusion of Integrated Pest Management tactics in the grain inspection and marketing system.

Previously, insects found in traps were considered filth and could be used by regulatory agencies as indicators of unsanitary conditions during routine sanitary examinations. This precluded the use of insect traps in bulk commodities and warehouses. However, insect traps are an important part of IPM for monitoring species presence and numbers. In 1990 the FDA in a written statement to FGIS, acknowledges that insect traps could be used without concern as part of a preventative pest management program. Insects found in traps in a condition sanitary exam would not be used as part of a negative sanitation assessment as long as the traps were not neglected and became a source of infestation. Insect sampling devices such as plastic pitfall traps used in bulk products and insect food/pheromone traps used in food warehouses permit the monitoring of the product for changes in insect population size and species.

In 1991, diatomaceous earth was approved for insect control in bulk grain. Diatomaceous earth kills insects with its abrasive qualities and does not leave unacceptable pesticide residues as do some contact insecticides. Diatomaceous earth is now acceptable as an additive to grain as long as its presence is written on the label of the submitted sample. If diatomaceous earth is identified as an unknown foreign substance in an inspection, the grain can be labeled as sample grade, the lowest designation. Consequently, it is advisable to identify the diatomaceous earth when the grain is submitted for grading.

Another development in policy for integrated pest management in grain in 1991 was the exemption from tolerance by the EPA of predatory and parasitic insects used as biological control agents in grain. Only certain species of hymenopterous parasitoids and predators are included in the EPA ruling, but difficulty in identifying the insects to species in the field may result in a broader acceptance of predatory and parasitic as long as the insects are used for biological control. Some of these biological control agents are represented in the FGIS Interpretative Line Slides for Insects.

USDA FGIS Interpretative Line Slides for Insects


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