Indian Meal Moth
One insect is found more often than any other on stored food and grain in the United States. This is the dreaded Indian meal moth (IMM). Since they feed on any items containing grain or cereal products, the Indian meal moth alone is responsible for most of the insect problems associated with the food industry. This is one of the easiest stored food insects to identify because the adults have a colorful appearance and the larvae (which look like yellow worms) leave a silken webbing trail wherever they crawl. Newly emerged adults have bi-colored (copper and tan) wings and measure about ½” from top to bottom. The IMM most commonly feeds on grain products including flour, cereal, nuts, bird seed and pet food.
The larvae of the Indian meal moth, Plodia interpunctella, under some conditions are also good penetrators and may be the most serious pests of packaged foods (Mullen and Highland 1988, Mueller 1998).
*Courtesy Insects Limited, Inc.
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The Indian meal moth is a handsome moth with a wing expanse of nearly three-quarters of an inch. It is easy to distinguish from other grain pests by the peculiar markings of the forewings; they are reddish brown with a copper luster on the outer two-thirds, but whitish gray on the inner or body ends. The hind wings lack distinctive markings and are more or less uniformly gray. Adults can be seen resting on the grain surface or grain bin walls. The adults fly at night and are attracted to lights.
The eggs of the Indian meal moth are whitish, ovate and very small. Because of their small size, they are difficult to see without the aid of a microscope. Eggs are deposited on the grain surface singularly or in groups of twelve to thirty.
Newly hatched larvae are very small and difficult to see. Larger larvae are usually yellowish, greenish, or pinkish. Fully grown larvae are one-half to five-eights of an inch in length with a brownish head capsule. Larvae have three sets of legs near the head (thoracic legs) and five sets of prolegs on the abdomen. Larvae of the meal moth spin a web as they become fully grown and leave behind silken threads wherever they crawl. The webbing is often sufficiently abundant to attract attention. Loosely clinging webbing on the grain is characteristic of this pest.
As long as the temperature within a grain bin or building where grain is stored remains above 50° F, the Indian meal moth can survive and reproduce. A typical life cycle (egg to adult) is completed in forth to fifty-five days. A potential for seven to nine generations per year exists; however, because of cool temperatures during the winter months fewer generations are usually completed. Under optimal conditions, the entire life cycle can be completed in approximately twenty-eight days.
A mature female lays 100 to 300 eggs on food material, either singularly or in groups of twelve to thirty. Larvae begin to hatch in two to fourteen days, depending on environmental conditions. Newly hatched larvae feed on fine materials within the grain and are small enough to pass through a sixty mesh screen. For this reason, it is difficult to exclude larvae from most packaged foods and grain.
However, larvae cannot chew through packages, so they must enter through a hole or at the seam. The larval stage lasts from two weeks to one year, and is responsible for grain losses. In grain, larval feeding is usually restricted to the top one to two inches. Large larvae feed on the grain germ. When mature, larvae spin a silken cocoon and transform into light-brown pupae. The cocoons and pupae can be seen on the grain surface and walls of grain bins. Adults emerge in four to thirty days, mate, and females lay the next generation of eggs. Adults live from five to twenty-five days.
Direct damage to grain is the result of larvae feeding on the seed germ. In grain to be sold for human or animal consumption, meal moth feeding reduces the dry weight. At the same time, grain weight may actually increase because of water absorption; with an increase in water content mold can become a problem. The biggest reduction in value is the result of contamination by larvae that leave droppings and silken webs in the grain. The presence of live insects and insect parts can result in dockage of the grain when sold.