Controlling Indianmeal Moths
in Shelled Corn and Soybeans

Copyright ©  2002  Regents of the University of Minnesota. All rights reserved.

Identification and Biology

The Indianmeal moth, Plodia interpunctella (Hubner), is a common insect pest of stored grain (especially shelled corn) and is the only storage insect pest of harvested soybeans in Minnesota. The adult moths have a wing span of about ¾ inch and are easily distinguished from other stored grain moths because the outer two-thirds of their front wings are reddish-brown with a copper luster. The remainder of their wings is whitish-gray.

Each female moth can lay 100 to 300 eggs on the surface of the grain and within 3 to 5 days, depending primarily on the temperature, small larvae (caterpillars) emerge from the eggs to feed. When fully grown, the larvae are about ½ inch long and dirty white, although they may have greenish to pinkish tints depending on the food they eat. Young larvae are difficult to detect, especially if only a few are present. Infestations are usually established before the infestation is detected unless special monitoring methods are used.

The larvae feed primarily on broken kernels, grain debris, chaff, etc. They generally stay within 18 inches of the surface of the grain and spin a silken web. They also infest aeration ducts and below perforated bin floors in the plenum area. Undisturbed surface grain or soybeans with heavy infestations may become completely webbed over. Underneath and within this webbing the larvae are partially protected from changes in their environment. Because the webbing protects larvae from the application of insecticides, it must be removed before treatment. This webbing also prevents uniform grain aeration. In addition to the webbing and feeding, these insects contaminate the grain with their feces and cast skins.

The larvae spin cocoons when they become fully grown. Many of the cocoons are formed on the grain's surface, but some of the larvae move out of the grain and make their cocoons on bin braces, on rafters, or beneath perforated bin floors. The moths emerge from the cocoons, mate, and lay eggs to continue their life cycle. Moths live for about one week. During the summer, this cycle can be completed in 4 to 6 weeks.

Adult Indianmeal moths and larvae on corn

Adult Indianmeal moth

Preventing Infestations

The most favorable temperature for Indianmeal moths is about 80°F, although they become active following winter when grain or air temperatures above the grain rise to 60°F. Since they infest surface grain, which is usually the first to warm up with increasing spring temperatures, they become active relatively early compared to beetles and weevils that may have penetrated deep within the grain mass.

Malathion is no longer effective against most populations of Indianmeal moths. Research in Minnesota documents that the dosage must be increased 300 times to obtain 50% control. No such malathion application is legal. Consequently, other alternatives must be used.

Currently, the dichlorvos "Farm Pest Strip" is still registered for control of adult moths. Suspend one dichlorvos strip per 1,000 cubic feet of overhead space above the grain.

Pirimiphos-methyl (Actellic^®) is a grain protectant that is now registered in the United States for use in preventing insect infestation in corn and grain sorghum. Actellic must be applied as a spray solution when the grain enters storage. Apply 12 fluid ounces of Actellic in 5 gallons of water to each 1,000 bushels of grain. Make only one application per crop, calibrate spray equipment periodically, and do not store diluted Actellic in your spray tank more than 48 hours before use.

Phosphine, methyl bromide, and chloropicrin are registered for application on corn and stored small grains. All are toxic to the life stages of Indianmeal moths. Phosphine is the only fumigant that can be used both on soybeans and stored grain. It may be added as the grain is being binned, or it can be probed into the grain surface after the bin is filled. For details, see Fumigating Stored Grain (FS-1034).

Detecting Indianmeal Moths

As stated earlier, when the grain surface warms to 60°F in the spring, the larvae of the Indianmeal moth migrate to the top of the grain mass in search of a place to pupate and become adults. As these adults emerge, an effective way to detect them is by the use of pheromone traps (see Figure 1). Pheromones are chemicals secreted by the female moth to attract the males to a particular site. After entering the trap the males contact a glue-laced surface and are unable to escape. These traps should be suspended inside the storage structure near the roof but within 10 feet of the grain surface. The moths will be attracted to these traps for as long as six weeks but they should be checked for moth activity every 5 to 10 days.

Do not consider these traps as a means of moth control. They are only effective to detect and monitor moth infestations. Factors affecting the number of moths caught in pheromone traps are:

1. Temperature—Air or grain temperatures less than 60°F inhibit moth activity.
   
   
2. Life Stages—Normal life cycle fluctuates in the moth populations to produce peaks of adult moth activity. Visual observations of the larval stage supplement the trap detection.
   
   
3. Placement—Putting traps too far away (more than 10 feet) from the grain or placing them in direct air drafts reduces trap efficiency.
   
   
4. Dust and Damage—Dust can prevent the glue from holding adult moths and a mutilated trap cover or base may inhibit moth entrance.
   
   

These traps are sold by many distributors of grain fumigants and protectants.

Phil Harein and Bh. Subhramanyam
Entomology

Issued in furtherance of cooperative extension work in agriculture and home economics, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Patrick J. Borich, Dean and Director of Minnesota Extension Service, University of Minnesota, St. Paul, Minnesota 55108.

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