By John W. Van Duyn and Steve Bambara
Many types of insects attack small grains in storage. Because small grains are usually harvested in late spring and early summer and stored when temperatures are high, insects can develop rapidly within the grain. Therefore, insect problems in storage are more severe in small grains than in other grains, such as corn, that are harvested and stored during cooler fall and winter months. If a problem occurs, the first step is to identify the insect pest. Producers not familiar with stored-grain pests should consult their county Extension agent for assistance.
Weevils
The rice weevil and maize weevil are common pests. These weevils look similar, although the maize weevil, usually found in corn, is somewhat larger and darker than the rice weevil, which is more often found in small grain (Figure 12-1). Both are small snout beetles, about 1/8 inch long, and are reddish brown to almost black. The wing covers are usually marked with four reddish or yellow spots. Eggs are laid within individual kernels, and the grub-like larvae consume the grain from within. Pupation occurs in the kernel, and adults emerge through a small round hole, leaving behind a hollow kernel. During warm weather, an entire generation may be completed within 26 days; thus, stored grain may be severely damaged within a month of harvesting. Infestations may start near the top of a storage bin (as a result of insects that fly in from outside) or near the bottom (caused by insects that migrate up through the perforated floor). Weevils are very mobile and may be found anywhere within the grain mass.
Figure 12-1. Adult rice weevil
Lesser Grain Borer
Adult and larval stages of this insect feed on and within kernels. Grain produced in North Carolina is only occasionally infested by this insect. Also, grain shipped in from the Midwest may be infested. The beetles have a slender, cylindrical form, with the head turned under the body. They are dark brown or black and are slightly less than 1/8 inch long. Eggs are laid in the grain. After they hatch, the young larvae feed upon debris or flour produced by the boring beetles. In a short period, larvae bore into the kernels and feed from within. Lesser grain borer populations can build up rapidly in warm weather and can cause significant losses. The beetles can develop throughout the grain mass and cause weevil-like damage.
Grain Moths
Indianmeal moths and Angoumois grain moths (Figures 12-2 and 12-3) usually feed on the exposed surface of the stored grain mass. They rarely penetrate more than 1 foot below the surface. Therefore, their damage potential is somewhat limited. However, damage and contamination from these insects can cause an economic loss.
The Angoumois grain moth is a small, buff-colored or yellowish brown moth with a wingspan of about ½ inch. Infestation may occur in the field or within bins. Under normal circumstances, eggs are laid on the outside of the grain and the larvae bore into and develop within the grain. The larvae are small, white caterpillars with yellowish heads and grow to 1/5 inch long. Important in identifying this insect is the small round emergence hole it produces in each infested kernel.
Indianmeal moth caterpillars feed from the outside of the kernel and primarily destroy the germ. They also feed on dust, chaff, and broken kernels. Thus, they are more of a threat to seed grain than grain intended for feeding purposes. The moths have a wingspan of about ¾ inch with reddish brown to copper-colored markings on the outer two-thirds of the front wings. Larvae are dirty white, about ½ inch long, and may produce a great deal of webbing. They stay on the surface of the grain and do not develop deep within the grain mass. Severe infestations are covered with large amounts of surface webbing that may clog unloading and grain-handling equipment.
Figure 12-2. Indian meal moth
Figure 12-3. Angoumois Grain Moth
Secondary Feeders
Many insects are secondary feeders on grain, but in North Carolina this group is usually limited to one or more bran beetles. Most likely to cause problems are the red flour beetle, the saw-toothed grain beetle (Figure 12-4), and the rusty grain beetle (small reddish beetle with head and antennae pointing forward). These insects do not attack whole kernels and are limited to feeding on grain fragments or damaged grain, so they are more likely to damage milled products than in stored whole grain. However, these insects can be important sources of contamination in stored grain. Indianmeal moth larvae, discussed in the primary insect section, may also become serious secondary feeders.
Figure 12-4. Adult saw-toothed grain beetle
Prestorage Procedure
Insect management for stored grain depends upon good sanitation and grain storage practices. Clean grain-handling equipment before harvest, and discard or feed to livestock the first few bushels that come through the combine and auger. Clean nearby feed storage areas, feed rooms, and similar areas to reduce the potential for insect migration into the new, noninfested grain.
Also, before harvest thoroughly clean inside, around, and under the empty bin. Although it may be diffcult and time-consuming to remove and clean under the perforated floor, most insect problems originate in carryover material from this area. Take up the floor periodically, if possible. Spray the bin walls, roof, and floor to the point of runoff with Tempo, Reldan, methoxychlor, or malathion according to label guidelines. Be sure to treat cracks, crevices around doors and behind false partitions and similar voids. Sprays will not successfully control insects hiding in grain debris below nonremovable perforated subfloors. However, chloropicrin fumigant (sold as Larvacide, Quasar, and by other names) will control insects below the perforated floor if used properly. Chloropicrin tear gas is a restricted-use pesticide, and the applicator must be properly certified. Before applying chloropicrin, read label carefully and use proper safety equipment during the application. After cleaning and spraying the bin, seal the fan with polyethylene film and duct tape or elastic bands, and similarly seal other openings in the lower half of the bin to make them gas tight. Because chloropicrin is much heavier than air, no sealing is needed around the top of the bin.
Post fumigation warning signs on entrances to the bin. Signs are available from the dealer. Climb to the top of the bin and pour in 1 pound of chloropicrin per 1,000 cubic feet of underfloor area through the center roof opening, letting it splash onto the bin floor. Choose the right size container for the bin, and pour in all of the contents at one time. It has a very strong odor, so pour at arm’s length, and keep your head out in the fresh air. Wait at least 48 hours; then unseal the fan and run it at least 24 hours before adding grain. Run the fan until the odor is gone.
Stored grain should contain minimal foreign material and a moisture content of 13 percent or less. Do not mix new grain with old grain. Grain bins should not be overfilled. Once the bin is full, a load or two should be removed and fed to the livestock or stored in another location and used as soon as possible. This removes many of the hard-packed fines in the center of the bin and also makes leveling easier. The leveled grain surface should be at least 8 inches below the lip of the bin; this allows for a topdressing application of grain protectant and also makes effective examination and fumigation easier.
Once the grain is dried to 13 percent moisture or less, cool it as soon as possible by running aeration fans on cool nights. Reducing the grain temperature to less than 60ºF stops insect reproduction and lowering it to less than 50ºF stops insect feeding. While this may not be possible in June or July, the sooner the temperature can be lowered, the better. Aeration fans may be run whenever the air is cooler than the grain. Cooler air will not wet dry grain, but the grain must be dry (13 percent moisture or less) before it can safely be cooled.
Protecting Stored Grain
Apply liquid grain protectants to the grain as it is being augured into the bin to ensure adequate coverage. Malathion is no longer registered for direct application to grain going into storage. Reldan or pyrethrum can be used on small grains and grain sorghum. Actellic can be used on corn or grain sorghum but not on small grains. Reldan gives the longest protection, up to 1 year, and is effective on all common grain-infesting pests except the lesser grain borer. Pyrethrum has very short residual protection and is not often used for long-term storage. It also is often in short supply and hard to obtain. No effective protectant for the lesser grain borer in stored small grains is available.
After applying the protectants, topdress with additional pesticide if allowed; always follow label instructions. To keep the pesticide barrier intact, do not disturb the grain after treatment. If pyrethrum is used, apply an additional liquid or dust topdressing of Bacillus thuringiensis (Bt) to control Indianmeal moths.
As stated earlier, cool the grain below 50ºF as soon as possible. Stored-grain insects generally do not reproduce at temperatures below 60ºF or feed below 50ºF.
Inspect the grain at monthly intervals (weekly when the temperature is greater than 60ºF). Use probes and appropriate equipment to monitor temperature, moisture, and insect presence at several sites and depths. Even when outdoor temperatures are low, moisture, insects and sunlight may produce areas within the grain mass that are warm enough to allow insect development. Therefore, be sure to inspect the grain frequently and thoroughly.
Handling Infested Grain
If grain becomes infested, the best option may be to cool it to less than 50ºF and feed it to livestock. Seriously damaged grain may be discarded. If only secondary (surface-feeding) insects are involved, it may be possible to treat the grain with grain protectant as it is moved into a clean bin. This, of course, will not control insects feeding within individual kernels, such as weevils or Angoumois grain moth larvae. Contrary to popular belief, it is a waste of time to try to apply such grain protectants by using an aeration fan; they simply will not penetrate far enough to do any good.
Fumigation
Infested grain that cannot be handled as suggested above should be fumigated. It is strongly advised that this be done only by professional fumigators. They can do a better job and the overall cost may be less. Aluminum phosphide (Phostoxin, Fumitoxin, etc.) is best in most circumstances. Methyl bromide may also be used, but it is not as practical under most farm storage conditions. Five days in a very tightly sealed storage are normally required for aluminum phosphide fumigation, depending on temperature. Methyl bromide may give control in 24 hours, but recirculation, which is difficult in most farm bins, may be required.
Fumigation is tricky and potentially dangerous. It should be done professionally unless the producer knows how to do it right, has the equipment to do it properly, and is properly certified. Careless (or uninformed) fumigators die young!
For more information on grain protectants and fumigation, refer to the latest edition of the North Carolina Agricultural Chemicals Manual, which is available on the Web at: http://ipm.ncsu.edu/agchem/chptr5/504/pdf, and follow all label directions. A properly managed grain storage operation rarely needs fumigation.
When using any agricultural chemical, be sure that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your county Extension agent.
Some original materials authored by Dennis Keever and Rudy Hillman, retired Entomology specialist.
This is a chapter from Small Grains Production Guide, 2004-05. Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by the NC Cooperative Extension Service nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your county Cooperative Extension Center.
Return to Index: http//www.smallgrains.ncsu.edu/Guide/cover.html
Published by
North Carolina Cooperative Extension Service
Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. North Carolina State University and North Carolina A&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, or disability. In addition, the two Universities welcome all persons without regard to sexual orientation. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
9/04—3M—JMG (Revised) AG-580 EO4-43980
Last Revised Sept. 2004