Managing Stored Grain Insects in Fall and Winter

October 4, 2002 - Managing Stored Grain Insects in Fall and Winter

Bill Wilcke, Minnesota Extension Engineer
Colleen Cannon, Minnesota Extension Entomologist

We tend to get the most questions about problems with insects in stored grain in late summer and early fall. Problems are most common during warm weather in grain that has been held over from the previous year. Farmers discover the insects when they get ready to move grain or to prepare bins for harvest of the new crop. What causes insect problems, what can be done about them, and how can they be prevented in the future?

The primary factor for development of stored grain insects is grain temperature. When grain temperature is between about 60 and 90F, insects grow and reproduce rapidly. If grain temperature can be kept below 60F, insects develop slowly, or not at all. In the upper Midwest, it's relatively easy to aerate grain to keep it below 60F in fall and winter, but it's a challenge during summer months - which is probably why we see more problems at the end of summer. Other factors that can contribute to stored grain insect problems are:

The presence of insect populations in or near the bin at the time the new crop is added. Some stored grain insects don't fly very well or very far, and often many of the insects that enter new crop grain come from the old grain and dust in the grain storage and handling facility.
Broken grain kernels and other fine material in the grain. Fine material is a problem because some insect species can only feed on broken kernels and because fine material tends to accumulate in concentrated areas that are difficult to aerate.
The presence of moldy grain in the bin. Some insects actually feed on mold, but also, mold growth produces hot spots that attract insects.

Considering that warm grain temperatures, initial presence of insects in the bin, fines, and moldy grain all contribute to insect problems in stored grain, the best strategies for managing insects include:

Cleaning bins and handling equipment thoroughly before harvest to remove the old grain, grain residue, and dust that can be the source of insect infestation.
Cleaning grain before storage to remove broken kernels and other fines, or at least filling bins in a way that minimizes concentration of fines under the bin fill spout (this includes using grain spreaders to distribute fines more evenly, moving the fill spout frequently during bin filling, or withdrawing some grain from the bin through the center unloading sump during bin filling to remove fines that accumulate in the center of the bin).
Making sure that stored grain is dry enough so that it won't mold (13% moisture for small grains; 15% moisture for corn stored through the winter, 14% for corn stored through summer, and 13% for corn stored more than one year).
Aerating grain as needed during cool weather to keep grain temperature at 20 to 30F during winter months and 50F or less during other times of the year.

There are several insecticides that are registered for treatment of empty bins and some that are registered for use on stored grain, but before buying insecticides, compare the cost for insecticide treatment compared to other insect management options and make sure that potential buyers will accept grain that has been treated with an insecticide.

What can be done if insects are discovered in stored grain in late summer? Farmers who have livestock could try to feed the grain as rapidly as possible before the problem gets too bad. Farmers who don't have livestock could consider selling the grain to a neighbor who feeds livestock. It is important, though, to be honest with the neighbor about the fact that the grain contains insects and to be prepared to accept a reduced price for the grain.

In cases where the plan is to sell old crop grain before the new crop is harvested, or where the plan is to keep old crop grain through winter, running the grain through a grain cleaner to remove broken kernels and some of the insects could reduce insect populations and improve grain storability. Cleaning the grain won't remove all the insects, but it might reduce populations enough to allow sale of the grain, or at least to buy some time for other insect management options.

If average outdoor temperatures are well above 60F and weather below 60F is not expected for some time, consider fumigating the grain. But use grain fumigation only as a last resort. Grain fumigation is costly, the chemicals used for grain fumigation are potentially hazardous to human health, and special licenses, equipment, and procedures are needed to apply fumigants. Also, since grain fumigants do not provide any residual protection against insects, fumigation is only a temporary control measure. If the insect problem developed because the grain is warm, moldy, or contains fines, insect populations will build up again soon after the fumigant dissipates.

If insect problems are discovered at a time of the year when outdoor temperatures dip below 60F at least part of the day, run aeration fans during the coolest parts of the day - even if that's just at night, to cool the grain. Keep track of grain temperatures and insect activity and operate the fans as needed to cool the grain to around 40F during fall. If the grain will be stored through winter, aerate the grain again as the weather gets colder to reduce the grain temperature even further. Most insects go dormant at around 40F and many will die if they are held at low temperatures for long periods of time. The lower the temperature, the faster insects will die. If grain is cooled to well under 30F, it might be a good idea to aerate grain again in late winter to bring its temperature back up to 30 to 40F to avoid excessive temperature differences between the inside and outside of the bin during warm spring weather.

The time required to cool a bin of grain depends on the airflow per bushel that can be provided by the fan. A rough estimate of the number of hours that a fan needs to operate to cool a bin of grain can be found by dividing the number 15 by the airflow delivered by the fan expressed in cubic feet per minute per bushel of grain in the bin (15 divided by cfm/bu). Many storage bins are equipped with fans that can provide 0.1 cfm/bu, which means that about 150 hours (15 divided by 0.1) of fan operation are required to completely cool the grain in those bins. Long-term weather records for southern Minnesota indicate that, on average, there are about 100 hours when the outdoor temperature is below 60F during the month of August, 150 hours when the temperature is below 50F in September, 200 hours when the temperature is below 40F in October, and 350 hours when the temperature is below 30F in November. This information can be useful for deciding what fan size might be appropriate for a desired aeration management strategy and can indicate what grain temperatures might be achievable with an existing fan.

For more information about managing stored grain and selecting fans, see the Biosystems and Agricultural Engineering Department's postharvest website at www. bae.umn.edu/extens/postharvest, or call Bill Wilcke at 612- 625-8205 or Colleen Cannon at 612-625-4798

URL: http://www.soybeans.umn.edu/crop/harvest/storage_insects.htm

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