Warm Grain Temps Encourage Insect Growth In Stored Grain
University of Minnesota Extension Service
Insects are most likely to show up in stored grain when the weather is warm and the grain has been held over from the previous year.
Grain temperature is the main factor in the development of stored grain insects. When grain temperature is between 60º and 90º F, insects grow and reproduce rapidly. Insects develop slowly or not at all when grain temperature is kept below 60º. In the upper Midwest, it's fairly easy to aerate grain to keep it below 60º in fall and winter, but it's a challenge during the summer.
Other factors that can contribute to stored grain insect problems are:
--Insect populations in or near the bin when the new crop is added.
Some stored grain insects don't fly very well or very far. 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.
--Moldy grain in the bin. Some insects actually feed on mold. Also, mold growth produces hot spots that attract insects.
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. If this isn't feasible, fill bins in a way that minimizes concentration of fines under the bin fill spout. This may mean using grain spreaders to distribute fines, 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.
--Aerating grain as needed during cool weather to keep grain temperature at 20-30º F during winter months and 50º F or less at other times.
Several insecticides are registered for treatment of empty bins and some are registered for use on stored grain. Before using insecticides, though, compare the cost for insecticide treatment with other insect management options, and make sure potential buyers will accept grain that has been treated with an insecticide.
If you find insects in stored grain in late summer and want to sell it or keep it through the winter, you might consider running it through a grain cleaner. This can remove broken kernels and some of the insects, reduce insect populations and improve grain storability. It won't remove all the insects, but it may reduce populations enough to allow sale of the grain, or at least buy some time for other insect management options.
If average outdoor temperatures are consistently above 60º, fumigating grain is an option. However, it should only serve as a last resort. Grain fumigation is costly, and the chemicals are potentially hazardous to humans. Also, because fumigation provides no residual protection against insects, it is only a temporary control measure. If the insect problem is due to grain that's warm or moldy or contains fines, insect populations will rebuild soon after the fumigant dissipates.
Most insects go dormant at around 40º F, and many will die if the grain temperature stays low for an extended period of time. The lower the temperature, the faster insects die. Therefore, running aeration fans is an effective insect control strategy when the weather is below 60º at least part of the day. If necessary, run aeration fans only at night to cool the grain to around 40º during the fall. If the grain will be stored through the winter, aerate it again as the weather gets colder to reduce grain temperature even more.If grain is cooled to well under 30º F, it may be a good idea to aerate again in late winter to bring it back up to 30-40º. This is 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 the fan can provide. A rough estimate of the number of hours a fan needs to run 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 have fans that can provide 0.1 cfm/bu, which means that about 150 hours (15 divided by 0.1) of fan operation is necessary to completely cool the grain. Long-term weather records for southern Minnesota show that, on average, there are about 100 hours when the outdoor temperature is below 60º F. during August. There are 150 hours when the temperature is below 50º F. in September, 200 hours when the temperature is below 40º in October, and 350 hours when the temperature is below 30º in November. This information can be useful for deciding what fan size might be appropriate for a desired aeration management strategy. It can also indicate what grain temperatures may be achievable with an existing fan.
For more information about managing stored grain and selecting fans, go to the U of M Biosystems and Agricultural Engineering Department's postharvest website at www.bae.umn.edu/extens/postharvest  You can also get more information from Bill Wilcke at 612-625-8205 or Colleen Cannon at 612-625-4798.