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Insects and Fumigation

Sunday, August 1, 1999
filed under: Harvest/Storage

"The Best Way to Avoid Both is to Make Sure Seeds Are Cooled Down Before They Can heat up"

No one enjoys a pleasant 75- or 80-degree fall harvest season more

than a producer bringing in a bountiful crop - no one, that is, except

those species of insects which infest stored grain.

Warm grain temperatures provide ideal conditions for insects that

feed on stored grain, notes North Dakota State University extension

entomologist Phil Glogoza. Some grain insect pests, like the lesser

grain borer, red flour beetle, flat grain beetles and Indian meal moths,

are common across the Upper Midwest, he notes, and will fly from storage

site to storage site. Other insects, such as granary weevils, the

saw-toothed grain beetle, mealworm beetle and spider beetle, pose the

biggest threat when new grain is stored in bins that were previously

infested and then not adequately cleaned.

Glogoza and NDSU ag engineer Ken Hellevang encourage close

monitoring of grain conditions to prevent insect problems. "Check the

surface of the grain; then use a probe to check the moisture and

temperature at a variety of locations. Pay attention to the look, smell

and feel of the grain. Your senses can tell you a lot about its

condition," Hellevang remarks.

Frost or condensation on the inside of bin roofs is a clear

indication of excess moisture in the grain, Hellevang notes. He

suggests examining samples under good light (perhaps on a light-colored

cloth) to make insect identification easier. Bringing cold samples into

a heated area will increase insect activity and thus facilitate


Temperature is the key to controlling insects in stored sunflower

and other grains, Hellevang emphasizes. As noted in the accompanying

article, insect reproduction slows once the interior bin temperature

drops below 70 degrees F. By 60 degrees, reproduction has ceased; at 50

degrees the insects become dormant - and most will die if the bin

temperature falls to freezing and remains there for an extended period.

Contributing to the threat of insect infestation is grain's

proficiency as an insulator. Cereal grains have an R-value of one per

inch, meaning "grain at the center of an 18-foot bin is insulated in

excess of R-100," Hellevang notes. That compares to an R-20 insulation

rating for a typical North Dakota home. While sunflower's insulation

rating is less than that of cereal grains, it is still substantial.

Fumigation - traditionally touted as the standard method of

eliminating insect infestations in stored grain - is not effective or

economical in many instances, Glogoza and Hellevang advise. Here's why:

* Fumigants do not work well once grain temperatures drop below 60

degrees F. For a fumigant to work, it must volatilize and spread

throughout the storage structure; and temperatures must be above 60

degrees for that to occur. Also, the entire bin must be at or above

that temperature, Hellevang adds. "If you have a pocket that's at 80 or

90 degrees and the rest of the bin is at 40, the fumigant will

volatilize in that pocket," he explains, "but insects in cooler areas of

the bin won't be hit nearly as hard." An important related point:

having the warm temperatures required for effective fumigation runs

counter to other basic management strategy, i.e., cooling the grain to

avoid insect and mold problems.

* Fumigants are expensive, a hassle - and product options are very

limited. (Phostoxin is essentially the only alternative left, Hellevang

says.) For a fumigant to work properly, the bin must be tightly sealed

to avoid leaks.

* If temperatures are too cold during fumigation, remnants of the

fumigant could remain in the grain. Along with making the grain

dangerous to handle later on, detectable amounts of fumigant could

result in rejection of the grain at the elevator.

The best strategy, Glogoza and Hellevang emphasize, is to avoid

situations under which insect populations can develop. Even if the

binned crop is cooled over winter, a large population of dormant insects

will be prepared to "explode" upon the arrival of warmer spring

temperatures. Cooling the binned grain down as quickly as possible

after being placed into storage is the most effective way to keep the

troublesome bugs at bay.

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