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.