The practice of aerating wheat in storage bins has long been recognized as an integral part of any integrated pest management program — and ultimately, a good grain quality control measure. This is especially important in South Central states such as Kansas, Oklahoma, Texas, Nebraska, New Mexico and Colorado, where warm summer weather heats up the grain’s surface, creating an inviting den for insects and other pests.
Pressure aeration — using fans to push ambient air from the bottom of the bin upwards — has traditionally been the method used by elevators and other wheat handlers. But researchers from the USDA Agricultural Research Service (ARS) Center for Grain and Animal Health Research, located at Kansas State University in Manhattan, KS, set out to determine whether suction aeration would be more effective at managing insects in stored wheat. Suction aeration involves reversing the fans to pull air from the top downward to keep the grain at 60 F or below, the optimal temperature for pest control.
Conclusions from two ARS field trials, conducted by research entomologist Frank Arthur and agricultural engineer Mark Casada, were that suction aeration cools the top of the grain mass more quickly than pressure aeration.
“There is a stigma about suction aeration,” Casada says. “Especially up North where suction aeration in the winter can pull the bin roof inward if the aeration vents are frozen over. However, if you follow proper design and management criteria, successful suction aeration should also be possible in colder climates.”
Structural damage concerns aside, there is the fear that suction aeration will pull moist air from the headspace down throughout the bin, thus increasing the moisture content of the grain.
Arthur says this was not true in either of their field trials because the airflow rates used for aeration are low, and states that for South Central U.S. wheat handlers, suction aeration could become a new option for pest control.
ARS field trials
The researchers conducted their first field trail from July 2004 through March 2005 and the second from July 2006 through March 2007. They used six 1,250-bushel capacity metal storage bins filled with wheat. Three bins were randomly assigned to pressure aeration and three were assigned to suction aeration.
Aeration was accomplished at an approximate airflow rate of 0.2 to 0.3 CFM (cubic feet/minute per bushel) and was done by adjusting thermostatic controllers to operate the aeration fans when ambient temperatures fell below specified thresholds.
They monitored the temperature of the grain throughout the bin using cables inserted at depths of 1, 3 and 6 feet from the top surface in the center, north and south positions.
Using standard plastic pitfall traps, they sampled seven insect species populations including rusty grain beetles, foreign grain beetles, hairy fungus beetles, red flour beetles, saw-toothed grain beetles, rice weevils and lesser grain borers.
Suction aeration prevails
The data showed that, during the summer, suction aeration cooled the surface zone — the upper portion of the bin — more quickly than pressure aeration did. Cooling the top portion of the wheat is essential in controlling insects because that is where they enter the bin.
So why is suction aeration more effective at cooling the surface zone?
“When the cool air from outside is pulled in and mixes with the warm headspace air, the warm air will be cooled and cool air will be pulled down through the grain mass, thereby cooling the surface off much more quickly than could be achieved by pressure aeration,” Arthur explains.
Cooling the surface zone with pressure aeration can take nearly 200 hours for the cool air to pass completely through the grain mass, and by then an infestation could be developing.