Controlling maize weevils in stored corn is the focus of ongoing research being conducted by entomologists and engineers with the University of Georgia.
"If you dry corn in the field, the corn will become infested with maize weevils," says Extension agricultural engineer Paul Sumner. "Once a weevil goes to an ear of corn — to the kernel — it'll emit a pheromone which will attract other maize weevils to that area. They subsequently lay eggs in the mature kernels, and adult weevils emerge within three to four weeks."
By delaying harvest in the field, growers allow a second infestation of weevils in the corn, says Sumner. "It's going to occur anyway in our part of the country. Do you leave corn in the field and let it dry, or do you harvest it after it has field dried? Then, do you store it or sell it immediately?"
If you plan on storing corn, then you'll have to place some insecticide on it to prevent a regular occurring infestation of weevils inside the storage bin, he says. Among the insecticides available are Actellic, which is applied at 9.2 to 12.3 ounces per 30 tons of material. The translates into about six cents per bushel, says Sumner.
Researchers have looked at several different methods of drying corn, says Sumner, including layer and bin drying and batch and bin drying.
"Then you have just the batch process. This is where you dry it, cool it down, put it in the bin, and run a continuous flow dryer. If you're spraying a material like Actellic, and corn is coming out of the dryer and into the bin, it needs to be cooled down. The heat given off by the corn as it comes out of the dryer — at 180 or 200 degrees — will affect the material you're spraying. So if you're using an insecticide, you really need to cool down that product as it's going into the bin," he says.
Sumner says he conducted a test where he placed corn samples in a small oven at 110, 140 and 180 degrees F. "We pulled out six containers — at one hour, three hours, six hours, 12 hours and 24 hours. Then we evaluated them for adult weevil emergence by placing these samples in plastic bags and waiting for the weevils to emerge in about three weeks after going through that exposure to the hot temperatures. We did the same test in larger containers with forced-air drying," he says.
There was a good infestation of weevils in the test, says Sumner. "We stored grain from 2003 in 20-gallon barrels, and we allowed them to go through several cycles so we would be certain of having a good infestation in the corn. The containers were placed in the oven after we sifted through the corn, getting rid of the adult weevils. After exposing the corn to the heat, we placed it in bags and waited."
In the check part of the test, there were eight to 25 weevils, depending on the time the test was conducted, which might have been at the beginning or end of a cycle of emergence. "It was interesting, that after we ran the first test at 180 degrees, there weren't any weevils left after an hour. So when we ran the second, third and fourth test at that temperature, we pulled out samples at 30 minutes to see if we could find a cut-off period. In the 110-degree test, we were seeing weevils up until six hours of exposure to the 110-degree temperature.
"After that six-hour period, we next would pull a sample in 12 hours, after which no weevils emerged. The same thing occurred with the 140-degree test. Weevils would emerge after being exposed to the heat for an hour."
Researchers also harvested about 600 bushels of corn, directly from the field, and stored it in the grain bin for about six weeks, says Sumner. "We took the weevils left over from the oven-dried test and dropped them in on top of the bin. We modified a peanut trailer and placed four square bins in the trailer. We ran the tests for 110, 140 and 180 degrees, pulling samples at 15, 30 and 45 minutes. Then we went to one hour, three hours, six hours and so forth to see where we could catch the emergence that was being shut down by exposure to the heat. We pulled samples, placed them in sealed plastic bags, and left them there for three weeks."
For 110 degrees F., there was a good infestation of weevils, he says. But after six hours, there was no weevil emergence. At 140 degrees F., the mortality rate actually dropped compared to the oven-dried test.
"That is because in the oven-dried test, it was just exposure to heat. The forced-air test was more like normal drying, blowing heated air up through the product and heating the corn faster, causing the maize weevils to die. At the 180-degree test — we chose 180 degrees because that was the highest temperature we could get from the peanut trailer — we no longer had weevils after 30 minutes.
"This tells me that if I'm going to harvest wet corn, and I need to dry it, I need to hold it in the dryer for at least a certain amount of time if I want to dry it at 110 or 140 degrees. It'll take at least a couple of hours to get it from harvest moisture down to 15 percent, put it in the bin, and then dry it on down to 12 percent. If you change the temperature — say you're using 130 degrees F — it's going to require more than an hour."