Insecticides effective in protecting stored grain

History tells us that storing corn can be advantageous for growers in the lower Southeast. But any advantage is lost if insects make a meal of the grain before it is sold.

It’s important, says University of Georgia Extension Entomologist Steve L. Brown, that growers do all they can to manage risks in storage, and that includes protecting the grain from damaging insects.

"We’re talking here about protectants — insecticides that we put directly on the corn at the time we’re loading the bins," says Brown. "You may use fumigants later on, and they can be effective, but they’re dangerous and a lot of trouble. If we have something that’ll give us good, long-term protection, that would be the way to go."

Brown conducted trials this past year in grain bins that simulate conditions on the farm.

"Environmental conditions are so different in a grain bin, and that’s why it’s important that we do this research in these situations. We get the natural fluctuations in temperature and moisture, and that all factors in to what we do," he says.

The maize weevil, says Brown, is the key pest of stored grain in south Georgia. "We find many different insects in a bin of corn, but this one hurts us the most, at least in south Georgia. We get maize weevil infestations in the field. We’re harvesting infested corn, more so in some years than in others, and in some situations more than in others," he says.

This past year, Brown conducted a study using three grain bins. In the first bin, SpinTor or Tracer was applied at a rate of 3.5 ounces per 1,000 bushels. In the second bin, SpinTor was used at 3.5 ounces along with Actellic at 5 ounces per 1,000 bushels. The third bin was an untreated check, and Actellic was used at 10 ounces per 1,000 bushels in the fourth bin.

SpinTor, says Brown, was granted an experimental use label a couple of years ago to use on stored products. It wasn’t intended originally for beetle or weevil control, but activity was being seen on beetles.

"The company behind SpinTor petitioned the EPA for an experimental use permit, and this permit has expired. But SpinTor probably will get a new label for use on stored products. It will be sold as Secure by Gustafoson. They’ve told me that they’ll try to keep the cost at about 3 cents per bushel, and it could receive a label by the time corn is being harvested."

Actellic, says Brown, has been around for a long time, but it isn’t used to a large degree in Georgia. "It is expensive and difficult to find. When we went to buy Actellic for this experiment, the cost was $900 per gallon. That works out to about $70 per 1,000 bushels, and we don’t need to be using that kind of rate. You probably can find it cheaper than that, at about $40 per 1,000 bushels."

Researchers placed probe traps in the four grain bins and monitored the traps on a weekly basis, says Brown. This was done throughout the storage season, from October into March.

The life cycle of the maize weevil is about 30 days under ideal environmental conditions, he says. "We saw a peak in November, which probably was the first generation after the field generation. There also was a small peak in January. They’re still feeding in cold weather, but they’re not as mobile, and we didn’t catch as many in the traps. There probably are more of them in the bin than the traps indicate. In late February, we saw massive infestations."

In addition to maize weevils, researchers also looked at Indian meal moths, red flour beetles and a group of scavenger beetles known as cryptolestes, says Brown. All treatments worked well on the pests until February, he adds.

"They started to break down when we got into February and March. But that’s pretty good control, if you can protect the grain from September through February. There wasn’t much difference between the treatments. The protectants worked very well."

Brown also notes that beneficial insects can be found in a grain bin. These parasitoids are tiny wasps that feed on the insect pests, he says.

These insecticides will not be as effective, he says, if they are not applied properly.

"You must make the extra effort to put them on correctly. When it’s hot and humid, and you’re harvesting corn, you usually have a million other things to do. But you need to make sure, as you’re loading the bin, that everything is calibrated correctly and the right rates are going in.

"This can be tricky — you must know how much corn is flowing through that auger. Then, you have to adjust your spray rig to compensate for that. It’s difficult to do. But if you take the time to do it right, these protectants will hold for several months."

Researchers also looked at the test weight of the corn to determine what effect the insects were having on the grain’s value, says Brown. "From the time we placed grain in the bin in September and took it out in March, we had a drop in the test weight of the untreated grain of about 1 pound. We saw a trend indicating that the untreated grain probably lost weight due to the fact that insects were hollowing out the kernels."

Aflatoxin also was noted at a higher level in the untreated corn, says Brown. Other tests showed that four months after treating the grain, the insecticides still were active and killing weevils, he says.

The insecticides can pay dividends, he says, if growers are certain that they’ll be holding the grain for several months. "These protectants can be worth the money, and they may prevent you from having to fumigate. They’ll break down eventually, but they should hold for four to six months."

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