Insect scouting critical as grain sorghum moves closer to harvest

Grain sorghum producers in the central and upper Mid-South are entering that “witching hour” when they almost have the 2015 crop made, but not quite.

While harvest has begun in the southern parts of the region, the bulk of the grain sorghum acreage in the central and north Delta is approaching the stage where growers will soon begin applying desiccants to prepare it for harvest.

In most crops, that would be a simple matter of applying sodium chlorate or glyphosate, waiting a few days for the crop to dry down and starting the combine through the field. But, in those other crops, growers aren’t dealing with white sugar cane aphids.

“What folks are facing now is ‘do I wait until the harvest aid needs to be applied’ to spray for sugar cane aphids?” said Drew Ellis, market development specialist, Dow AgroSciences, who is based in Memphis, Tenn. “Can I wait even seven to 10 days or do I need to spray Transform or another product now.”

The situation is complicated because this pest will migrate from the older leaves on the bottom upward to find fresh, new leaves, and once the desiccant is applied to prepare the crop for harvest it will move up to the grain head.

“The stem up near the head is one of the last things that remain green,” said Ellis, a weed scientist by training. “So they are able to remove sustenance that they’re feeding on from the grain sorghum stem or head and secrete the sugary honeydew in the head. You get that pulled through the combine, and that’s where we get those harvest issues.”

Pathway for glyphosate

The key, said Ellis, is to make sure glyphosate, which is the most commonly used desiccant or harvest aid, is able to get into the crop. “Any time where you have those heavy infestations where honeydew followed by the sooty mold or black mold forms, it can inhibit the glyphosate from entering the plant tissue.

“It kills the efficacy,” he noted. “We have seen in worst-case scenarios where you have to make a repeat application of the glyphosate.

Many times rainfall will wash the honeydew or black mold off the plant – if growers can stop the aphids from feeding with an at-harvest application that includes Transform.

“Let’s say a rainfall event happens just before they put their glyphosate out, and it kind of cleans up that mold, that helps the glyphosate get in the plant,” he noted. “You would still need Transform because the aphids start building back.

“When you apply the glyphosate, you have about a two-week period before the grain is harvestable. That works really well with our Transform Section 18 (emergency exemption) label, which allows for a 14-day, pre-harvest interval.

If a grower is about five days out from applying glyphosate to desiccate the crop; is seeing 30 percent of the plants infested with 50 to 100 aphids, which is the treatment threshold; and begins to see the leaf surface covered with the honeydew, he probably needs to begin spraying with Transform and stop that infestation from creating the sooty mold, says Ellis.

Stop the honeydew

“Then, if you get a rain you’ve got the aphids taken care of, and they’re not continuing that process of secreting the honey dew, it can wash it off, and the glyphosate will work better. On the opposite side of that, if you don’t have that significant level of honeydew, but your populations are building, maybe you can wait four or five days til you get closer to black layer and apply that harvest aid.”

If growers don’t have sooty mold present, they might want to mix the glyphosate with Transform, “control the aphids from then on, and the glyphosate will go in better. That’s the most common and best-case scenario.”

The good news, he says, is that if you don’t get a rain, the sooty mold or black mold will dry and start flaking off  -- if the sugar cane aphids are no longer secreting fresh honeydew.

Dow AgroSciences has submitted for a Section 3 label for Transform and expects to receive full federal registration in time for the 2016 crop year.

University and Extension entomologists are happy that farmers will have two new products to apply for sugar cane aphids once Transform is fully labeled for grain sorghum. (Bayer CropSciences previously received registration for Sivanto 200 SL in several crops, including grain sorghum. https://www.bayercropscience.us/products/insecticides/sivanto/labels-msds.)

“We’re certainly excited from the standpoint of longevity to have multiple products because when you’re dealing with an aphid that has been notorious for being able to develop resistance in a very short amount of time, having a product you can rotate with ours benefits the grower and it benefits the market,” says Ellis.

To read more about sugar cane aphids in grain sorghum, go to http://www.uaex.edu/publications/FSA-7087.pdf.

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