Grain sorghum producers whose crops are approaching or are in boot stage take note: React quickly to any signs of sugarcane or white aphids building to treatable numbers.
That’s the advice growers attending the Northeast Research Station’s Pest Management and Crop Production Field Day received from Sebe Brown, Extension entomologist for LSU’s Northeast Region. Brown and David Kerns, LSU research entomologist, gave an update on the pest.
“If sugarcane aphids infest grain sorghum very heavily at boot to pre-boot, you can see some catastrophic losses,” said Brown. “Last year, some guys let the aphid populations outrun them, and they were behind the eight-ball all season.
“In extremely-infested fields, the grain sorghum heads never emerged. Essentially, the heads were blank, and they rotted inside the boot. That’s why we’re stressing you have to be on time with this aphid. You have to follow recommendations because if you let them get ahead of you you’re playing catch up the entire season while you’re losing yield.”
Many Mid-South farmers are growing grain sorghum for the first time or for the first time in a long time in 2015. USDA won’t release its survey-based estimates of grain sorghum plantings until June 30, but most believe the acreage will be up significantly in Louisiana and other Mid-South states.
That means those growers haven’t seen how sugarcane aphid populations can suddenly explode. Brown and Dr. Kerns, who is also based at the Macon Ridge Research Station in Winnsboro, La., gave a crash course in grain sorghum insect management at the field day Wednesday (June 17).
Based on research in Texas, where sugarcane aphids first became an economic problem in grain sorghum, and in the LSU AgCenter’s Northeast Region at the Macon Ridge Research Station at Winnsboro, university entomologists are recommending consultants and growers treat when they find 50 aphids per leaf on 20 percent of the leafs in a field.
Louisiana, Texas research
“That’s our preliminary threshold that we’ve found correlated very well with the data in Texas,” says Brown. “We had two completely separate locations with the same test design, and we got very similar results so we feel we can stand behind that threshold.”
Louisiana and Mississippi have received Section 18 emergency use exemptions for the application of Transform on sugarcane aphids in grain sorghum the last two years. The total that can be applied per season is 3 ounces per acre.
“You can split that into three applications, but you get three ounces per acre per season,” said Brown.
Bayer CropScience, meanwhile, has released a new insecticide called Sivanto that the company said will target damaging pests while safeguarding beneficial insects such as wasps that can parasitize sugarcane aphids.
“There is some data out of Arkansas and Georgia that says that the five-ounce rate of Sivanto looks pretty good,” said Brown. “Unfortunately, our aphid populations crashed last year before we got a chance to look at it. We will have some good efficacy data on that as the season progresses.”
Sivanto is the first insecticide in the newly created Insecticide Resistance Action Committee subgroup category called the Butenolides. It reportedly provides excellent control of neonicotinoid-resistant aphids and whiteflies.
Multiple modes of action
“The good news is we do have more than one mode of action to rely on to control this pest,” said Brown.
Chlorpyrifos or Lorsban is another product with activity on sugarcane aphids, but its use may be complicated when sorghum midge is also present at threshold levels. Brown says entomologists have changed their midge recommendations.
“We’re not recommending a pyrethroid for midge unless you have heavy midge populations, and grain sorghum planted on time typically won’t see midge populations severe enough where you’re going to have to use a pyrethroid insecticide,” says Brown.
“When you spray a pyrethroid over the top of grain sorghum it essentially opens the door and lets the aphids explode because all of the beneficials that David (Kerns) was talking about are very susceptible to pyrethroids. When you clean the slate and open that door for aphids they exponentially increase uncontrolled.”
Last season, some farmers “tag-teamed” pyrethroid and Transform applications, leading to a control failure. “So you’re taking two steps forward and one step back,” said Brown. “Pyrethroids are something we can use in a management system if you plant late and you have high midge populations.”
That means scouting for midge is critical, he noted. “Just going out and spraying at 30 percent heading is not going to be the way to go about it. We actually need to get out in the field and look because if you apply a pyrethroids unnecessarily you’re actually shooting yourself in the foot when it comes to aphids.”
Applying a pint of chlorpyrifos or Lorsban will give good control of low to medium populations of midge and sugarcane aphid suppression, says Brown. Applying a quart of Lorsban, on the other hand, will control aphids at a low to medium population.
Growers must observe a 60-day preharvest interval for a quart of Lorsban. “That means you can’t apply a quart at the soft dough stage and be within the preharvest interval,” says Brown. “So the window for Lorsban, especially at the quart rate, will be at pre-boot or boot stage.”
Transform, the other insecticide Mid-South entomologists are recommending for sugarcane aphids, is relatively short-lived. “With Transform we can see efficacy for about five days,” Brown notes. “That doesn’t mean you have to go out and spray again, but don’t expect to see very long residual with Transform.”
For more information grain sorghum pest management, see http://varietytesting.tamu.edu/grainsorghum/otherpublications/2000FieldGuidePests.pdf