Texas AgriLife specialists recommend scouting for sugarcane aphids

In Southwest Farm Press

Sugarcane aphids are not building up on Texas grain sorghum as rapidly as they did last summer, but from experience over the past two years producers know populations can explode, making scouting a critical part of sorghum management.

Texas AgriLife specialists believe rainfall and cooler temperatures so far this season may be keeping sugarcane aphid populations low enough to allow beneficial insects to provide some control.

Continued vigilance and routine scouting will be necessary, says AgriLife Extension entomologist Allen Knutson.

Knutson, addressing a Stiles Farm Field Day crowd gathered Tuesday in the Taylor, Texas, Knights of Columbus Hall, recommended weekly scouting as long as no sugarcane aphids are identified. “If the aphids are found, scout twice a week,” he said.

“Look for honeydew on the leaves,” he said, “and look for aphid populations. Also check Johnsongrass near sorghum fields.”  Johnsongrass is another host for the sugarcane aphid and the pests may move from the weeds into sorghum fields.

Knutson says research and Extension specialists have refined treatment recommendations since the aphid first appeared on sorghum two years ago. Recent trials to assess threshold levels for insecticide application show that waiting too long can result in significant yield losses as well as severe harvest problems associated with the honeydew, a sweet, sticky secretion from the aphids that “gums up” combines and causes trouble at grain elevators.

Knutson says if producers wait too long to apply insecticides after thresholds are met, yield loss can be severe, as much as 37 bushels per acre in research trials. Also, when populations build to high numbers, knocking numbers back to an acceptable level becomes much more difficult and may require additional insecticide applications and more expense.

The honeydew also serves as a growth platform for a sooty mold.

Research shows 50 to 125 sugarcane aphids per leaf justifies applying an insecticide—Transform WG, available through a Section 18 special exemption, or Sivanto 200 SL, just labeled for sugarcane aphid control.

Both products are specific for sap feeder control and are soft on beneficial insects. Producers should be aware of pre-harvest intervals—up to 21 days if harvested for grain, seven days for forage sorghum—following application.

He says producers should make certain they get the products into the canopy for adequate control.

Neither of those products is effective on other sorghum pests, such as midge, so Knutson suggests they might add one of these to usual treatments for midge and other pests.

Gaucho and Poncho also provide some aphid control but usually are ineffective by the time sugarcane aphids move in, typically mid-season. Pyrethroid applications for midge and other pests also may be problematic. They are effective but are hard on beneficials and may flare Sugarcane aphid populations.

Harvest challenges also occur with high sugarcane aphid populations. “Managing sugarcane aphids near harvest is more art than science,” Knutson says, as producers try to balance knocking populations back with insecticides while watching those pre-harvest restrictions. They also may consider applying harvest aids such as glyphosate to knock off the leaves, depriving the aphids of their food source and also getting rid of some of the honeydew. “But sometimes the aphids move into the head,” he said.

Forage sorghum producers may opt to graze out the crop early if they have heavy infestations. “Forage sorghum is vulnerable to sugarcane aphid damage and the pest is harder to control in forages because of heavier foliage.”

New hybrids may offer hope for more efficient control, Knutson says. “Some new hybrids have resistance to sugarcane aphid. These hybrids are not immune but populations build on them less rapidly. We may need to apply an insecticide and we will need to scout.

“But this is the future of sugarcane aphid management. I think we will see a resistant hybrid on the market soon.”

The Stiles Farm filed Day was held off site and inside for the first time in the event’s 52-year history because of concerns that a pending tropical storm would  make walking the fields at the farm even more difficult than it already was following recent rainfall.

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