Sugarcane Aphid Advances

After Slow Start, New Sorghum Pest Causes Problems

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Adult sugarcane aphids give birth to 30 to 60 pregnant nymphs at a time. They reach maturity in just three short days, allowing populations to increase exponentially and catch growers off guard. (Photo courtesy Gabriela Esparza-Diaz, Texas A&M AgriLife Extension)

ST. LOUIS (DTN) -- You can't keep the sugarcane aphid under the weather for long.

After inflicting major damage in 2014, sorghum's newest pest appeared to be struggling for a foothold during the wet start to the 2015 crop season. Now the pest has made a late-season comeback, infesting fields at an impressive rate as parts of the Midsouth and Midwest dry out.

Diligent scouting now and for the rest of the season will be key for sorghum growers, said Louisiana State University entomologist David Kerns. "You don't want to scout any less than once a week because they can blow up so quickly," he told DTN. "You can have just a few aphids one week, and the next week they will be everywhere."

The aphids first showed up on sorghum in Texas in 2013 and spread as far north as southern Kansas and as far east as South Carolina by 2014.

The pest is now staking out new territory in Missouri, Kentucky, and North Carolina. According to Moneen Jones, a research entomologist with the University of Missouri's Fisher Delta Research Center, the aphid is migrating roughly 40 miles north through southeast Missouri every week.

Late-planted sorghum fields will be most at risk, Kerns said. Although the aphids can cause problems at any time of the season, the plants are most vulnerable at the pre-boot stage, when the head is still tucked away inside the stem.

"If you have a heavy infestation of aphids at that time and it is not controlled, it will sterilize and kill that head," Kerns said. Most of the nation's sorghum is beyond this stage; the USDA's most recent crop progress reports estimated that 57% of the crop was already headed and 29% of the crop was turning color.

"I think most of our milo is safe for this year, except for the later plantings," Kansas State University entomologist J.P. Michaud said of the aphid's presence in the state's southern counties of Butler and Sedgewick.

However, the aphid can do damage well into the final stages of maturity, Kerns warned. They secrete a sticky substance called honeydew onto the plants, which can cause harvest problems. "All that sticky honeydew and even the aphids themselves will clog the screens on the combine, so the seed won't sift and it just throws all the seed out the back," he said. "And you can get so much honeydew on your belts and pulleys that it will burn them up."

Two years of research have helped establish a preliminary spraying threshold for the aphid at 50 aphids per leaf with 20% of the plants in the field colonized in the boot to milk sorghum development stage. For treatment thresholds at other stages of development, see this chart from North Carolina State University here: http://bit.ly/…. A scouting guide from the Sorghum Checkoff can be found here: http://bit.ly/….

For now, growers have two treatment options for the sugarcane aphid. This year, 12 states have received approval for the use of Dow AgroScience's insecticide Transform WG under a Section 18 emergency use exemption, according to the National Sorghum Producers. A new Bayer insecticide called Sivanto is also now available to growers.

Kerns said both products have worked well on the aphid this year, but growers shouldn't assume one application will take care of the problem. Many fields, including Kern's research trials, have required two insecticide passes already this summer. "They have a propensity to resurge," he said. "Keep scouting. They'll come back."

Beneficial predatory insects have proven a helpful ally in the fight to control the aphid this year, Kerns added. Both Transform and Sivanto aren't too harmful to the beneficials, which has been a boon for Louisiana growers, he said. "One of reasons we haven't had the issues as severe as last year is the beneficials," he said, noting that tiny parasitoid wasps have been especially numerous.

As a result, growers who spray pyrethroids to control midge problems may find themselves with a newly vulnerable sorghum crop devoid of these predatory insects.

"Pyrethroids are extremely hard on the natural enemies, particularly these little parasitoid wasps and they don't do anything to the aphids," he said. "It's almost like pouring gas on a fire -- aphids will really take off following a pyrethroid application."

Kerns recommended holding off on midge treatments if populations are low.

The aphid's resurgence this year suggests it may be a permanent new cost for sorghum growers. Transform applications run about $7 to $8 an acre, and Sivanto is closer to $10. Growers whose acres are too wet or numerous for ground application will have to add the costs of aerial application to that, he said.

For next year, sorghum growers should consider seeking out aphid-tolerant varieties. Researchers are sorting and testing a number of naturally tolerant hybrids and more data will be available after this summer, Kerns said. For the most current research on which hybrids are showing promise, see this Texas Sorghum Producers' publication: http://bit.ly/….

For more information on the most recent research on the sugarcane aphid, see this Louisiana State University news release: http://bit.ly/…. For help with identifying, scouting, treating and managing the aphid, see this Texas A&M guide: http://bit.ly/….

Emily Unglesbee can be reached at emily.unglesbee@dtn.com

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Emily Unglesbee