Frances Gould, LaBauve, Randy | 8/8/2015 12:16:19 AM
Sugarcane aphids in Louisiana transitioned from sugarcane to grain sorghum in 2013 – some of the first such colonization in America. Since then, this invasive insect species has rapidly spread to all sorghum-producing parishes, decreasing quality and yield and causing catastrophic crop losses in 2014.
Researchers have been exploring various strategies for curtailing the aphids’ destruction through a delicate balancing act between many variables, including attempts to preserve beneficial insects that attack aphids. However, development of resistant hybrids and use of effective insecticides are the most important tools in the arsenal.
One collaborative project includes researchers from the LSU AgCenter and Texas A&M AgriLife. For the past two years, they have been refining foliar insecticide applications into a threshold action plan for spraying the right chemicals at the right time in the right place.
"A lot of research we’ve done to control these pests chemically over the past two years includes how to use those products and how to develop an integrated approach that considers other pests, like sorghum midge," said David Kerns, LSU AgCenter entomologist at the Macon Ridge Research Station in Winnsboro.
Kerns, who has researched insect pest management on corn, cotton, grain sorghum and soybeans, said spraying for midge influences what happens to aphids. Pyrethroids have been effective for controlling midge, but the chemicals help sugarcane aphids proliferate.
"When we kill all the natural enemies, you end up with aphid infestations that have enormous population growth potential," Kerns said. "In just a matter of days you can be covered up with them."
In Louisiana, 100 percent of the sugarcane aphid population is female, and the aphids reproduce asexually. Adults give birth to 30 to 60 live, pregnant nymphs, which become mature adults in only three days. After five or six days, their population can grow to several hundred, according to Kerns.
Combined grain sorghum results from the LSU AgCenter and Texas A&M have established a preliminary action threshold at the boot to milk sorghum development stage as 50 aphids per leaf with colonization of 20 percent of the plants in the field before spraying insecticides.
AgCenter researchers have been testing seed treatments and analyzing post-emergence insecticides for combating sugarcane aphids. There are presently only two new products that work well – Transform, available through a Section 18 emergency exemption, and Sivanto.
"Those products are effective, but because there are many generations of those pests, we don’t know how long it can last because they develop resistance very fast," said Fangneng Huang, an LSU AgCenter entomologist. "If we detect a product that is no longer efficacious, then we need to start looking at different alternative chemistries," Kerns said.
LSU AgCenter grain sorghum variety trials at the Northeast Research Station in St. Joseph, the Macon Ridge Research Station and the Dean Lee Research station in Alexandria are being monitored for rapid reproduction of sugarcane aphids. A related sampling project, which combines collaborative results from the AgCenter, Texas A&M and the University of Arkansas, is determining patterns of aphid distribution in fields and using that knowledge to improve sampling techniques.
Huang, who typically works with corn pests, began focusing efforts on the sugarcane aphid last year because of the potential for devastation of sorghum. Huang found fewer sugarcane aphid populations and a later arrival of the pests in 2015 than he did in 2014, but suggested lower temperatures in 2015 may be the reason they came later.
The plant stage just before seedhead emerges (boot stage) is probably the most critical period. At that point, high numbers of aphids can remove sap from the plants and cause sterile heads, which will rot. Besides affecting the weight, number and size of seeds, sugarcane aphids produce a sticky, syrupy honeydew that clogs combine belts and screens.
Researchers have advised producers to plant their grain sorghum crop as early as possible. Planting dates in the state range from early April and May for south Louisiana to mid-April and May for north Louisiana .
"Really, the key to controlling the aphids, not considering resistant hybrids at this point, is to stay on top of them and not let them get numerous," Kerns said.
But LSU AgCenter researchers say effective hybrid varieties are the ultimate defense against an onslaught of sugarcane aphids. Gerald Myers, an LSU AgCenter plant breeder and genetics professor, had to transition from his normal crop, cotton, and begin efforts to develop good-yielding sorghum hybrids suitable to Louisiana’s climate with resistance to the aphids. His first step was to obtain sorghum lines representative of the majority of sorghum diversity worldwide. Researchers planted 320 lines in Alexandria and Winnsboro in 2014. From those evaluations, a smaller subset was identified.
"Louisiana is one of only a few land-grant universities in the Southeast with grain sorghum breeding programs," said Myers.
Sugarcane aphids and the plant disease anthracnose are main concerns to the breeding program, according to Myers.
"Breeding is not one of those disciplines in which you identify a problem and have a result six months later," said Myers. "You identify a problem, and it can be six or seven years before you have a solution."