Southwestern corn borers were a big concern in Mississippi and other Delta states in the 1960s, but largely disappeared when Mid-South growers turned to other crops in the 1970s and 1980s.
With the rebound in corn acres brought on by Freedom to Farm and rotational considerations, Southwestern corn borers once again have been spreading across the Delta region, according to Extension specialists.
“We’ve been seeing tremendous populations this year,” said Don Parker, Extension entomologist with Mississippi State University. Based at the Central Research and Extension Center in Raymond, Miss., Parker works with growers to control insects in corn throughout the state.
Speaking at the Monsanto-Allendale Planting Co. Technology Field Day in Shelby, Miss., Parker said Southwestern corn borer populations normally start small early in the season and then build with the second and third generations.
“This year, we started seeing high numbers of moths with the first generation in south Mississippi,” said Parker. “In the Glen Allan area (of the south Delta), we were catching 500 to 800 moths per trap early on.”
Parker displayed enlarged photographs of the minnow-shape egg masses laid by Southwestern corn borers, but acknowledged they can be difficult to find because of their size and because moths lay them down in the whorl of the corn plant.
“As a result, we began a monitoring program using pheromone traps to alert growers to the presence of Southwestern corn borer moths and the potential for egg laying in their fields.”
Tunneling by corn borer larvae causes “dead heart” in stalks. When that occurs, plants are unable to provide nutrients to fill out the ears. Later generations will come back up out of the soil and girdle the plants a few inches above the soil line, When the stalk falls over, it creates a terminal dome that shelters the insect during over-wintering.
Parker says the failure to detect and treat for Southwestern corn borer populations early can be costly.
“We had corn in the Tupelo area that was planted two to three weeks late because of the weather last spring,” he noted. “Some of those growers decided not to treat because of the economic situation. Now 70 percent of the stalks are infested with Southwestern corn borer, and they’re seeing massive tunneling.”
Because of the increasing numbers, Parker and other Extension specialists began a large-scale study of Southwestern corn borers in 2001. The study included test plots in Bolivar, North Washington, South Washington, Calhoun, Holmes, Hinds and Yazoo Counties.
“We included some locations that were not traditional areas for Southwestern corn borer infestations,” he said. “But we ran into high Southwestern populations in those areas, as well.”
The study was expanded to more locations in north Mississippi in 2002.
The test plots included corn varieties with and without the Bacillus thuringiensis or Bt gene. The Bt varieties had 0 percent infestation of Southwestern corn borers. Corn borer infestations in the non-Bt varieties averaged around 50 percent, “but some locations were much higher.
“The old literature said that most Southwestern corn borer damage came from lodging caused by the third generation,” said Parker. “But we’re seeing yield losses from earlier generations now.”
He said yield losses ranged from 15 bushels an acre at study locations with high infestations of Southwestern corn borer, 6 bushels an acre at locations with moderate infestations and 2 bushels in locations with light infestations.
The Bt corn varieties produced an average of 10 bushels per acre above the non-Bt varieties across all the test locations in 2001. “With the $9 per acre technology fee taken into consideration, growers could still see a $20 per acre advantage with the Bt varieties.”
Parker urged growers to check their fields for signs of tunneling in stalks following harvest. “Cut into the stalks and where you see high numbers of stalks with tunneling, I would plant that field in a Bt variety next year. Because they over-winter in the soil, Southwestern corn borers are likely to be back in that field next year.”
Parker reminded growers that EPA refuge requirements prohibit them from planting more than 50 percent of their corn acres in Bt varieties.
“That’s why I think it’s important that you plant fields with a history of corn borer infestations in a Bt variety,” he noted. “The other half of your corn acres will have to be managed with conventional insecticides.”
Monsanto representatives said southern corn growers would have more new varieties containing the company’s YieldGard gene and other Bt genes in 2003 than have been offered in the past.