Photo courtesy of the University of Georgia
By Clint Thompson
Pyrethroid resistance in corn earworm is a threat to the sweet corn industry in South Georgia. Among all the other insect pests that impact sweet corn production in Georgia, none compares to the impact of corn earworm.
Corn Earworm
The usual chemical applications that growers have depended on to control corn earworm have become less reliable. This has forced farmers to depend on other insecticides that are far more costly and not totally dependable, explains Stormy Sparks, University of Georgia Cooperative Extension vegetable entomologist.
“The big thing right now is corn earworm and pyrethroid resistance. It’s a threat to the industry in the Southeast. Typically, the way we control corn earworms, particularly from mid-summer on, is you’re spraying pyrethroids daily for 18 to 20 times. That’s about $3.50 an acre,” Sparks says. “If the pyrethroids aren’t working, and if you must spray with the other products, you’re talking $20 to $25 an acre. The finances get ugly in a hurry.”
Corn earworm is an annual problem for growers. They don’t even need to scout for the insect’s presence. They scout just to verify that the insecticides they’re applying are successful.
“It’s been building over the last five years. The previous season and last year it got really bad,” Sparks says. “The problem is not only finances because you’re going to more expensive products, but all these other products have application limitations. You can run out of legal applications really quick.”
Sparks added that if fields sustain 9% or 10% damage, growers will not even follow through with harvests. The insect attacks ears but also lays eggs on silks. The larvae travel down to the silk panel and eat on the tip of the ear.
Wes Briggs, a crop consultant in South Georgia, scouts about 5,000 acres of sweet corn every year. He has seen firsthand the insect’s impact, especially during the summer.
“Resistance to corn earworm has been a tough one in sweet corn. We have not been very successful, especially with the July moth flight, which seems to be the heaviest. Any corn silking after July 10 has been a major problem in South Georgia, controlling worms,” says Briggs. “That corn is sprayed every day. We’re using all of our better insecticides. We shouldn’t be seeing the pressure that we’re seeing. We shouldn’t be seeing the escapes, not with the program that we’re on.
“With the standards that are in place right now, to have good quality corn and not getting turned down, it’s definitely a threat (to the industry).”
Cornsilk Fly
While corn earworm is a top concern for Georgia producers, cornsilk fly remains a concern for South Florida.
Julien Beuzelin, associate professor at the University of Florida Institute of Food and Agricultural Sciences Everglades Research and Education Center in Belle Glade, explains the impact and challenge that cornsilk flies present to Florida producers.
“One of the species, the most problematic, has reduced susceptibility to insecticides. Even if you do everything right, you do not always have high mortality,” Beuzelin says. “We are also limited to insecticides that are effective. The main tool is pyrethroids. Because of that, we mainly target adults. So that means whenever they lay eggs, and the maggots start infecting the ears, we cannot get to them.
“Even if you do everything right for 2.5 weeks, but you have a few days where you cannot spray, you can still have an issue. If we miss a window and cannot get to the adults, and they lay eggs and result in maggots, it’s game over.”
Cultural control has shown effectiveness in limiting the cornsilk flies’ impact.
“We’ve shown that when corn matures late in the season, mid-to-late May, because populations are very high then, it’s highly susceptible. The earlier planting dates are safer,” Beuzelin says. “We know that the weeds surrounding the fields, for example, can harbor the flies. Keeping the fields and their surroundings clean is important.”
Photo courtesy of the University of Florida
Fall Armyworm
One pest problem that both Florida and Georgia growers share is fall armyworm. Both Sparks and Beuzelin agree that producers have been effective in controlling the pest with the current available insecticides.
“Fall armyworms are also a concern and tend to be worse in the fall. They can be a problem in other growing seasons. If they are not managed, they can result in crop failure. As of now, we have numerous effective insecticides. If used according to the label, they usually take care of the problem,” Beuzelin says.
As with all crops, adequate scouting can lessen an insect’s impact and decrease the finances needed to fund an expensive spray program.
“For fall armyworms, you can scout and spray as needed, especially before you have ears. You can tolerate a little bit of infestation,” Beuzelin says. “If you have nothing or very low levels of infestation, it can be tolerated early when you have young sweet corn plants. When you have ears, you must be more aggressive. If you don’t have armyworms, you can really save some sprays by scouting. Spray more aggressively if you have high armyworm pressure.
“For silk flies, it’s more challenging because of their behavior. Scouting is important to adjust the spraying. The philosophy is different because there’s more prevention with the silk flies. The risk is higher.”