You might want to think twice this year before taking a mower or a herbicide to those lush, green turnrows. They could be harboring populations of insects that, given the opportunity, will move readily to adjacent cotton.
The same cloudy, wet conditions that have adversely affected cotton fruiting in the Delta have been ideal for grasses and weeds, and destroying those wild hosts could result in exploding insect populations in your cotton, says Gordon Andrews, entomologist at Delta Research and Extension Center in Stoneville, Miss.
“We're not paying for our water with dollars, but with sunlight,” he says. “This is showing up in the fruiting behavior of cotton, with the cotton in some fields not fruiting until node eight and nine.”
And, Andrews says, fewer bolls means it takes fewer insects to cause enough damage to lower yield potential. “Cotton plants at node 10, which should have seven to 10 squares, may only have two squares,” he says. “If an insect attacks one square on a plant with 10 squares, that plant has a 90 percent fruit set. However, if an insect attacks one square on a plant that has only two squares, that plant will have a 50 percent fruit set.”
It all comes down to the effect sunlight, or a lack thereof, has on plant development. Too few langleys — which measure the solar radiation equivalent to 1 gram calorie per square centimeter of irradiated surface — and the cotton plant must adapt itself to capture what little sunlight is available. To do that, plants will put on more green foliage in attempt to capture available sunlight, and the energy that would normally go into boll production instead goes into vegetation.
“In the seven-day period ending June 19, we averaged 379 langleys per day, when 563 langleys per day is normal,” Andrews says. Fewer than 300 langleys per day for three days or more can have a substantial effect cotton plant development.
Because reduced fruiting goes with an increased susceptibility to insect damage, Andrews recommends mapping cotton plants to determine how many squares are on plants. “If your cotton crop is not fruiting properly, you probably need to lower your insect thresholds,” he says.
In addition to mapping cotton plants, Andrews says growers should be alert to potential insect infestations from other sources, including turnrows, ditches, and neighboring fields.
Recent rains have kept many wild hosts lush, making that foliage more attractive to insects. The flip side to that, however, is that as host plants dry down, insect populations begin to move in search of another desirable home.
Between rains, Delta growers are busy playing catch-up, trying to rid their crop fields of weeds and cleaning up turnrows. At the same time, county and state maintenance crews are mowing along roads. What that means, Andrews says, is that many growers are inadvertently setting themselves up for an invasion of pests into their cotton fields.
“Because of the good growing conditions of the wild hosts, plant bugs may be moving from weeds that are killed or mowed to cotton over a longer period,” he says. “High square loss can occur in a short period, and extreme losses can occur in adjacent areas where plant bug host plants are destroyed.”
Historically, he says, growers have until about July 21, depending on their location, to make squares they expect to pick.
“Preferably by now, plant bugs should be controlled, cotton plants should be between nodes 10 and 13, and our beneficials should be building faster than our pest population,” Andrews says. “Know what is in your fields and use whatever free control is available while you can.”
He adds, “We're already somewhat late this year because of a relatively cool spring and less-than-desirable planting conditions; we don't need any additional delays in maturity between now and harvest.”