In the last 10 years, growers have seen a lot of new technology in corn production and insect control is no exception.
“The seed tag on a bag of corn is getting pretty complicated,” says Dave Buntin, University of Georgia Extension entomologist. “There are a lot of new traits, herbicide and insect-tolerant traits, and it’s getting even more complicated as new products come on line. Five to eight different genes have increased the price of a bag of seed over conventional corn by up to $70-75/bag.”
“There are a large number of seedling pests in corn, with no one being dominant. These can occur at any time in fields under different conditions. Most of these are enhanced by reduced tillage. Poor growing conditions and weedy conditions can enhance cutworms. The exception is the lesser cornstalk borer, and it likes a hot and dry environment in conventional tillage. Chinch bugs also prefer conditions that are hot and dry,” says Buntin.
This past year, in Georgia, and farther west, in the Delta the last two years, there were many problems with sugar cane beetles, he says. “And it is giving us fits where it has occurred because it’s difficult to control.”
There is no key insect pest of corn in Georgia causing serious damage in most fields every year, says Buntin. But corn is sensitive to plant population. “As little as a 10% loss in stand will reduce yield potential. Consequently, insect management in corn focuses more on seedling insect pests causing stand loss than in other crops. Once corn plants are established and past the seedling stage (six-plus leaf stage), the crop is quite tolerant to insect injury,” he says.
Corn can tolerate considerable leaf defoliation and some ear and kernel damage before significant yield loss occurs, he says. “Therefore, insecticide use in field corn in Georgia historically has been limited and aimed mostly at soil and seedling pests,” says Buntin.
Insect pest management in field corn, he says, consists of two approaches: 1.) prevention of insect damage by crop management and preventive insecticide use in high-risk situations and 2.) regular monitoring of the insect-pest infestations and treatment on a field-by-field basis as needed after plants have emerged.
“Historically low commodity prices for corn made routine preventive use of insecticides in Georgia a questionable practice. However, recent robust grain prices and availability of low-cost seed treatments make active pest management with insecticides more beneficial,” says Buntin.
Cornfields should be checked about two weeks after planting to verify that plants are emerging and to determine the kinds and numbers of insects that may be present and initiate controls if necessary, he says.
“Inspect at least 10 whole plants at each of 10 different locations for average sized fields. Sample the entire field. Yield loss occurs when as few as 10% of plants are destroyed or damaged so severely as to prevent normal stalk and ear development. Look for insects around the plants, on the plants, and in the soil around the stem and roots.
“Also, look for dead, dying and lodged plants. If insects are present, heavy damage to the young seedlings can occur in two to three days if not controlled, says Buntin.
Growers should check late-planted corn very carefully for the lesser cornstalk borer by looking for larvae (usually in a silken tube) boring into the plant just at the soil line.
Once corn plants reach the five to seven-leaf stage, they are large enough to escape damage by most seedling pests, says Buntin. Most insects of importance during the whorl stage defoliate the whorl and leaves. These include grasshoppers, armyworms, corn earworm, cereal leaf beetles and others.
Stalk-boring and root-feeding insects can be a problem on corn at mid-season, says Buntin. European corn borer, Southwestern corn borer and Southern cornstalk borer are caterpillars of moths that tunnel inside corn stalks during the whorl and ear fill stages.
“Western corn rootworm is present in the northern two thirds of Georgia, but the insect continues to spread southward. Larvae feed on root tips causing root pruning reducing root activity, and yield potential,” he says.
Stink bugs can cause feeding damage to small developing ears before silking, says Buntin. “This type of feeding injury usually deforms ears into a C or boomerang shape. These ears fail to develop properly and are more susceptible to infection by corn smut fungus.”
Stink bugs, he says, are a perpetural problem on a variety of crops, moving from crop to crop as it enters fruiting and seed set. “In research looking at stink bugs using pheromone traps in corn, we focused on brown stink bugs. We found two populations, one in early June that corresponds with silking in corn, and a second population as the corn begins to mature and dry out.”
Resarchers, says Buntin, are looking at thresholds using various approaches. “Based on corn prices of about $4/bu., it’s one bug per two plants. With current prices being higher, the threshold is probably even less. Brown stink bugs are a lot harder to control than green. Methyl parathion has been our standard where it can be used. It will be phased out in 2013. At that point, at least with stink bugs in corn and in soybeans, we’re going to have some significant control challenges.”
Corn rootworm adults, Japanese beetles and grasshoppers can clip corn silks and interfere with pollination, says Buntin. Silk damage or removal by insect feeding can cause poor seed set and partially filled ears.
Damage must be severe, he says, to justify control with insecticides. Insecticidal control may be needed if most ears are infested, if silks are being clipped to within ½ in. of the ear tip, and one to two or more rootworms or Japanese beetles per ear are present.
Maize weevils naturally infest corn in Georgia as the crop matures in the field, says Buntin. “Timely harvest is the most effective tool for minimizing maize weevil infestations in the field. Insecticide control before harvest is not recommended in the field. Instead, corn should be treated as it is placed in storage and managed to reduce the temperature of the corn in storage.”