Corn that is well established and past the seedling stage in Southeastern fields can be highly tolerant of insect damage, so most insecticide use in the region focuses on soil and seedling pests. But this doesn't mean yield-robbing insect damage can't occur in the later growth stages.
There are three key times for controlling insects in corn, says University of Georgia Extension entomologist David Buntin — at planting for seedling corn, at the whorl stage for worms, and at the ear formation stage.
Field corn in Georgia, says Buntin, is subject to attack by many different kinds of insect pests, and some are capable of completely destroying a corn crop. “However, there is no key insect pest of corn in Georgia causing serious damage in most fields every year. Indeed, in many years, most corn fields may completely escape serious insect damage,” he says.
Corn is sensitive to plant populations, and a stand loss of as little as 10 percent can reduce yield potential, says Buntin. “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, corn is quite tolerant of insect injury. Corn can tolerate considerable leaf defoliation and some ear and kernel damage before significant yield loss occurs,” he says.
Historically, low commodity prices for corn made routine preventive use of insecticides in Georgia a questionable practice, he says. “However, the recent increase in grain prices and availability of low-cost seed treatments make active pest management with insecticides more beneficial.”
Once corn plants reach the five to seven-leaf or whorl stage, they are large enough to escape damage by most seedling plants, says Buntin. “Most insects of importance during the whorl stage defoliate the whorl and leaves. These include grasshoppers, armyworms, corn earworms, cereal leaf beetles and others.”
Whorl-stage corn, he says, is very tolerant to defoliation. “Almost 50 percent leaf loss can occur at the five to seven-leaf stage before any yield loss is expected,” he says.
Whorlworms — including fall armyworms, corn earworms, true armyworms and other armyworms — infest whorls where they chew large holes in expanded and unfurling leaves. These caterpillars as a group are sometimes called budworms, says Buntin. “Armyworms lay masses of eggs on the leaves, whereas corn earworms lay single eggs. Small larvae cause window-pane or shot-hole type injury before moving to infest the whorl. As they feed and grow, they tunnel in the whorl causing large holes to develop as the leaves unfold and expand.”
Armyworm and earworm infestations during the boot stage often may cut all or part of the tassel before it emerges, he says. Controls should be initiated when 30 percent of the plants in a field are infested.
“Use ground equipment and apply at least 20 gallons of finished spray per acre directed down into the whorls. Cone-type nozzles producing large sized droplets will aid in control. Bt corn, both YieldGard corn borer and Herculex will prevent damage by whorlworms, but Herculex is more effective than YieldGard-CB under heavy infestations.”
The cereal leaf beetle, says Buntin, is a pest of winter small grains, where larvae defoliate leaves in the spring. Newly emerged adults leave small grain crops as they mature and move to adjacent grass crops such as corn. Adults chew long irregular lines in leaves of seedling and whorl-stage corn.
Corn fields located immediately next to small grain fields are most heavily infested, he says. Beetles occur initially along the field edge and often can be controlled by treating the first 50 to 100 feet of the corn field edge.
Grasshoppers feed on many different plants and usually are a problem in dry years, says Buntin. “Adults are very mobile and hard to control. Nymphs should be controlled if they are causing excessive defoliation and are numerous.
“Reduced-tillage situations tend to have greater grasshopper infestations than clean-tillage fields. Grasshoppers typically occur along the field edge initially and often can be controlled by treating the first 50 to 100 feet of the corn field edge.”
European corn borers, Southwestern corn borers and Southern cornstalk borers all are caterpillars that tunnel inside corn stalks during the whorl and ear fill stages, he says. Eggs are laid in masses on leaves, and small larvae feed on foliage before tunneling into the stalk. Once in the stalk, they cannot be controlled using insecticides.
“Stalk borers usually are not serious insect pests of corn in most of Georgia. The Southwestern corn borer only occurs in the northwestern part of the state and usually causes significant stalk damage in later plantings. Bt hybrids are very effective in controlling these insects.”
Western corn rootworms are present only in north Georgia, but the insect continues to spread southward, says Buntin. Larvae feed on root tips causing root pruning which in turn reduces root activity and yield potential. In severe cases, most of the roots are destroyed, causing the plants to lodge or fall over in a “gooseneck” appearance.
“Western corn rootworms only feed on corn,” he says. “Adults are attracted to silks where they feed and lay eggs in the soil in corn fields. Eggs over-winter and hatch the next year and damage the following corn crop. Therefore, crop rotation is very effective in controlling this insect.
“At-planting insecticides are recommended for preventive control in continuous corn fields with a history of rootworm damage. Hybrids with Bt rootworm technology also effectively control Western corn rootworm.”
Corn insect damage also can occur during the ear formation, tasseling/silking and kernel-fill stages, says Buntin. Stink bugs can cause feeding damage to small developing ears before silking. This type of feeding injury can deform ears into a “C” or boomerang shape that usually curves away from the stalk, he says.
“These ears fail to develop properly and are more susceptible to infection by corn smut fungus. Growers should treat during the ear elongation stage — before silking and pollen shed — if one stink bug per 20 plants is present. After ear elongation and seed set, stink bugs only damage individual kernels and control is not warranted unless populations reach one bug per 10 ears.”
Brown stink bugs, says Buntin, are less susceptible to pyrethroid insecticides than Southern green stink bugs. “Growers should use pyrethroid insecticides such as Capture, Warrior, Decis, Baythroid or Mustang MAX, if green stink bugs are prevalent. If brown stink bugs are prevalent, it is recommended that growers use methyl parathion — Methyl 4E or Penncap M — but they should not apply methyl parathion during pollen shed.”
Corn rootworm adults, Japanese beetles, corn earworm larvae and grasshoppers all can clip corn silks, he says, thereby interfering with pollination. “Silk damage or removal by insect feeding can cause poor seed set and partially filled ears. Damage must be severe to justify control with insecticides. Later plantings also tend to suffer more silk clipping than early plantings.”
Insecticidal control may be needed, says Buntin, if: 1.) Most ears are infested; 2.) Silks are being clipped to within ½ inch of the ear tip; and 3). Five or more rootworm beetles or two or more Japanese beetles are present per ear or corn earworm larvae are present on most ears.
Aphids seldom require control on field corn in Georgia, he adds. “Corn leaf aphid is the most common aphid occurring on field corn in Georgia. Natural enemies such as ladybugs and parasites are usually effective in regulating them at non-damaging levels. Consider control if heavy aphid infestations occur and leaves appear to be drying and dying over large areas of the field, or if aphids on the tassels and silks appear likely to interfere with pollination.”
Corn earworm and fall armyworm larvae feed on developing kernels on corn ears, according to Buntin. “Corn earworm feeding damage usually is confined to the tips of the ears. Several small larvae may infest an ear, but because larvae are cannibalistic, usually only one larva completes development per ear. Corn earworm feeding activity tends to open up the husks to provide points of entry for kernel diseases and secondary insects such as sap beetles.
“Later plantings have greater infestations than earlier plantings. Infestations of 60 percent to 100 percent of ears can occur in some years and in later plantings in most years, but yield loss generally is less than 5 percent.”
Fall armyworm damage, says the entomologist, is similar to corn earworm damage, but several fall armyworms may complete development in a single ear. “Therefore, damage during armyworm outbreaks can be much more severe than by corn earworm. Early planted corn often escapes ear infestation by fall armyworms. Because larvae are protected within the husk, using insecticides to control corn earworms and fall armyworms in the ear is not feasible in field corn. Bt corn is helpful in partly preventing ear damage by corn earworms but growers should not expect more than 50 to 75 percent control.”