Corralling southwestern corn borers (SWCB) can be challenging in Arkansas, Alabama, Tennessee or Texas. Growers must fight Bt refuge limits of 20-50% and sometimes juggle their insecticides to prevent outbreaks of other menaces and obtain control of others.
SWCB causes lodging due to its larvae tunneling in the stalk and girdling the plant. Postharvest tillage can help manage this pest. But in an age of no-till farming, where growers want a residue blanket to control soil and water erosion, a good spray program is vital.
“Some areas obtain good control by spraying with bifenthrin (Capture), but we prefer Intrepid (methoxyfenozide) because of the problems we have with spider mites being resistant to bifenthrin,” says Roxanne Fegley, Texas AgriLife Extension agent, integrated pest management (IPM) at Dumas in the northern Texas Panhandle.
The four-county area she covers produces about 400,000 acres of corn annually under irrigation, and yields often exceed 250 bu. So poor bug control means a bust.
Second-generation SWCB — that do the most damage — are creating bigger headaches. “SWCB has become a bigger control problem for us because of a very extended time that egg-laying moths fly,” says Fegley. “We see some lodging where they aren't controlled. We have to apply more insecticide.
“However, our 2008 data shows that the second-generation SWCB has a seven-week moth flight. Our insecticides usually don't last that long. So it requires a second application.”
At $20+/treatment, control is doubly expensive. “We're trying to combine the applications with other treatments to save on aerial spraying costs,” she says.
IN THE SWCB cycle, moths emerge from corn stubble and weed hosts in the spring to lay first-generation eggs on corn whorls. First-generation larvae mature and pupate in the stalk. Moths emerge about mid-July and lay eggs of the second generation, usually after tasseling.
Texas A&M entomologists say that about three-fourths of these eggs are laid on the upper surfaces of the middle seven leaves (the ear leaf, two leaves above and four leaves below the ear leaf).
Eggs are laid singly or in masses of two, three or more. Eggs overlap like fish scales or shingles. Freshly laid eggs are creamy white. One day later, three red bands appear across each egg.
Mature larvae reach 1-1½ in. long. They are dull white and have a regular pattern of raised black dots over the body. As plants mature, larvae prepare for overwintering in the base of the stalk by girdling the plant from 1 to 6 in. above the ground. Wind can easily lodge girdled plants.
For growers using minimum tillage, a single tandem disk cultivation or shredding will expose larvae to cold and dry winter conditions while leaving sufficient residue to prevent soil erosion. Double disking and deep plowing are effective if soil erosion is not a problem.
However, every producer must cooperate by destroying stubble to effectively reduce SWCB populations area-wide. Insecticide should be applied when 20-25% of the plants are infested with eggs or newly hatched larvae.
Different regions have different insect problems. Since the northern Texas Panhandle sees spider mite resistance, growers must apply a separate pesticide, usually Comite, Oberon or Onager, in conjunction with the SWCB treatment.
“We also have a western bean cutworm problem, which sees its insect flight before the second-generation SWCB,” says Fegley. “That can require up to three Intrepid treatments, which is certainly not cost effective.”
Other suggested insecticides for SWCB treatment include carbaryl (Sevin 7), chlorpyrifos (Lorsban), esfenvalerate (Asana) and permethrin (Ambush and Pounce). Fegley encourages growers to consultant their local IPM agent or Extension entomologist for which SWCB control is best in their areas.