Hordes of true armyworms have invaded much of the Northeast in the last two weeks, already causing extensive crop damage. That's the word from John Tooker, Extension Entomologist at Penn State University.
"The damage we've seen and the number of reports of damage we've received has been remarkable. From nearly every portion of [Pennsylvania] and in New York, we've heard of true armyworms chewing up corn fields, clipping the heads in wheat fields, and marching into grass hay fields. It's the most substantial influx of armyworm in years," he adds.
All fields are at risk, so get out there and look for damage. True armyworm damage to corn begins from the edge of the leaves, and often looks ragged with large pieces of tissue removed. But armyworms rarely eat the leaf midrib. In heavy damage, little more than the midrib of corn leaves will be left.
Armyworms feed at night and hide in the corn whorl during the day, where their brown, wet, mushy feces accumulate. The majority of feeding damage occurs when the larvae are nearly mature, which accounts for much of the damage seemingly appearing overnight.
In wheat, armyworms first feed on leaves, then progress upward to the head, which they can clip off as they try to get enough food. During the day, they hide at the base of plants. Clipped cereal crop heads are good signs of their presence.
Bt hybrids and seed treatment don't claim to provide significant control of true armyworm, notes Tooker. "So the best control option is to scout fields and apply rescue treatments.
When scouting corn fields, look for leaf feeding and presence of caterpillars in the whorl, where they hide during the day. Control efforts are usually not economical unless 10% or more of the plants are infested.
While a number of insecticides control true armyworm, control gets to be more challenging as caterpillars grow to be one-inch long or longer. Available and effective insecticides, in no particular order, are those with active ingredients of cyfluthrin, lambda cyhalothrin, zeta-cypermethrin.
For growers wanting to conserve natural enemies in their fields, Tracer and Intrepid 2F provide good control of armyworm and have little activity against predators and parasitoids.
Organic growers can turn to Entrust. Use higher rates the heavier the infestation and the larger the caterpillars.
Given frequent rains, it's probably best to take advantage of any spray window that comes around. Keep in mind that a stretch without rain will help improve insecticide efficacy.
For control details, click on insecticide options in Penn State's Agronomy Guide. Be sure to consult labels for specifics for each product. Also see the fact sheet: true armyworm.