Armyworm in Organic Corn – Ron Hammond and Andy Michel

July 1, 2012

The true armyworm, a pest of cereal grasses and corn, was a significant pest in conventional crops in parts of eastern Ohio and states to our east this past spring.  They began their feeding in wheat, rye, and oats and then moved to corn.  The larvae that caused these problems were from adults that had migrated into the state in early spring.  The next generation of larvae that will occur over the next month or so is not usually considered a problem in conventional crops because the cereal grasses have been harvested, and with corn, the crop is usually too big to be damaged in mid-summer, and seldom do you see a lot of grassy weeds in fields.

The grassy weeds are important because adult moths prefer to lay their eggs on grassy weeds, followed by larvae subsequently moving to the corn.  The concern is in organic corn fields that have a significant amount of grassy weeds present because of insufficient weed control.  Those fields might be prime egg laying sites for the moths, whose larvae, after feeding on the weeds, could move to the corn.  And the corn, if late planted, might be small enough to be very susceptible to armyworm feeding. 

We saw this scenario in 2010 in Wayne County when an organic corn field with poor grass weed control was nearly destroyed by the mid-summer generation of true armyworms.  Because of the abundance of adult, true armyworm moths, the potential for a problem exists this summer in fields having a mixture of grass weeds and corn.  Scouting for larvae and their feeding might be warranted.  If control is necessary, there are a number of OMRI listed materials available, including, but not limited to, Entrust (a spinosad product) (Note: Tracer which is also a spinosad formulation is NOT OMRI listed), Dipel (and other Bt formulations), and Pyganic.  Although Neem products have many of the armyworm species on its labels, the true armyworm is not one of them.  Timing of these materials is extremely, important; they need to be sprayed early and perhaps multiple times when larvae are small.  Growers should follow all label directions.  Although this insect usually has numerous parasitoids that attack it and various pathogens that can often keep them under control, current environmental conditions (hot and dry weather) might prevent those beneficials from preventing economic damage if pest populations are high.