Armyworm Damage in Corn Fields

Armyworm Damage in Corn Fields

August 2006

CCES 1014


Armyworms Ravage Pastures and Field Corn in Southwestern Oregon


By Amy Peters, Extension agent and Glenn Fisher, Extension entomologist

Last year, hundreds of acres of grass pastures in Coos County literally disappeared in about a week!  It appeared the same thing was happening this year.  Fall of 2005 marks the second and even third consecutive seasons that armyworms (Pseudaletia unipuncta Haworth) have defoliated pasture and field corn in Coos County and established tall fescue seed crops in the mid-Willamette Valley.  Populations of over 30 larvae per square foot were recorded at both locations.  If they were uniformly distributed, that would be about 1,300,000 larvae/acre with a biomass approaching ten tons!

The term “armyworm” generally refers to the immature stages (worms, caterpillars, larvae) of many species of moths.  They occur suddenly and in large numbers and cause extensive defoliation of grasses and corn over broad areas.  Armyworms feed on above-ground plant parts by night and hide in soil cracks and under vegetation by day.  When they devour all leaves and consumable plant stems in an area, they move like an army to eat adjacent plants until they either mature and turn into moths or are controlled biologically or chemically. 

Armyworms are uniform in color and markings, characterized by brown and black lines running the length of the body.  They are 1 to 1½ inches long when mature.  The armyworm moth is about one inch long, has a wingspan of about 1½ inches and is tan to grayish brown.  Each of its uniformly light brown  forewings has a single tiny white dot in the center.  The moths fly at night and are often seen around bright lights on porches or outbuildings from late spring through mid-fall. 

In the past, armyworms would be found in pastures every 10-15 years.  Cold, wet winters and natural predators helped keep them under control.  Now, however, we think they are over-wintering in Western Oregon.  There may be two overlapping generations of this pest per year in Oregon.  Heaviest infestations occur in late July or early August, with noticeable damage observed from early August through September.

The few insecticides registered for armyworms in grass pastures just don’t control this pest.  On-farm trials were conducted in Coos County in 2005.  We applied for a special use permit to determine effectiveness of a new product from Dow AgroSciences, Success® (for conventional milk producers) and Entrust® (for organic milk producers). These biological insecticides have an active ingredient called spinosyn, a byproduct formed by the fermentation of bacteria.  Our results showed 90-95% control with Success and 50% or less control with carbaryl and Bt.  Cost to apply Success or Entrust was $10-20/acre, similar to the cost of the other registered products that don’t work as well.

If you noticed damage to pastures or field corn this year, you may want to scout for armyworms next year.  Begin scouting in July to determine presence of armyworms.  During the day, check for pupae and worms along fences by parting grasses and soil litter.  Look for reddish-brown pupae and curled up armyworms on the soil surface.  In the evenings, scout pastures for moths flying near lights or tall objects such as buildings.  If worms or signs of armyworms are found within five minutes of scouting, develop a pest management control plan. 

Your local Extension agent, Extension entomologist, or product representative should be able to provide additional information.        


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