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Armyworms Invade Lawns

Posted: June 8, 2012

This spring Penn State has received calls from Erie to Chester County and everywhere in-between about armyworm infestations in grass pastures, corn and small grain fields.
Armyworms, showing color and size variations, and feeding damage.

Armyworms, showing color and size variations, and feeding damage.

They’re back….

Extension offices and lawn care companies across Pennsylvania have received numerous calls over the last few weeks about a plague of worms.  These infrequent invaders are armyworms, which may be found in large numbers crawling across lawns, consuming the grass as they go.

So where did they come from?  According to Penn State entomologist Dr. John Tooker, most people want to tie their outbreaks to unusual spring weather, but no one really knows why they become so abundant some years.  This spring he has received calls from Erie to Chester County and everywhere in-between about armyworm infestations in grass pastures, corn and small grain fields.  They are most likely to show up in lawns that are near those fields.

Armyworms do not overwinter in our area, but are prevalent in the south.  The adults, which are moths, migrate northward and lay eggs on and around grass plant hosts.  Farmers and extension educators in the state use special traps to monitor for the arrival of these pests.  Armyworm adults showed up in traps around State College back in May.  A quick search on the internet shows there have been outbreaks in other states this year as well. 

The only life stage that is a problem is the caterpillar, when abundant.  The female moths can lay as many as 2000 eggs during their life, so populations can build up quickly.  The larvae are smooth caterpillars, not hairy like gypsy moth larvae or woolly bears.  These “worms” can reach 1½ inches in length.  The main body color varies among individuals from almost yellow, to yellow-green, to dark brown or black—and there are distinctive stripes running along the length of the armyworm.  The stripes are alternating dark and a salmon pink.

Armyworms actively feed on plants in the grass family.  Their feeding creates brown patches in turf that first appear like heat and drought stress.  The grass leaves themselves are ragged from where the armyworms feed on them.  In large areas the blades are clipped right to the ground.  When the population is high, feeding produces bare circular areas.  If you walk over the lawn during mid-day, you may miss them since the worms rest during the heat of the day in the thatch.  In the late evening and early morning, or anytime on cloudy days, they will be actively feeding.  Many people have described “seeing the lawn move” as caterpillars crawl over the turf looking for food.  Commonly, lawns are damaged along mulched beds or other protected areas, since the armyworms retreat to these sheltered areas to spend the day and then invade the lawn to feed.   Although it can be intimidating to watch the mass of caterpillars demolish the lawn as they feed, they do not eat everything (like chairs, houses, or decks).  Many times clover and other broadleaf weeds are left as the grass around them is consumed.

What to do?  The armyworm invasion will likely end as quickly and unexpectedly as it began.  Dr. Tooker predicts that they may be around for another few weeks or so.  Natural enemies, like flies and wasps are attracted to areas infested by armyworms.  The good news is that armyworm feeding rarely kills the turf, since they are just eating off the tops.  However, additional stresses from heat or drought can combine with armyworm damage to kill grass.  For most lawns, a light fertilization and watering if weather turns hot and dry can help the lawn recover.  Squashing caterpillars with a roller or tractor tires works, too, but it’s not for the faint of heart.

For large populations, insecticides labeled for armyworms in lawns include the active ingredients spinosad, indoxacarb, chlorantraniliprole, and pyrethroids.  All work best on small larvae.  Treatment can be limited to the perimeter since the armyworms will advance through the treated zone.  Discuss insecticide options with your lawn care professional or extension office. 

Armyworm images

Here is additional information:

Armyworm as a Pest of Field Corn - Penn State Extension Fact Sheet

Armyworms and cutworms in turfgrass - USU Extension Fact Sheet

Armyworms, Annual Bluegrass Weevils, White Grubs - UMass Extension Fact Sheet