The armyworm (Pseudaletia unipuncta) has recently infested turf
areas throughout Massachusetts, from Cape Cod and Plymouth County in the
east to Southwick in the Connecticut River valley. In each case the infestation
has appeared with little or no warning. The information in the following
"frequently asked questions" has been compiled from e-mail exchanges
with Dr. Paul Heller (Pennsylvania State University) and Dr. Dave Shetlar
(Ohio State University), as well as from three turf insect references:
Vittum, Patricia J., Michael G. Villani, and Haruo Tashiro. 1999. Turfgrass
Insects of the United States and Canada, Second edition. Cornell University
Press, Ithaca, NY.
Potter, Daniel A. 1998. Destructive Turfgrass Insects. Ann Arbor Press,
Chelsea, MI.
Watschke, Thomas L., Peter H. Dernoeden, and David J. Shetlar. 1995.
Managing Turfgrass Pests. Lewis Publishers, Boca Raton, FL.
What are the caterpillars that are covering my lawn (or golf course,
or athletic field)?
The incredible infestations that are being reported throughout Massachusetts
are huge populations of the armyworm, Pseudaletia unipuncta. There are
many species of "armyworms", most of which exhibit very typical
behavior - appearing in overwhelming numbers over a short period of time,
congregating and feeding in densely packed groups that migrate from one
site to another. The accepted common name of the species we are seeing
on Massachusetts turf is the "armyworm", but some people call
it the "common armyworm", to avoid confusion with some of the
other species.
What is different about these caterpillars?
Young armyworm caterpillars are pale green, and look like inchworms, "looping"
across the grass. Full-grown caterpillars are about 1.5 inches long, dark
green or green gray on the top (some describe it as navy blue or black).
There are two pale stripes along each side of the body and another straw-colored
broken stripe down the middle of the back. When the caterpillars are disturbed,
they tend to curl into a tight ball. The head of the caterpillar is light
brown with a series of intertwining dark lines. The caterpillars are relatively
smooth (not hairy, like gypsy moths).
Where did these armyworms come from, and why us?
It is not clear whether the armyworms can survive winter conditions in
New England, but there is little doubt that the moths can fly into the
region in large numbers each spring. We believe that moths arrived on
one of the frontal systems that delivered thunderstorms in late May or
early June, marking the end of the six-week mini-drought that much of
New England experienced. Armyworms often attack corn, but corn had not
yet germinated because the soils were so dry. So we suspect the moths
sought an alternative, and found lawns and other expanses of turfgrass
to be very suitable.
Heavy armyworm infestations are not unheard of in New England, but they
are quite uncommon. We do not yet know why this year has produced a banner
crop, but we think the dry conditions when the moths arrived induced moths
to lay eggs in turf, and the subsequent wet conditions may have been beneficial
to caterpillar survival.
What can we expect in the next week or so? And will the grass recover?
Armyworm caterpillars tend to move en masse from one location to another.
Homeowners whose lawns are currently inundated with caterpillars (variously
described as "Medusa's hair, or something from Hitchcock's "The
Birds") can expect that caterpillars may persist for several days
before they move on to a neighboring site. The caterpillars may be less
active in the heat of the day, and may migrate short distances to stay
in shade. As evening approaches, they begin to move back to the turf areas
to resume feeding.
Most damage appears to be relatively superficial - the caterpillars feed
on grass blades and upper parts of stems, but seldom feed on the crowns
of plants. This means that many turf areas should be able to recover reasonably
well if they can be irrigated during the next couple weeks.
If most caterpillars in an area are an inch or more in length, attempts
at controlling populations may not be very successful. But the good news
is that large caterpillars will be moving on relatively soon - or will
pupate in preparation for emerging as moths.
What can we expect for the rest of the year?
The armyworm probably completes two generations a year in southern New
England, but may be able to complete a third generation this year (because
the spring, overall, has been quite mild). In most cases, only one generation
of caterpillars will devastate a given location. In other words, even
though we seem to be blessed with populations of biblical proportions
right now, there is a good chance that subsequent populations will move
on to other areas - perhaps corn or field crops, or blown downwind. We
can hope they are blown out to sea!
Should I treat my lawn?
It is impossible for us to make a blanket recommendation regarding treatment
of lawns or other turf areas. Each person has different conditions, expectations,
and views about insecticides. Application of a lawn insecticide that has
armyworms on the label should provide some relief for those who cannot
tolerate caterpillar activity - but the treatment window is very narrow.
In other words, if you are going to treat, do it soon! There is some evidence
that liquid formulations (once that are diluted in, and sprayed with,
water) are slightly more effective, partly because they provide more uniform
coverage on the foliage.
For those who are reluctant to use insecticides, keep in mind that many
of the armyworms will be moving reasonably soon, or will be pupating.
In either case, they should not persist too much longer. Of course, there
are no guarantees.
What is the long-term prognosis?
Given that many of the turf professionals in eastern Massachusetts have
never seen such an infestation, even in 20 or 25 years of experience,
we presume that this is a "once in a lifetime" event. We believe
the crazy weather patterns this spring somehow were perfect for armyworm
explosions. Areas that have experienced similar plagues of armyworms in
previous years normally have NOT seen outbreaks in subsequent years. Again,
there are no guarantees, but at this point it would be difficult to justify
preventive applications of insecticides to protect against future outbreaks.
Scouting regularly (especially about two or three weeks after we observe
large numbers of moths flying) should enable turf managers to determine
whether there will be another explosion this year, and whether further
control actions will be necessary.
