An armyworm infestation is upon us
LAMBERTON -- The mere mention of armyworms can cause angst in those who have experienced the severe
outbreaks of the early 80s and earlier and the news of armyworms in the area causes can
trigger unnecessary insecticide applications. Fortunately, other than taking some time, scouting for armyworms is fairly straight forward and they are easily controlled with insecticides.
There is a small problem with using black light trap captures to predict armyworm (Mythimna
unipuncta, formerly known as Pseudaletia unipuncta, Haworth) outbreaks in Minnesota. The
moths captured can predict when a problem is likely and when it will occur but not where it will
occur. Immigrant moths are particularly likely to re-migrate. For example, at Lamberton, where we had the high trap captures, we can't find any larvae but yet they are present east and north.
From what I have been hearing from some of you in the field, there an big arc of infestation from SE Minnesota through East Central and Central MN into North Dakota. Minnesota counties reported problems include Wabasha, Goodhue, Chisago, Wright, Stearns, Brown and Wilkin. I am passing on information that is limited and the geographic scope could be larger. Reports have been mostly from corn but I know of wheat, sudangrass and grass hay needing treatment.
All the corn reports have been from fields with grassy weed problems.
Most fields do not have an armyworm problem.
Moths target specific areas to lay eggs. Areas of dense grasses are favored egg laying sites. Field edges near lodged grassy areas, lodged small grains and corn that had earlier areas of heavy grass weed problems should be checked.
Armyworms are native to eastern North America but they cannot overwinter in MN. Each spring, they migrate north like their black cutworm cousins. They seem to be more abundant in cool, wet years. Heat and dry weather are hard on armyworm eggs and small larvae.
Armyworm larvae are often heavily parasitized by flies and wasps and they can be infected by fungal and virus diseases. Eggs of fly parasites can sometimes be seen behind the heads of larvae and cocoons parasites cover some infested larvae.
These are not the tent caterpillars (called armyworms by some) that feed on broadleaf trees and shrubs. The true armyworm prefers
grasses. In previous infestations, I have seen an armyworm infestation clean out the weedy
grasses in a sunflower or bean field and ignore the broadleaf crop. However, they have
occasionally been reported as a pest on some broadleaves. This may be a result of larvae
migrating when depleting their food. Hungry larvae will move a fair distance to find a new food source. The "armies" can easily cross
a road and feed well into a field on the other side in a single night.
Armyworms have multiple but distinct generations in Minnesota. The larvae can range from tan
and olive to nearly black in color. The pattern of a dark band flanked by white bordered pink to
orange bands on along the side is a distinguishing character as is the net-like pattern on the
There are six larval instars (stages) and most of the vegetation is consumed during the last
week of larval life. Larvae are approximately 1 ½ inches long when mature. When these
larvae move underground to pupate, this year's risk is over.
Scouting and management
Chewing damage on crop leaves and the presence of frass (insect fecal pellets) on plants and
the ground indicate that an insect was present. The presence of live armyworm larvae should
be confirmed before an insecticide is applied.
Armyworm larvae are most active at night and cloudy days. During the heat and bright
sunlight, larvae often hide under leaf litter on the ground. Scouting and insecticide applications
are often more effective near dawn and dusk and on cloudy days. When disturbed, armyworms
drop to the ground and curl into a C-shape to "play possum". Preliminary scouting for
armyworms in small grains and field edges can be done with a sweep net.
Wheat, barley, oats: Pay close attention to areas that are lodged, have grassy weeds or near
lodged grass borders when trying to detect larval populations. When an economic armyworm
infestation is suspected in a small grain fields populations per square foot should be estimated.
Shake the plants and look for larvae on the ground in a square foot area. In small grains the
treatment threshold is 4-5 larvae/square foot. Check under debris and soil clumps. Do this in at
least 5 locations within the field.
The larvae occasionally clip heads and when significant can require treating at lower
populations. Head clipping is a behavioral change and usually occurs after leaves have been
defoliated or senesce.
In spite of the preference for broadleaves, anyone, including an armyworm, can make a
mistake. I'd be a little nervous with an alfalfa under seed being undamaged. A barley (or
wheat) cover crop may have more armyworm pressure than oats.
Corn: Grassy weeds are attractive to egg-laying moths. When scouting, pay close attention
to field borders and within-field or areas with current or past high grass weed pressure.
Examine plants for feeding damage and larvae. Larvae can often be found in the whorl and the
nighttime feeding often occurs in the whorl.
Treat whorl stage corn when 25% of plants have 2 larvae/plant or 75% of plants have one larva
or more. On tassel stage corn minimize defoliation at or above the ear leaf.
Lepidoptera Bt traits are tested and labeled with fall armyworm, a different species found to the
south. While there may be some benefits from the traits that are effective on other members of
the family Noctuidae (black cutworm, corn earworm, fall armyworm, common stalk borer) true armyworm is not on these labels. We are typically dealing with larger, less susceptible larvae
moving from weeds and field borders into corn. Secondly, insects must eat the Bt to be
affected. Even if the protein were effective, damage would occur with very high armyworm
populations on the move.
Do not base treatment decisions solely on field edge populations. Treatment of populations that
are near or starting to pupate or are heavily parasitized is not recommended.
Partial field or border insecticide treatments for armyworm are often sufficient when infestations
are well identified by scouting and armyworms populations are found early are the armyworms
are migrating. Treat several boom widths ahead of the infestation.
Long insecticide residuals are not needed because of the short time a larval generation is
damaging. Many products are labeled and effective. Refer to the insecticide label for rates. At
this time of year it is important to check the pre-harvest interval of any small grain pesticide.
Take precautions to protect pollinators, particularly as corn nears tassel stage.
Thanks to those agriculturalist that provided a heads up on the presence of damaging larval
populations. You have my appreciation and should have the appreciation of MN agriculture as
I hope that this year's armyworm infestation affects a minimal number of fields. High
temperatures, modern improvements in weed control, crop size and other factors are in our
favor. Any excitement should be over within a week.