Update: Variegated Cutworms Delaying Alfalfa Regrowth

       
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Variegated cutworms are still active on alfalfa with the commonly reported injury
being  delayed regrowth of the second crop.  The infestation appears to be statewide
with the most recent reports coming from Tripp and Gregory counties.

Variegated cutworms are often mistaken for armyworms.  It is important to differentiate
between the two since the word "armyworm" evokes unnecessary panic among growers
and the public.  Remember, not all larvae are "armyworms."

Please refer to the June 11, 2001 Issue or the pictures below to be able to differentiate
between variegated cutworms and armyworms.

     
            (University of Nebraska)

The economic threshold for variegated cutworms on alfalfa is expressed in terms of
the number of days of delayed regrowth that could be tolerated.  This threshold is
the same for any other insects that may cause delayed regrowth in alfalfa such as
the alfalfa weevil.

The thresholds vary with the cutting schedule of the grower, cost of treatment, and
the market value of the alfalfa on the field.

For example, at $80 per ton alfalfa market value, $12 per acre insecticide-plus-application
cost, and a harvest schedule based on bloom-stage, then the grower may wait 8 days
of delayed regrowth without incurring economic loss.  Spraying may be considered if
it has been 8 days since the first cutting with little regrowth on the field, and if (and only if)
the variegated cutworms are still actively feeding and are less than an inch in length.

A thorough discussion and examples can be found in my Web site on the alfalfa
weevils under the "There is no regrowth! How long can I wait?" section at:

http://plantsci.sdstate.edu/ent/entpubs/awvl_EIL.htm

The insecticide list for cutworms on alfalfa includes Baythroid, Lorsban, Pounce,
Sevin XLR Plus, and Warrior.  Read and follow label directions.

We have not seen armyworms on alfalfa this season.  However, we have confirmed an
armyworm infestation on corn near Platte last week.  The corn was directly planted on
herbicide-burned sod (a rather unique situation).  More information can be found below.

Armyworm Infestation Found on Corn Near Platte 

A severe infestation of armyworms on corn  was reported on June 20 by Jason Tiede (Agronomist,
Agland Coop-Delmont) and Bill Mayhew (Den Besten Seeds) near Platte, SD.  About 100 acres
of corn were affected, with 50 acres almost completely eaten by the armyworm larvae.

The infested corn was planted on sod that was burned using a herbicide in the spring.  I am of the
opinion that the grass was already infested with armyworm eggs or small larvae before the corn
was even planted.  When the grass was burned, the armyworms had nowhere to go but the corn
seedlings for food.

This is the only armyworm infestation that we have confirmed so far.  Again, variegated cutworms
on alfalfa are always mistaken for armyworms. We have not seen armyworms on alfalfa this season.

Armyworms can be very damaging to small grains and corn if present in high numbers.
As the name implies, armyworms can migrate in groups from field to field.

The economic thresholds of armyworms on corn are 2 worms per plant
on 25% of the stand, or 1 worm per plant on 75% of the stand.  On small grains,
the threshold is 5 worms per square foot.

Armyworm moths migrate into the state from the south in the spring.  Grassy fields appear to attract
egg-laying by the armyworm moths. And larval infestations usually starts in grasses injured by hail and
lodged to the ground. The armyworm larva may take about 4 weeks to attain its full length of 1.5 to
2 inches.  There may be more than one generation of larvae per year in SD. 

Asana XL (5.8-9.6 fluid ounces per acre) and Pounce 3.2EC (4-8 fluid ounces per acre) may be used
to control armyworms on corn.  On wheat, Warrior may be used at 2.56-3.84 fluid ounces per acre. 

Web posted on June 25, 2001 by Mike Catangui

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