Thistle Caterpillars Are Feeding on Soybeans
 
    
       (NDSU)                         (NDSU)

Thistle caterpillars (a.k.a. Painted lady caterpillars) prefer to feed on thistles. But
with the thistles gone as a result of spraying or mowing, thistle caterpillars will
also feed on soybeans and sunflowers.

Butterflies of South Dakota lists some other hosts of the thistle caterpillar.

Recommended insecticides on soybeans can be found in the link below:

http://entomology.unl.edu/instabls/soydefol.htm

The economic threshold will be the same as in other defoliators such as bean leaf
beetles and grasshoppers. At current insecticide costs and soybean market value,
treatment may be considered if 50% of the leaves of the seedlings are gone.

Are These Variegated Cutworms? Armyworms?

     
                                         (U. Nebraska)                  Armyworms                 (U. Nebraska)

Variegated cutworms are always a threat to alfalfa in SD. Alfalfa weevils are often
blamed for the injuries caused by variegated cutworms on the alfalfa regrowth.
This season, with alfalfa weevils not present in high numbers, we could more
clearly see the damage that variegated cutworms can do on alfalfa.

In addition to alfalfa, variegated cutworms  will also feed on the leaves of corn,
soybeans, turf, and many vegetables in home gardens.

Variegated cutworms are commonly mistaken for true armyworms which usually
occur in late-June through July in SD. It is important to be able to differentiate
between armyworms and variegated cutworms because the word "armyworm"
evokes unnecessary fear among growers and the public.

As you all know, armyworms can be very damaging to small grains and corn
if present in high numbers. Economic thresholds in 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.  I have no idea who created these thresholds
but seem to be the only guidelines available at the moment.

Armyworm infestations usually starts in grasses injured by hail and lodged to the ground.
Armyworm moths migrate into the state from the south.  Fields with a lot of foxtails also
appear to attract armyworm moths.  Numerous chemicals are available for use against
armyworm on corn.  Insecticides for use on small grains have a lot of restrictions and
so make sure you read and follow the fine prints on the label.

The pictures of variegated cutworms and armyworms above should help
in identifying larvae found on the field. The following sites also have good
information and insecticide recommendations for both insects:

http://entomology.unl.edu/fldcrops/pestipm.htm

http://www.exnet.iastate.edu/Publications/IPM60.pdf

Insecticides labeled for use on alfalfa, corn, and soybean to control cutworms include
Ambush 2E, Asana XL, Lorsban 4E, Pounce 3.2EC, and Warrior.  Sevin XLR PLUS
may be used for cutworms on alfalfa. The economic threshold of variegated cutworms on
alfalfa is 2-4 larvae per square foot.

Web posted on June 11, 2001 by Mike Catangui

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