Dan Marzu
UW-Extension Agriculture Educator
Lincoln & Langlade Counties
This past week I have been receiving phone calls regarding large groups of green worms invading lawns and corn fields throughout Lincoln County. These worms, called armyworms, are larvae of a white moth. They do not overwinter in Wisconsin and instead have to migrate from southern states. The larvae prefer grasses and small grains. Bryan Jensen, UW-Extension Nutrient and Pest Management Program, stated that it has been about a decade since we have seen this much damage to crops in the area.
This pest can be an economic pest for corn, small grains, and pastures. – as well as a pest to home gardens consisting of sweet corn and decorative grasses. Armyworms can have two to three generations throughout the year. The larvae will hatch seven to 10 days after the eggs are laid. The larvae are brownish green and have alternate dark and light stripes down their backs. The head is a pale brown with dark markings. They will feed for three to four weeks then pupate for two weeks and emerge as the adult moth.
Farmers should check their fields for worms. For corn, treat if 75% of the plants have one caterpillar or if 25% of the plants have two or more caterpillars. For small grains, treat if there is three per square foot. Harvest if the small grain is dry enough in order to control the larvae. If an insecticide will be used then be cautious if the pre harvest interval fits into the harvest timeline.
here are no thresholds for pastures. If an insecticide will be used be sure of the pre harvest interval prior to introducing livestock back into the pastures after application. Although not a preferred crop, if the larvae are found in soybeans use the general threshold of 20% defoliation during the reproductive stages. If the worms are over one inch long no treatment is recommended as they will begin to stop feeding and begin to pupate. Recommended insecticides can be found in the UW-Extension publication A3646 Pest Management in Wisconsin Field Crops. Contact the UW-Extension office for more information on insecticides.
For home landscaping and gardens there are no set thresholds to aid in treatment. The few phone conversations I have had with homeowners describing worms so thick you can’t walk without stepping on one might seem to warrant a treatment of an insecticide. However, most of the time once the damage is seen it is too late and the armyworm is too large to control with insecticides. If insecticides will be applied, purchase insecticides that contain the active ingredients of carbaryl, deltamethrin, halofenozide, pyrethrins, or spinosad. Formulations with Bt will only control small larvae. Be cautious of applying any insecticide with pollinators present as insecticides including organic insecticides may harm these beneficial insects. Always read and follow labeled directions before applying any insecticide.
For more information about armyworms or any other pest contact the Lincoln County Master Gardener Volunteers at 715-539-1073 or the Lincoln County UW-Extension office at 715-539-1072. The Master Gardener Volunteers are also at the T.B. Scott Library Thursday nights from 5-7 p.m.