From Ken Wise: Eastern NYS Extension IPM Specialist- Livestock and Field Crops
Mike Hunter (CCE Northern NY) is finding a lot of true armyworm in grass hay fields in Northern NY. He states the larvae are still very small (1 mm to 1.5 mm) and are in the 1st instar. These armyworm moths most likely have come on the recent storms from the south and laid their eggs in hay fields. If there is a lot in the field you will not see much damage until they reach the later instars. True armyworm larvae in their (6th) final instar will eat 80% of all the forage they will consume. Many times a hay field can look great in the evening and gone the next day if they all reach the 6th instar about the same time.
It is best to scout your fields ASAP and look for smaller larvae. Be proactive make sure you know if your fields are infested. True armyworm will feed on grasses, corn and small grains. There are economic thresholds for corn and small grains.
Recommended economic thresholds for corn:
- seedling: 10 percent or more plants show damage and larvae are still present.
- whorl-stage: apply an insecticide when there are three or more larvae per plant.
- Tall corn seldom needs treatment unless the leaves above the ear are also damaged.
Recommended economic thresholds for small grains:
- Wheat/small grains – 5 or more larvae per linear ft of row, larvae less than 1.25 inches and not parasitized, watch for flag leaf reduction or if grain heads clipped off – yield losses, a spray before soft dough to save the remaining 3 upper leaves is generally beneficial since these tissues are still important to grain filling
Recommended economic thresholds for grasses:
- Grasses – no specific guidelines available, need for treatment based on the level of damage observed in relation to the expected value of grass harvest
Most years, natural enemies—various fungal and viral diseases as well as parasites such as tachinid flies—help suppress armyworms. You cannot be sure when and where they occur.
Sometimes when armyworms are at very high populations they will march to new fields. They can be in a hay field and move to a corn or small grains field.
SPECIAL NOTE: if you spray for armyworm the CROP and True/Common Armyworm has to be on the label! READ THE LABEL!!!!!
Check the Cornell Guide for Integrated Crop Management for an insecticide labeled for use.
I have added some web links that have specific armyworm information:
Armyworm (Pseudaletia unipuncta Haworth)
Armyworm as a Pest of Field Corn
Cornell Guide for Integrated Field Crop Management
Armyworm Damage to Field Corn and Grass Hay and Pasture