Armyworms Come Marching
We are beginning to see an uptick of armyworms throughout the county and surrounding counties. With all the rainfall we have received and the abundant forage supply, this may not be that big of an issue for most of us.
I can find armyworms in most any pasture or meadow right now, but for the most part I have not seen high numbers. Many of us have all the hay we care to bale and excellent grazing, so small populations may not merit treating, but there can be a fine line and scouting is key.
If you have spent the money on fertilizer and are still making hay, then you will want to be extra diligent on scouting.
The larvae of the fall armyworm are green, brown, or black with white to yellowish lines running from head to tail.
There will be a white line between the eyes that forms a “Y” pattern on the face. At first, the fall armyworm caterpillar or larvae are very small about 1/8”- at this stage they cause very little damage and usually go unnoticed.
The larva will feed for 2 to 3 weeks growing to 1 to 1.5”. Once the armyworms reach ¾” in length, the quantity of forage they eat increases dramatically. During the final 2-3 days of feeding, armyworms consume 80% of the total forage during their entire development.
Once the armyworm completes feeding, it tunnels into the soil and enters the pupal stage. The moth emerges from the pupa in about 10 days and repeats the lifecycle.
Again, scouting is the key to controlling damage associated with the fall armyworm.
It is best to check fields late in the evening or early in the morning; this is when they will be feeding in the upper canopy.
Damage from the small larvae will leave a clearing or windowpane effect on the leaf, as the larvae grow, they will begin eating on the edges of the leaves.
For threshold determination, sweep nets are ideal but the old standby of running your hands over 1-2 square feet of grass then checking the soil surface below still works well.
Typical thresholds for either method would be 2 or more ½” or larger per sweep or visual check. Large numbers of small caterpillars would also merit treatment.
There are several products that will work on armyworms but may have different requirements for stage of treatment and different residual times.
Though it is more expensive, Vantacor is probably the best product; it has up to a 6-week residual and will work on mature grasshoppers and armyworms. With all the grass we have, you may just want a quick knockdown and do not require a long residual. If that is the case, any of the lambda cyhalothrin products are less expensive and will work well at killing mature worms and grasshoppers but offer only about a 7-day residual. I encourage you to check with your good local pesticide dealers and applicators to see what is working best in your area. Always read and follow all label instructions on pesticide use and restrictions.