What is the army cutworm?
The army cutworm larvae is an early season pest of several crops including alfalfa. They reach 1.5 to 2 inches in length. They are pale greenish-gray to almost brown in color with no real distinctive markings. They tend to be more of a problem in our western service region but outbreaks east of there are not uncommon. They begin to feed on alfalfa plants soon after plants break dormancy, preventing spring green up.
What is the life cycle?
Only one generation is produced each year. Moths emerge from the soil in late June and fly to mountainous areas and enter a period of inactivity through July and August. In the fall, moths fly back to the plains to lay eggs. Eggs hatch and larvae feed on alfalfa plants until late fall and then over winter in the soil. Fall soil moisture is required for larvae to survive. When spring arrives and the soil warms, larvae emerge and begin to feed. Once larvae are mature, they pupate 2 to 3 inches below the soil and emerge as adult moths in June once again.
What damage can army cutworms do?
They feed on stems and leaves of alfalfa plants, preventing spring green up. Although cutting of plants in established stands is unlikely, new seedings may be prone to cutting by the cutworm. Large numbers of cutworms can be very damaging and consume large amounts of vegetation. Outbreaks can appear suddenly in the spring and are favored by dry summers followed by a wet fall.
What is the economic threshold?
For established alfalfa stands, 4 to 5 larvae per square foot usually warrants treatment, while new seedings may require treatment at 2 larvae per square foot. Fields should be monitored as soon as conditions allow for alfalfa to break dormancy in the spring. Larvae will likely be just beneath the soil surface during the day as they prefer to feed at night or on overcast days.
How is this pest controlled?
Army cutworm larvae have many natural enemies, including birds, predatory beetles, and wasps which can reduce populations. Although helpful, these beneficial insects cannot be relied upon as a control measure. If the economic threshold is reached and the initial spring growth is being affected, insecticide treatment will provide the best results. The pyrethroids Warrior and Mustang Max are both very effective. Best control will be obtained if applied when the larvae are still small (1” or less).