Spring Miller Moth Invasion
It is once again the time of year when uninvited guests in the form of pesky moths can be found in and around our homes and other structures. These “miller moths” are the adult stage of army cutworm caterpillars that were found this spring in Nebraska winter wheat and alfalfa fields, as well as pastures and grassy areas. The moths are generally gray or light brown (Figure 1), with a wingspan of 1.5 to 2 inches. Each forewing is marked with one circular and one bean-shaped spot, as well as other dark and light markings that can be highly variable.
The moths that we see now are beginning their migration journey. Spring-emerging army cutworm moths migrate from east to west to eventually spend the summer months in mountainous areas of Colorado, Wyoming and Montana. The migrating moths feed in the evening on nectar from flowering plants such as lilacs, viburnums and linden trees. They do not cause any damage to the plants as they feed. During the daylight hours, the moths seek shelter in cracks and crevices, including those found in houses and other buildings. The moths begin to emerge from these locations at dusk to resume their feeding and westward migration.
The moths are attracted to light and may be noticed circling porch lights, where they can easily move into living spaces. Moths inside structures can cause human anxiety from their persistent movement around lighting. While they do not cause other harm (these moths will not be laying eggs or infesting pantries and clothing closets), they can leave droppings, resulting in small stains on surfaces. The stains are readily removed with a mixture of water and most types of common cleaning solutions.
The best management method is to keep the moths from entering structures.
- Seal cracks and crevices: Inspect doors, windows, utility openings and other potential entry points for any gaps. Seal them with caulk or weatherstripping to deny moths easy access.
- Install screens: Ensure that windows and vents have properly fitted screens to prevent moths from entering your living spaces while allowing fresh air to circulate.
- Use light management techniques: Reduce outdoor lighting or switch to yellow-colored bulbs (which are less attractive) near entry points, especially during the moth's peak season. Draw curtains or blinds after sunset to minimize the attractiveness of indoor lights.
- Close doors and windows: Be vigilant about keeping doors and windows closed, particularly in the evenings when moths are most active. Use door sweeps to block gaps under exterior doors.
The moth’s attraction to light can also be used as a method to remove those that have entered a building. A simple trap can be constructed with a desk lamp placed over a large bowl that contains soapy water. The moths that fly around the trap will soon be caught in the water where they will drown. The dead moths can then be dumped outside. Application of insecticides are not recommended to control adult moths.
The great hordes of millers in the spring are a result of their migratory nature. Their numbers depend on spring cutworm populations and environmental conditions. Moths emerging in Nebraska tend to remain in the area for two to three weeks but may stay for up to six weeks or as long as local plants are flowering. Cool, wet conditions during this time will extend their stay. Hot, dry conditions will encourage them to move westward.
The moths will migrate westward to higher elevations as they follow the progression in the initiation of spring flowering plants. During this time, with the aid of easterly winds, moth concentrations can increase dramatically. When the last trees finish flowering (e.g. locusts and lindens) and temperatures increase in the high plains, the moths move to the Rocky Mountains. There they escape severe summer temperatures and find alpine flowers, their primary food source. Interestingly, the moths can serve as a high-energy food for grizzly bears foraging in high alpine areas.
In September, the moths once again return to the plains. Army cutworm moths are noticed throughout Nebraska from mid-September through October. As they migrate eastward in the fall, they mate and lay eggs in barren or sparsely vegetated fields, especially winter wheat, alfalfa and grasslands. The eggs hatch within a few weeks and the larvae begin to feed. The cutworms enter a diapause state during the winter and resume feeding in the spring.