Large Yellow Underwing Moths

These moths may be larger than many others, but they are not yellow. They are brown. And their larvae can be devastating to your garden.

There are several types of yellow underwing. Whichever species you come across, these are airborne adult cutworms. Cutworms are destructive caterpillars that cut prized garden plants down at the soil level during the night. Native to Eurasia and North Africa, this invasive pest is now firmly established throughout North America.

Large yellow underwing (Dumi) CC BY-SA 3.0

Large yellow underwing description

As far as moths go, large yellow underwings are sturdy. They have some heft to them. While many specimens are brown, other members of this family can range from tan to nearly black. Like other moths, they rest with their wings held flat, in tabletop fashion, over the body. [Butterflies tend to hold their wings upright when at rest.] Large yellow underwings have a wingspan of 2” to 2-1/2” across.

The larvae start as tiny gray caterpillars but grow to 2” long. They can be green or gray-brown, with darkened heads. Brown cutworms have bands of gray or brown that run the length of the caterpillar. When disturbed, they curl up into a C-shape.

Large yellow underwing caterpillar (Ivar Leidus) CC BY 4.0

Large yellow underwing lifecycle

Adult moths generally fly from July to September, but those flight patterns have expanded in recent years. Females lay clusters of tiny, pearl-like eggs in the soil or on host plants. If you were to look closely, you would see that these eggs look ribbed or have a netting pattern.

Eggs start white but soon turn grayish-pink. After they hatch, these tiny caterpillars may feed a little bit, but the real damage occurs as temperatures rise. Just when your tomato, spinach, and strawberry plants are coming to life, cutworms emerge at night, devouring young leaves and severing new stems. After eating their fill, they return to the soil. There, they surround themselves with hard, rust-colored oblong cases where they pupate. Adult moths emerge 2-3 weeks later, and the cycle begins again.

Host plants

Adult large yellow underwings are attracted to butterfly bush, ragwort, and red valerian. But it is larval feeding that causes all the damage. Larvae feed on the stems and leaves of young plants. The following plants (and other members of the same families) are commonly used as food by large yellow underwing larvae:
•   Beets (Beta)
•   Broccoli and cabbage (Brassica)
•   Carrots (Daucus)
•   Grapes (Vitis)
•   Lettuce (Lactuca)
•   Rhubarb
•   Spinach
•   Strawberries (Freesia)
•   Tomatoes and potatoes (Nightshade family)

Carnations, chrysanthemums, dahlias, dandelions, freesia, gladiolas, sweet violet, and grasses are also vulnerable to cutworm feeding. But planting calendula in your garden will help to deter these pests.

Controlling large yellow underwing moths

These flying pests are attracted to lights at night. If you feel so inclined, you can sit on the porch with a butterfly net and put an end to their destructive ways with a container of soapy water or a garden shoe. Or, you can go out at night with a flashlight and handpick the buggers before too much damage is done. You might also use a garden claw to gently disturb the soil around affected plants. The larvae tend to stay near the surface, so you may be able to find and remove those pests that way. Drench tests can also flush cutworms out of the soil.