Overview
Large yellow underwing moths are common pests whose larvae, known as cutworms, feed on plant roots and lower stems. They are particularly damaging to young plants and can sever seedlings at the base.
They belong to a broader group of nocturnal feeders similar to Knot Grass Caterpillars.
The Master of the Flash: Large Yellow Underwing Moths
The Large Yellow Underwing (Noctua pronuba) is one of the most common and hardy moths in the Noctuid family. While the adult moth is a frequent visitor to porch lights across Europe and North America, its larval form—a type of “cutworm”—is a notorious subterranean pest that can wreak havoc on a wide range of garden vegetables and ornamental plants.
Dual-Purpose Appearance
The adult moth is a study in evolutionary camouflage. When at rest, its forewings are a mottled brown or gray, allowing it to blend perfectly into dried leaves or tree bark. However, if a predator disturbs it, the moth takes flight, revealing brilliant, bright orange-yellow hindwings with a contrasting black band. This “flash coloration” is designed to startle predators, giving the moth a split second to escape.
The “Cutworm” Threat
The larvae are thick, clay-colored or green caterpillars with distinctive dark dashes along their backs. They are true generalist feeders, but they are most destructive in the early spring. Their feeding habits include:
- Stem Cutting: True to their name, these cutworms often chew through the stems of young seedlings at or just below the soil surface, effectively “felling” the plant like a tiny lumberjack.
- Nocturnal Feeding: They hide in the soil or under mulch during the day and emerge at night to feed, making them difficult to catch in the act.
- Root and Tuber Damage: In later stages, they may bore into the roots of carrots or the tubers of potatoes, leading to secondary rot.
Management and Control
Because these pests hide in the soil, management requires a “ground-up” approach. For home gardeners, collars made of cardboard or plastic (such as a bottomless yogurt cup) pushed an inch into the soil around seedlings can physically block the larvae. Tilling the soil in late winter can expose the overwintering larvae to birds. In Tucson’s sandy soils, maintaining a clear “no-mulch” zone immediately around the base of sensitive plants can also reduce the damp hiding spots these cutworms prefer.
Damage
- Cut seedlings at soil line
- Root damage
- Reduced plant survival
Management
- Inspect soil around plants
- Use protective collars
- Encourage predators