Cutworms

Cutworms are only occasional pests of high desert, Central Valley, and intermountain alfalfa but are frequent pests in the low desert where alfalfa is sometimes planted on beds. The granulate and the variegated cutworms are the two species that most commonly attack low desert alfalfa.

Female moths lay white or greenish eggs in irregular masses on leaves or stems of plants, often near the base of the plant. Eggs darken as they approach hatching. Full grown caterpillars are about 1.5 to 2 inches and appear as smooth-skinned caterpillars of various colors and patterns. Larvae frequently roll into a C-shape when disturbed. Cutworms feed at night and hide during the day in soil cracks and under debris and loose soil.

Variegated cutworms may develop in weedy areas and migrate into seedling stands or occasionally mature stands. Injurious numbers usually occur from April to late June. Seedling alfalfa stands can be severely damaged by cutworms cutting the seedlings off at or just below the soil surface. Established fields are damaged when cutworms cut off new growth or feed on the alfalfa foliage. Sometimes they feed underneath the alfalfa windrows, causing severe damage.

Granulate cutworm is a devastating pest of bed-planted alfalfa and can also be a pest of alfalfa planted between borders. Low desert alfalfa fields are most severely attacked from May through October, but the pest occurs year round in fields. Established alfalfa fields can be severely injured when cutworms cut off new shoots at or below ground level following harvest. The pest often goes undetected after cutting and hay removal but the problem becomes apparent when the field is irrigated and there is little or no regrowth.

Tillage, flood irrigation, and weed control are important in cutworm management. When damage is severe in seedling fields, apply an insecticide.

Tillage helps to limit cutworm numbers prior to planting; seedlings in well-tilled fields—especially when there is an interval between crops—are less likely to have cutworm problems. Keep the field and field edges weed-free. Flood irrigation can drown many cutworm larvae, reducing their numbers. Flood irrigation during the day will attract many birds that prey on the cutworms as the advancing water forces them from hiding.

Only cultural control methods are available for organic production systems.

Cutworm infestations are sporadic, and treatment guidelines have not been established in California. Check for cutworms by looking under duff and carefully digging to a depth of 1 inch in loose soil near alfalfa crowns. Look for small chewing or bite marks in the alfalfa roots, which create entry wounds for secondary pathogens such as phytophthora. When cutworm numbers exceed one or two per foot of row or severe damage is apparent, it may be necessary to apply a pesticide.