Ayman Mostafa
There have been many cases of cutworm infestations in alfalfa in central Arizona over the last few weeks. Cutworms are frequent pests in the low desert of Arizona. The granulate (Feltia subterranea) and the variegated (Peridroma saucia) cutworms are the two most common species in alfalfa.
Full grown caterpillars are about 1.5 to 2 inches long and vary in color 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.
Cutworm populations may develop in weedy areas and migrate into seedling or mature stands.
Established fields are damaged when cutworms cut off new growth or feed on the alfalfa foliage. 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.
Seedling alfalfa stands can be severely damaged by cutworms cutting the seedlings off at or just below the soil surface. Cultural control is important in managing cutworms in alfalfa, especially for new stands. Cutworms are most injurious in fields with high plant residue. Tillage helps to limit cutworm populations; 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. Flood irrigation during the day will attract many birds that prey on the cutworms as the advancing water forces larvae from hiding. When damage is severe in seedling fields, apply an insecticide bait.
Treatment guidelines have not been established in Arizona. Check for cutworms by looking under duff and carefully digging to a depth of 1 inch in loose soil near alfalfa crowns. When cutworm numbers exceed one or two per foot of row or severe damage is apparent, it may be necessary to treat. If treating with baits, apply baits in late evening or at night when cutworms are on plants.
References:
http://ipm.ucanr.edu/PMG/r1300911.html
http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=10705