Development of Economic Injury Levels for Last-stage Variegated Cutworm (Lepidoptera: Noctuidae) Larvae in Alfalfa Stubble

The effect of the intensity of stubble defoliation by last-stage larvae of the variegated cutworm, Peridroma saucia (Hubner), on the yield, quality, and economic return of alfalfa was investigated. The 3-year study was conducted during the second alfalfa growth cycle. Six and greater last-stage larvae per 0.1 m2 completely suppressed alfalfa regrowth for 10–15 days. Densities of 1.5 and 3 larvae per 0.1 m2 only partly suppressed regrowth. Plant development was delayed in direct proportion with larval density. Stubble defoliation increased forage quality, as measured by crude protein and in vitro digestible dry matter, but reduced dry-matter yield. Production of dry matter was affected more than plant development and forage quality. Hence, yields of dry matter, crude protein, and digestible dry matter were reduced at the same stage of plant development. These results were significant in most years when larval densities were ≥ 6 larvae per 0.1 m2. Smaller larval densities did not consistently affect regrowth in most years. A replacement feed-cost analysis was conducted for harvest systems, where cutting is based on either plant stage or calendar date. Economic injury levels were calculated for both harvest systems, and management of laststage variegated cutworms in alfalfa is discussed.