Secondary Outbreak Induction of Beet Armyworm by Experimental Insecticide Applications in Cotton in California

Spodoptera exigua (Hubner) on cotton in California is typically a secondary pest, likely to become abundant after insecticide applications against Lygus hesperus Knight. In this study local outbreaks were generated experimentally in an area of one square mile of cotton, subdivided into 4 randomized blocks of 4 treatments. Applications of dimethoate triggered significant increases in abundance of beet armyworm larvae. Levels up to 4 times as high as those in the untreated control were reached. Analysis of results of population censuses of both beet armyworm and its natural enemies in the experimental area, supplemented by observational evaluation of predator activities on early developmental stages, indicated impairment of predation on eggs and newly hatched larvae as the calise of outbreaks. Especially important in this context were the hemipterous species Geocoris pallens Stal, Orius tristicolor (White), and Nabis americoferus Carayon, which were severely reduced by the treatments. Another important predator of eggs and caterpillars, the larva of Chrysopa carnea Stephens, appeared immune to the insecticide. Field observations further revealed some egg predation by the otherwise predominantly phytophagous L. hesperus . The relevance of this study to pest management procedures for cotton is discussed.