Control of the Corn Earworm on Sweet Corn in Southern California with a Nuclear Polyhedrosis Virus and Bacillus thuringiensis1

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Abstract
Effectiveness of a nuclear polyhedrosis virus and Bacillus thuringiensis Berliner for control of the corn earworm, Heliothis zea (Boddie), was investigated on successive sweet corn plantings in southern California in 1965, 1966 and 1967. Effectiveness of the treatments was determined by using the classification ‘marketable ears’ (less than 25% of the car from the tip injured by larval feeding) as delined in the Agricultural Code of California in the sections pertaining to marketing standards for green corn. In 1965, 4 applications of a 2.5 larval unit (LU) virus dust formulation at 3-day intervals, using the individual-ear, brush-dust method, resulted in an average of 35.8% increase in marketable ears and 42% ears free of larval injury. The virus dust was more effective than a Blacillus dust or a combination of the, two when applied to the silk. In 1966. 4 applications of a virus spray to the silk at 3-day intervals at the rate of 228 LU per acre resulted in a significant increase (27%) in marketable ears and a lower number of live larvae present at harvest. Virus sprays applied to the tassels were ineffective in reducing the larval population thereon, and a tassel + silk treatment was not significantly better than a silk treatment alone for control of the larvae affecting the cars. The 228 LU virus Tate per acre was consistently better than 114 LU per acre. Similar applications of a virus spray at 194 LU per acre in 1967 resulted in a significant increase (24%) in marketable cars and a lower number of live larvae at harvest. Effectiveness of the virus treatment was not increased significantly by the addition of mineral oil, emulsifier, or wetting agent. From 36 to 70.6% of the larvae, collected from cars in the control plots and reared individually in the laboratory, were killed by virus transmitted by ovipositing adults, moving from treated to untreated silk. During the 3-year study, 77 to 87% marketable ears were obtained with virus sprays and 91 to 100% with virus dust applications.

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