The gypsy moth, Lymantria dispar L., nucleopolyhedrovirus (LdMNPV) product Gypchek is a microbial pesticide produced by the USDA Forest Service. Gypchek is a mixture of LdMNPV genotypes produced in vivo. Commercial interests prefer to develop a stable, high-potency genotype that can be produced at low cost, preferably in vitro. We sprayed 2 LdMNPV strains and Gypchek at various doses, with and without the viral enhancer Blankophor BBH (Burlington Chemical Co., Burlington, NC), on oak foliage under field conditions to determine the relationship between application rate and larval mortality. We used strains previously isolated from Gypchek; strain 203 was produced in vivo and strain 122 was produced in vitro. When applied at a rate of 10¹² viral occlusion bodies (OB) per 379 L of water without enhancer, mortality was 26% with strain 122 and greater than 90% with both strain 203 and Gypchek. In addition, strain 203 killed larvae faster. At an application rate of 10¹¹ OB with enhancer, larval mortality was greater than 90% with all 3 viral preparations. Strain 122 produced in vitro at a lower cost than is currently possible for Gypchek production would allow compensation for its reduced virulence by the application of a higher dose. Alternatively, if applied with enhancer at a rate of 10¹¹ OB, there would be no significant differences in efficacy of 122 compared with Gypchek applied at that same rate.