Two granulosis viruses (GV) were tested as enhancers for the gypsymoth nuclear polyhedrosis virus (LdMNPV). Helicoverpaarmigera (Hübner) GV (HaGV) had no detrimental effect uponlarval growth and development, but in combination with LdMNPV itreduced both the LC₅₀ and the LT₅₀ for the NPV.In addition, the combination also adversely affected the growth anddevelopment of gypsy moth larvae. The LC₅₀ of LdMNPV wasreduced by as much as 300-fold (HaGV at 10⁻² dilution) andthe LT₅₀ was reduced by as much as 18% (HaGV at10⁻² dilution). Spodoptera frugiperda (J.E. Smith) GV reduced the LC₅₀ of LdMNPV by as much as13-fold but had no effect upon the LT₅₀.