Results of a study in 1999 were reported that assessed the effect of B.t.-transgenic cotton on the cotton bollworm-Helicoverpa zea nucleopolyhedrovirus interaction, and evaluated the effectiveness of lower rates of Gemstar against cotton bollworms on B.t.-transgenic cotton. The B.t.-treated diet used in the laboratory study reduced the growth rate of cotton bollworms and caused an additive mortality response when combined with Gemstar. Larval counts of H. zea in field studies were significantly lower on B.t. cotton for all treatments, while H. zea larvae in all virus treatments showed lower survival.