Isolates ofPandora neoaphidis(Remaudière&Hennebert) Humber from field collections in Kentucky (KY)and North Carolina (NC) were subjected to various temperature and humidity regimes to evaluate sporulation plasticity. Greatest conidial production occurred at 10–25°C and 100% RH or in free water. No conidia were produced at 30°C or below 98% RH. The optimal temperature for in vitro growth of the 2 isolates on modified egg-milk-agar media was 15°C. The NC isolate was more virulent than the KY isolate against tobacco aphid,Myzus nicotianaeBlackman, collected from the Lexington, KY, site. Alatae were more susceptible to P. neoaphidis than apterae when exposed to the same source of inoculum at 23°C, 100% RH, and a photoperiod of 16:8 (L:D) h. Conidia produced in vitro by both isolates, when showered onto tobacco leaf disks, were infectious toM. nicotianaefor at least 35 h at 100% RH and 23°C.