Nonlinear models were used to estimate nondiapause larval and pupal development under constant temperatures using laboratory (North Carolina and Pennsylvania) and field (North Carolina) strains ofPlatynota idaeusalis(Walker) reared on semisynthetic medium and excised apple leaves. Significant differences in degree-day requirements due to sex were found in the pupal, but not the larval stage. Strain and food significantly affected thermal requirements for development and the effect occurred during the larval but not the pupal stage. Strain differences showed that North Carolina field strains and Pennsylvania laboratory strains developed faster during larval stage than the North Carolina laboratory strain. When larval and pupal thermal requirements were combined, total thermal requirements for strains were not significantly different. Insects developed faster on semisynthetic medium than on leaves for larval stage alone and larval and pupal stages combined. Lower threshold temperatures and thermal requirements for individual instars reared on medium were determined. Using the thermal requirements for larval and pupal development of the North Carolina field strain reared on apple leaves (478 degree-days over a threshold of 10.2°C:) plus the thermal requirement for egg development (116 degree-days over a threshold of 13°C:), the preimaginal thermal requirement for development is estimated to be 594 degree-days (1,069 Fahrenheit degree-days).