Lymantria dispar; WEIGHT-; DIET-; Artificial, effect on biology & hymenopteran parasite biology; FOOD-PLANTS; Trees; Diet & hymenopteran parasite strain; GROWTH-RATE; Diet effect; HYMENOPTERAN-PARASITES; Cotesia melanoscela; Host diet & parasite strain, effects on host & parasite biology Cotesia melanoscela; SIZE-; DEVELOPMENT-; Time, effect of lepidopteran host diet; VARIATION-; Strain differences in performance using lepidopteran host; LEPIDOPTERAN-HOSTS; Lymantria dispar; Host diet & parasite strain, effects on host & parasite biology; POPULATION-SEX-RATIO; Lepidopteran host diet effect The effects of gypsy moth, Lymantria dispar (L.), larval diet on parasitism rates and performance by 2 strains of the larval parasitoid Cotesia melanoscela Ratzeburg were studied in laboratory trials. Gypsy moth larvae that were parasitized by C. melanoscela strains from France and South Korea, and an unparasitized control group, were allowed to develop on foliage from 5 tree species (red oak, tamarack, sugar maple, hybrid poplar clone NC5271, and white pine) and synthetic diet. Food plant had a significant effect on gypsy moth relative growth, development time, and pupal weight, Gypsy moth food plant affected C. melanoscela development time and size. Host plant influenced percentage of parasitism, but this effect varied between blocks. Parasitoid strain was a major factor in development time, size, and host selection, but was of marginal influence on parasitism rates and sex ratio. Plant suitability to gypsy moth, both in host species and phenology, was the strongest underlying factor in C. melanoscela parasitoid success, development time, and size. Plant and strain effects on C. melanoscela may be important factors in guiding biological control efforts against gypsy moth. none