The effects of short-term phenological changes in different tree species on development and survival of the gypsy moth parasitoidCotesia melanoscela(Ratz.) are examined. Larvae stung byC. melanoscelaand an unstung control group were allowed to develop on foliage from four tree species (red maple, mockernut hickory, American beech, and white oak) started on five different dates during a 16-d period after bud break. Survival ofCotesialarvae declined Significantly with start date on all four species; however, white oak supported high wasp survival for most of the period. Wasp development time also increased significantly with increasing start date on each species. Stung larvae suffered significantly higher mortality from causes other than wasp emergence than did control larvae. The implications for underestimating mortality attributable to parasite attack are discussed. In particular,C. melanoscelaprobably causes significantly greater mortality of gypsy moth larvae than estimated from parasite emergence.