A new species, Lestodiplosis pinei Gagné (Diptera: Cecidomyiidae), from the United States is described from males, females, pupae and larvae. Larvae are egg predators of Pineus strobi Hartig, Pineus pini Goeze and Pineus cembrae (Cholodkovsky) (Hemiptera: Adelgidae). This is the first North American record for P. cembrae, which was found feeding on Pinus cembra L. planted at Arnold Arboretum in Massachusetts. Lestodiplosis juniperina (Felt) is redescribed as similar to L. pinei and for having once been considered to include specimens now identified as L. pinei. Lestodiplosis cerasi Felt and Lestodiplosis novangliae Felt are new synonyms of L. juniperina.