Significance Phenotypic plasticity is common in nature, yet the mechanisms by which environmental inputs are detected and alter developmental programs are not well understood. Even less explored are the mechanisms underlying transgenerational plasticity. Here we address the problem of how asexual aphids produce winged or wingless offspring in response to external cues. We find that gene expression patterns suggest the importance of the hormone ecdysone in this polyphenism. We show that aphids injected with ecdysone or its analog produce fewer winged offspring, and aphids with inhibited ecdysone signaling produce more winged offspring. Our results provide insight into the hormonal control of alternative aphid morphs, which have long been a textbook example of polyphenism, but whose control mechanisms have long proven elusive.