SignificanceIn animals, carotenoid pigments fulfill a number of essential roles in vision, protection from stresses, and signaling. Although carotenoids are synthesized primarily by plants and some bacteria and fungi, carotenoid biosynthetic genes have been acquired by horizontal gene transfer in some insects and mites. In the two-spotted spider mite,Tetranychus urticae, as well as in a related species, we show that a horizontally transferred carotenoid biosynthetic gene is required for normal pigmentation. Spider mites can therefore synthesize their own carotenoids. Further, in aT. urticaestrain, the gene is required for the induction of diapause, a key for overwintering of this widespread and important agricultural pest.