Monogeny is a phenomenon where a female gall midge produces unisexual broods within galls. Although it has been reported in other cecidomyiid gall midges, it has not been described in the genus Asphondylia. We report that not only do female Asphondylia prosopidis Cockerell induce a new floral bud gall type on mesquites (Prosopis spp.), but also predominantly produce unisexual galls (89%) on mesquites in Arizona and New Mexico. Among mixed galls (11%), the sex ratio was approximately 2:1 between females and males. When considering the use of Asphondylia spp. and other gall midges as biological control agents against invasive plants, it is critical to identify and release both male and female-galled racemes during initial gall establishment.