Adults emerge in spring before yellow starthistle has begun to bolt. Adults often feed on nectar of vetch (Vicia spp.) or other plants with early developing flowers while waiting for their target plants to develop to the appropriate stage for oviposition. Adult females lay eggs singly beneath bracts of closed buds. After hatching, larvae tunnel into seed heads and feed on receptacle tissue and developing seeds through three instars. Pupation occurs in puparia within flower heads inside chambers made of pappus and chewed seeds. Adults emerge in early to midsummer, mate, and lay eggs in more starthistle buds. Mature larvae of the summer generation overwinter within yellow starthistle seed heads. There are usually two generations per year, though three generations are possible where the growing season is sufficiently long. Chaetorellia australis will frequently oviposit in buds of the earlier-flowering invasive weed bachelor’s button (Centaurea cyanus); adults emerging as part of the next generation will then utilize yellow starthistle.