Adults emerge in spring when knapweed is still in the rosette or bolting stage. Adults feed heavily on nectar when knapweed flower heads bloom. Mating begins with the onset of warm weather and continues throughout summer. In summer and early fall, females lay one to several eggs between florets in young flower heads. The female lays an average of 80 eggs in her lifetime and often marks the bracts of the flower head with a substance to discourage egg-laying by other females. Eggs hatch in 3–5 days, and larvae feed on ripening seeds and receptacle tissue through three instars. Larvae overwinter within seed heads, then pupate in chambers made of pappus in spring. Weather conditions determine the number of generations (one or two) of Terellia virens; however, only one generation has been confirmed at most North American sites.