Seasonal phenology and impact of Urophora sirunaseva on yellow starthistle seed production in California

The gall fly, Urophora sirunaseva, is one of six insect species released as classical biological control agents to control the invasive weed, yellow starthistle. Two study sites were established in Northern California to evaluate the per capita impact of galls on seed production. Evaluations during two years at each site used small cloth bags placed over developing seedheads to contain developing seeds and insects. Larger seedheads produced more seed but also supported more galls. Within-seedhead gall densities as high as 15 were detected. The impact of a single gall was estimated to reduce seed number by between 2.1–2.9 seeds per gall. A regression model was developed that estimated percentage seed loss as a function of seedhead size and number of galls. Individual galls were estimated to cause a 5–11% decrease in seed production compared to ungalled seedheads. The impact of the gall fly was greatest in smaller seedheads at both sites. Gall densities did not reach levels needed to exert significant control of seed production. Seed production in non-galled seedheads was not affected by the presence of galls in seedheads elsewhere on the plant. Gall flies were more likely to attack plants already supporting galls. Two generations of the fly were detected at both sites. The occurrence of overwintering larvae began in mid- to late-July depending on site. By early-August, all new galls held only overwintering larvae. Follow-up samples at one location eight years later showed the gall fly still present but at less than half of the population level observed earlier.