Bunching is the protective aggregating behavior of cattle against Stomoxys calcitrans (stable fly), where cattle bunch in a group with their heads to the center. Stable flies have a painful bite leading to stress which impacts productivity and welfare. Our objectives were to estimate stable fly intensity on dairies, threshold required to induce bunching, and the association of bunching with management and environmental factors. Between April and July 2017 we enrolled a convenience sample of 20 California dairies (herd size 2466 ± 1050), 13 Holstein, 4 Jersey and 3 were mixed. Data about feeding, manure management and cow cooling were collected using an in-person survey. Stable fly activity was recorded weekly using Alsynite traps and counts on cows. Bunching behavior was recorded weekly while recording fly activity. Data was analyzed using linear mixed models. At the dairy level bunching was associated with mean stable fly counts ≥150 flies/trap/week, months May and June (versus July), feeding wet distiller grains, and presence of wheat/corn or alfalfa crops on >2 sides of the dairy. Higher weekly mean ambient temperatures and cleaning the fence line manure were protective against bunching. At the pen level bunching was associated with a stable fly count >1 fly/cow leg, >50 flies/trap-week on traps closest to the pen, ambient temperature ≤30°C, freestall pens (versus open lot pens), dry and lactating cows (versus close-up cows), pens fed a ration containing molasses (versus not) all had higher odds of bunching; however relative humidity >50% was protective. Findings from our study show that bunching on the study dairies varied by stable fly activity, environment, facility design, dairy surroundings and management factors, including feeding and manure management.