Diflubenzuron is an insect growth regulator labeled for application to pastures and rangeland to suppress grasshopper (Orthoptera: Acrididae) populations. Livestock are permitted access to land immediately after treatment. We hypothesized the development and survivorship of horn fly Haematobia irritans (L.) larvae feeding on manure resulting from these animals would not be impacted due to the presence of diflubenzuron residue. Survivorship to the adult stage and percent pupae deformed were recorded for horn flies developing on manure samples from three pastures treated with 59 ml Dimilin 2L^® /0.4 hectares. Pastures not treated served as the controls. Manure samples ranged in age from three to 31 d post treatment. This study was replicated in 2004 and 2005. Results were highly variable between site and year. Horn fly survivorship to the adult stage in 11 of 15 sample dates taken was lower in treated than the control manure samples. Accordingly, a significantly greater percentage of deformed pupae was recorded for samples from the treated than the control sites ≤17 d post treatment. Based on this study, our null hypothesis was rejected. Using Dimilin 2L to suppress pasture and range land pests can also impact horn fly populations associated with cattle feeding in these pastures. However, care should be taken to apply adequate pasture coverage ensuring appropriate Dimilin 2L^® levels are consumed to achieve the suppression of associated horn flies.