Fusarium ear rot, primarily caused by Fusarium verticillioides, causes losses in grain yield and quality and can result in contamination of grain by mycotoxins, primarily fumonisin B1. Disease severity and fumonisin B1 contamination vary considerably among maize-growing regions and from year to year. A 2 year field study was conducted in six locations in the USA, to evaluate the roles of planting date, maize hybrid, rainfall, temperature and insect pests in the variation in fusarium ear rot symptoms and fumonisin B1 contamination. Grain samples were inspected to determine percentage of kernels with fusarium ear rot symptoms, categorized as ‘moulded’ or ‘starburst’; grain was also analysed by ELISA for fumonisin B1. Hybrid and planting date frequently had significant effects (P ≤ 0·05) on fusarium ear rot and fumonisin B1 contamination. Earlier planting consistently resulted in lower ear rot severity, fumonisin B1 levels and insect damage. Mould symptoms were highly correlated with thrips populations (Frankliniella occidentalis) (r = 0·78) and with fumonisin B1 concentration (r = 0·89). The starburst symptom was not as closely correlated with thrips (r = 0·33) or fumonisin B1 (r = 0·18). A multiple linear regression model identified highly significant effects on fumonisin B1 for thrips, lower average daily precipitation after flowering, and location. These results strengthen the evidence that locations with high populations of ear-infesting thrips and dry conditions after pollination have an increased risk of fumonisin contamination, and suggest that high fumonisin levels are much more likely in visibly moulded kernels compared to those with ‘starburst’ symptoms.