Reduction of crop diversity, on farm and landscape levels, has been suggested as a factor that leads to lower biodiversity in agricultural systems, and thus makes them more prone to pest damage. To determine whether the relationship between increase of corn production and reduction of crop diversity and the proportion of cropland treated with insecticide found previously in the Midwestern states is universally applicable, we applied spatial panel model analysis to USDA Agricultural Census data for 1997, 2002, 2007, and 2012 using county-level data for the entire continental USA. The 7 Midwestern states and the remaining 41 states were analyzed separately. Simpson's diversity index was used as the metric for crop diversity. We also examined the effect of temperature, the proportion of all cropland that is corn and average crop market value as additional predictor variables. The results show that expansion of corn production, together with the market value of crop products, and accumulated amount of heat could be key factors driving the increase of acres treated with insecticide. This phenomenon is observed in the entire continental USA, indicating that it was not a reduction of crop diversity in general, but specifically the predominance of corn that is likely driving increased insecticide use in the Midwestern states in recent years.