Pesticides overuse is a serious threat to ecosystems and wildlife, human health, and agricultural sustainability. So far, however, social scientists have not produced systematic evidence on the political–economic determinants of pesticides overuse. We argue that the agrochemical industry, as a profit-motivated interest group, will only mobilize politically to avoid reductions in pesticides use when regulatory institutions are potentially capable of correcting a market failure. If regulatory institutions are weakened by corruption or other factors, pesticides overuse occurs with or without the influence of the agrochemical industry. We test this interactive theory systematically against quantitative data on pesticides use in 24 Organisation for Economic Co-operation and Development countries, 1991–2003. Using corruption and other indicators to capture bureaucratic quality, we find substantively large and statistically robust interactive effects. The agrochemical industry is a crucial determinant of pesticides use in nations with low corruption, whereas the agrochemical industry has no effect on pesticides use under corrupt regulatory institutions. Troublingly, these results imply that reduced corruption may not improve actual regulatory effectiveness unless political institutions can somehow constrain the influence of special interests.