Researchers show that ozone levels, such as those found in many places on hot summer days today, destroy the sex pheromones of fruit fly species. As a result, some natural mating boundaries maintained by species-specific pheromones no longer exist. The research team has shown in experiments that flies of different species mate when exposed to ozone and produce hybrid offspring. Since most of these offspring are unable to reproduce, the results could provide another explanation for the global decline of insects.