Increasing neonicotinoid use and the declining butterfly fauna of lowland California

This correlational study related the presence or absence of 67 butterfly species at four lowland Northern California sites to the use of five neonicotinoid insecticides as well as the four most widely used non-neonicotinoid insecticides. The study also accounted for summer temperatures and land conversion. In part, the study was undertaken to better understand the dramatic decline in the numbers of butterfly species observed at these sites since the late 1990s.

Researchers found a negative relationship between neonicotinoid use, which began in the region in 1995, and annual variation in butterfly species observations. At the level of individual species, the strongest negative associations with neonicotinoid use were for species that were smaller bodied with fewer generations per year. Land conversion had an equally strong negative association to neonicotinoid use, but was not concomitant with the post-1990s decline in the number of butterfly species.

This study, which is one of a few correlational studies, uses a broad approach to document negative effects of neonicotinoids beyond narrow spatial and temporal windows. This approach provides insights to understand the spillover effects of pesticide use on populations of non-target species as well as indirect effects on insectivorous species such as birds and bats.