Insect declines driven by insecticides, not climate change or habitat loss

Abstract

Insect declines are largely attributable to habitat loss and climate change, with agricultural pesticides (insecticides and herbicides) also suspected to be a major driver. We evaluated the relative influence of these factors on butterfly declines using 17 years of data covering 81 counties across the Midwestern United States. We found that declines in butterfly abundance and species richness were most strongly associated with insecticides. Butterfly species richness was also more strongly associated with neonicotinoid-treated seeds, a special case of insecticides. This was the case even when including climate as a covariate. We show that assessments of insect declines must consider all plausible drivers and that the lack of recent neonicotinoid data releases in the US will impede future research.

Introduction

Like many other insects, butterfly populations are showing widespread signals of decline. Globally, overall declines in insect abundance and biomass for many insect groups have been reported at rates of 2-4% per year, rates that compound over two or more decades to as much as 50% loss of total abundance. For butterflies, declines have been widely reported, including in the UK, Netherlands, and grassland habitat across Europe. Evidence regarding the primary drivers of these declines remains incomplete. The three most widely recognized global drivers of insect declines are land conversion, climate change, and agricultural pesticides (insecticides and herbicides). Yet, the relative impact of each has been shifting over time and also interacting in ways that make it challenging to tease apart their individual contributions.

Here, we present a comprehensive analysis of the drivers of insect declines, incorporating the impacts of all three putative drivers, across a large spatial extent and over a long time span. We used a novel database we developed that harmonizes butterfly survey data, land use, climate, and multiple classes of pesticides across the Midwestern United States. We used this database to evaluate the relative influence of insecticides, climate, and land use on butterfly declines.

Results

We found that declines in butterfly abundance and species richness were most strongly associated with insecticides (Fig 1). Declines in species richness were most strongly associated with neonicotinoid-treated seeds. This included the abundance of the migratory monarch (Danaus plexippus), whose decline is the focus of intensive debate and public concern. Insect declines cannot be understood without comprehensive data on all putative drivers. The cessation of neonicotinoid data releases in the US will impede future research.

Conclusion

Butterfly species richness was also more strongly associated with neonicotinoid-treated seeds, a special case of insecticides. This included the abundance of the migratory monarch (Danaus plexippus), whose decline is the focus of intensive debate and public concern. Insect declines cannot be understood without comprehensive data on all putative drivers, and the 2015 cessation of neonicotinoid data releases in the US will impede future research.

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