Insecticides rise to the top as leading cause of butterfly decline in Midwest

(CN) — Global rates of butterfly and insect population decline are reported at 2% to 4% a year, equaling 30% to 50% loss in the past two decades, and researchers now say in a study that insecticide use has eclipsed other causes of butterfly population declines in the American Midwest.

In a study published in the scientific journal PLOS ONE, researchers examined weather patterns, land use, and pesticide applications across five Midwestern states over a 17-year period from 1998 through 2014, investigating major changes in pesticide usage patterns to separate out the impacts of the individual drivers on butterfly abundance and species diversity.

While there is mounting evidence of population declines, there has been less clarity around the degree to which specific causes contribute to the overall decline.

Generally, there are three main drivers of declines in butterflies and insects: climate change, land use change or the conversion of forests and wildlands to agriculture or urban use and pesticide use, which can affect insects directly through the application of insecticides or indirectly when the application of herbicides impacts a food source.

Results of the study indicated that butterfly species diversity and abundance both declined by eight percent over the study period and insecticides were primarily responsible for the negative effects.

“Evidence is accumulating that butterflies specifically and insects in general are declining and declining. And the key question is, what is causing insect decline. And what this paper points to is insecticides, and a specific class of insecticides neonicotinoids as the cause of decline,” said Nick Haddad, professor at the Kellog Biological Station and Department of Integrative Biology, Michigan State University and a co-author of the study said in an interview.

The 16-year study period saw the introduction and widespread use of seeds coated with these neonicotinoids — a potent class of insecticides. The researchers were able to use differences in the timing of these widespread adoptions along with location and usage data collected from farmers to isolate the effects of individual pesticides from the effects of weather patterns and land use change.

"These small, but highly variable effects, that account for farmers’ substitution between alternative approaches to pest control, appear in our counterfactual to accumulate to a persistent negative trajectory. Further, these effects appear to trigger the decline starting in 2003, the year that neonicotinoid-treated soybean seed became available in the Midwestern US," researchers wrote in the study.

Scientists honed in on the Midwest in part because of intensity of agricultural activity in the region but also because the region has the most well-developed network of butterfly monitoring groups in the country.

“Of all insects, butterflies are the ones that we have a hope of monitoring,” Haddad said. “Because people who have excitement and interest can learn butterflies and go out week after week, year after year to document the information. It speaks to the power of getting people involved in collecting the best data to understand decline.”

The study also examined the monarch butterfly species in particular and found a 33% decline over the study period with both herbicides and insecticides as major factors in their decline.

The research data spans a period in which several pest control methods were widely adopted. Genetic engineering had recently allowed for the development of crops resistant to glyphosate, known commercially as Roundup.

The monarch butterflies' primary source of food, milkweed, is one of these glyphosate resistant crops, but researchers say that the herbicide may not have had as much of an effect on the butterflies as expected, and other factors were likely more to blame for the decline of remaining populations.

The researchers also conceded that major land use changes in North America had already occurred before the study period, and did not factor so much into their results. However, weather patterns tended to affect populations in individual years but did not show longer term trends,

"On the other hand," they wrote, "we do see a dominant influence of weather on yearly population sizes, which is not surprising because most butterfly species’ abundances are known to fluctuate substantially from year to year, and these fluctuations are generally governed by the yearly variability in weather."

The also noted that recording of crucial data for the study ended in 2014, just before a series of record-breaking high temperatures were recorded around the world.

Though researchers identified insecticides as the current leading cause of butterfly declines during the study’s timeframe, the impacts of climate change are expected to play an increasing role as global temperatures continue to rise. The study authors note that it will be important to collect data on all pesticide usage to create policies that that can address each contributing factor appropriately.

These dramatic changes can have ripple effects throughout the world — like the documented decline in bird populations, possibly linked to the decline of availability of insects, both of which may even contribute to economic losses as crops lose a natural source of pollination.

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