by What is causing drastic declines in insects?
Although a growing body of research shows that many insect populations are in decline, scientists have found it difficult to disentangle the possible causes. Are insects suffering from habitat loss as natural areas are plowed and paved? Are they doing climate change in? What about pesticides?
The latest insight comes from a study of butterflies in the Midwest, published Thursday in the journal PLOS One. His findings do not ignore the serious effects of climate change and habitat loss on butterflies and other insects, but indicate that agricultural insecticides have had the greatest impact on the size and diversity of butterfly populations in the Midwest during the period studies, 1998 to 2014. .
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Particularly harmful, the researchers found, was a class of widely used insecticides called neonicotinoids that are absorbed into plant tissues.
“It’s a story about unintended consequences,” said Scott Swinton, a professor of agricultural economics at Michigan State University and one of the study’s authors. “In the development of technologies that have been very effective in controlling soybean aphids and certain other agricultural pests, non-target species, particularly butterflies, have been harmed.”
Europe largely banned neonicotinoids in 2018, citing risks to bees. The new findings come as US wildlife officials weigh whether monarch butterflies, which range from coast to coast, should be placed on the endangered species list. (They have already found that such protections were justified but said they were hindered by higher priority needs.)
As well as enjoying people and pollinating plants, butterfly species are a vital food source for other animals, especially birds, during their caterpillar life. In fact, research has linked some bird declines to insect declines.
For the new study, the researchers integrated multiple data sets and used statistical analysis to compare potential drivers of decline across 81 counties in five states. They found that pesticide use in the average county over the 17-year study period was associated with an 8% reduction in butterflies compared to a situation where pesticide use remained unchanged over the same period. For monarchs, that comparative drop was 33%.
The authors note that these pesticide-related declines began in 2003, coinciding with the appearance and rapid adoption of neonicotinoid-treated corn and soybean seeds throughout the Midwest.
Matt Forister, an insect ecologist at the University of Nevada, Reno, who was not affiliated with the study, praised its authors for their “detective work” and for the number of factors they included in the analysis: six groups of pesticides, climate change and changes in use land. The results of the study about neonicotinoids, he said, could be crucial to help combat the decline of butterflies.
“We often talk about, well, it’s a big concern for the Anthropocene, everything is piling up, it’s bad,” Forister said. “But when we see one particular thing being bad, as bad as it looked in the early 2000s, it’s kind of hopeful because it means you can make other choices.”
Earlier research by Forister found that climate change played a significant role in butterfly declines in the American West. The authors of the new study were careful to point out that they were unable to assess recent impacts from climate change because they had to end their study period in 2014; after that year, the data on neonicotinoid use was no longer available, so they were no longer able to make the comparisons.
“The last 10 years have been the warmest 10 years on record,” said Leslie Ries, one of the authors and a professor of ecology at Georgetown University. “So what impact has it had over the last 10 years? We need to continue to study that, but it’s hard to study it in its entirety when we don’t have neonicotinoid data.”
The Environmental Protection Agency did not respond to questions seeking comment about the study and seeking clarification on the status of neonicotinoids in the United States.
Climate change is not the only factor that was not as significant in this research as it might be in general. Another thing that happened before the study period: the huge shift in land use from natural ecosystems to industrial agriculture.
And in a seemingly surprising result, the study found no reductions in monarchs from the use of glyphosate, a herbicide commonly sold under the brand name Roundup. Glyphosate destroys all types of weeds including milkweed, the only food source for monarch caterpillars, and its use is widely believed to be the cause of the overall monarch decline. The authors do not object to that agreement; rather they say that starting in the early 2000s, the impact from glyphosate “had largely disappeared since the biggest decline in milk had already occurred”.
“That damage is done, and it is still anchoring monarchs at lower populations than in the past,” said Ries. “But it’s not explaining declines or changes over that 17-year period.”
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