What is causing this dramatic decline in insect populations?
While a growing body of research suggests that many insect populations are declining, scientists have a hard time pinpointing possible causes. Are insects losing habitat as natural areas are cultivated and paved over? Is climate change taking its toll on them? What about pesticides?
The latest insights come from a study of Midwestern butterflies. The study was published Thursday in the journal PLOS ONEThe results do not ignore the severe impacts climate change and habitat loss are having on butterflies and other insects, but they show that agricultural pesticides had the greatest impact on the abundance and diversity of butterfly populations in the Midwest during the study period (1998 to 2014).
The researchers found that a widely used class of insecticides called neonicotinoids is particularly harmful because it is absorbed into plant tissue.
“This is an unintended consequence,” said Scott Swinton, an MSU professor of agricultural economics and one of the study’s authors. “While developing very effective technologies to control soybean aphids and certain other agricultural pests, non-target species that we care about, especially butterflies, are being harmed.”
Europe largely banned the use of neonicotinoids in 2018, citing risks to bees. The new findings come as U.S. wildlife officials weigh whether to put monarch butterflies, which grow from coast to coast, on the endangered species list. (They have already found Such protection is reasonable But said they were excluded due to higher priority needs.)
In addition to bringing joy to humans and pollinating plants, butterflies are an important source of food for other animals, especially birds, during their larval stages. In fact, studies have linked declines in some bird populations to declines in insect populations.
In the new study, researchers combined multiple data sets and used statistical analysis to compare different potential drivers of decline in 81 counties across five states. They found that in the average county, pesticide use led to an 8% decline in butterfly numbers over the 17-year study period, compared with a scenario where pesticide use remained constant. For monarch butterflies, the decline was as high as 33%.
The authors note that these pesticide-related declines began in 2003, coinciding with the emergence and rapid adoption of corn and soybean seeds treated with neonicotinoids in the Midwest.
Matt Forister, an insect ecologist at the University of Nevada, Reno, who was not involved in the study, praised the study authors’ “detective work” and the many factors they included in their analysis: six groups of pesticides, climate change and land-use changes. He said the study’s findings on neonicotinoids could be key to helping address butterfly declines.
“We often say that all this is Anthropoceneeverything keeps accumulating and it’s all bad,” Dr. Forest said. “But when we see something particularly bad, even though it looked bad in the early 2000s, it’s actually a hopeful thing because it means you can make other choices.”
Dr. Forister’s early research found Climate change plays big role in butterfly decline in western U.S.The authors of the new study are careful to note that they could not assess the recent impacts of climate change because they had to end the study period in 2014; after that year, data on neonicotinoid use was no longer available, so they could no longer make comparisons.
“The last 10 years have been the warmest on record,” said co-author Leslie Ries, a professor of ecology at Georgetown University. “So what has been the impact of the last 10 years? We need to continue to study it, but it’s hard to fully study it without data on neonicotinoids.”
The EPA did not respond to questions seeking comment on the study or to explain the status of neonicotinoid pesticides in the United States.
Climate change isn’t the only factor that’s less important in this study, but it’s probably the most important one in the broader context. Another factor is what happened before the study period: a major shift in land use from natural ecosystems to industrial agriculture.
Surprisingly, the results did not find that the use of glyphosate, an herbicide commonly sold under the brand name Roundup, caused the monarch population to decline. Glyphosate kills all weeds, including milkweed, which is the sole food source for monarch caterpillars, and its use is widely believed to be responsible for the overall decline of monarchs. The authors do not dispute this consensus; instead, they say that the effects of glyphosate “largely disappeared as milkweed abundance had already experienced its largest declines” starting in the early 2000s.
“The damage has been done and monarch numbers are still lower than they were in the past,” Dr Rees said. “But that doesn’t explain the decline or change in monarch numbers over that 17-year period.”
