New study shows popular insecticide damages bird populations

Published: October 14, 2020
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A popular pesticide is causing bird species to decline at an alarming rate in the United States, according to a recent study co-authored by an Auburn University researcher and published in the journal, Nature Sustainability.

“There have long been concerns that the use of neonicotinoids—a group of synthetic insecticides that have gained popularity in seed coating, foliage spray and soil drenches in agricultural and residential areas in the past 25 years—may harm non-targeted species such as pollinators, birds and mammals, including perhaps even humans,” said Ruiqing Miao, assistant professor in the College of Agriculture’s Department of Agricultural Economics and Rural Sociology.

Earlier studies focusing on the impact of neonicotinoid use on birds are based mainly on evidence from laboratory experiments or focused on other countries, Miao said. This most recent study fills the gap, as it is a national-scale analysis based on historical data observed in the United States.

Based on county-level pesticide-use data and the observation-route level bird count data from 2008-14, all obtained from the U.S. Geological Survey, the study statistically analyzes nearly 15,000 observations of bird-pesticide pairs across the contiguous U.S. to examine the causal relationship between neonicotinoid use and decline in bird biodiversity.

The study found that, on average, a 100-kilogram increase in neonicotinoid use in a county will decrease grassland bird population in that county by 2.2 percent and decrease non-grassland bird population by about 1.4 percent, Miao said.

Species richness in a county, measured by the number of bird species, will be decreased by about 0.5 percent for both grassland and non-grassland birds for every 100-kilogram increase in neonicotinoid use in the county, he said. The same amount of increase in neonicotinoid use will cause a 0.4-percent decrease in species evenness (a measure of the distributional evenness of population across various species) for grassland birds.

When birds eat the pesticide-coated seeds or insects that have pollinated neonicotinoid-treated plants, the chemicals can harm bird development. Over time, they decrease birds’ abilities to reproduce.

The chemicals can have an effect for years after birds consume them. Neonicotinoids have increased in popularity among farmers because they do not have to be reapplied once plants are growing. However, past studies have also linked the pesticides to decreases in important pollinators, like bees and butterflies, which prompted the European Union to ban nearly all neonicotinoids.

“For non-grassland birds, however, the negative impact on spices evenness is not statistically significant,” Miao said. “Similar, but slightly smaller, negative effects are found for populations, species richness and species evenness of insectivorous birds and non-insectivorous birds.”

The study further quantifies the overall impact of neonicotinoid uses over the period 2008-14 in the United States, he said. It shows that over this period, on average, the neonicotinoid uses cause a 4 percent and 3 percent annual reduction in grassland bird population and insectivorous bird population, respectively.

For non-grassland bird and non-insectivorous bird populations, the annual reduction caused by neonicotinoid uses in the period is about 2 percent.

When the dynamic effect of bird population is taken into account (i.e., reduction in bird population in a year will further reduce the reproductive capacity of the population in the next year), then the annual reduction impact caused by the neonicotinoid uses from 2008-14 will be much higher, reaching 12 percent and 5 percent for grassland birds and insectivorous birds, respectively.

Geographically, the negative impact on bird populations occurs mainly in the Midwest, southern California and Upper Great Plains regions of the United States.

The study also compares the impact of neonicotinoids and that of non-neonicotinoids, Miao said. “The negative effect of a 100-kilogram increase in neonicotinoid use is about 40 times larger than that of the same amount increase in non-neonicotinoid use.

“For instance, a 100-kilogram increase in neonicotinoid use in a county will decrease the grassland bird population by 2.2 percent, whereas the same amount of increase in non-neonicotinoid use decreases the grassland bird population by only 0.05 percent. Having said that, one must note that neonicotinoids for only about 1 percent of pesticides in terms of total weight in the United States.”

The study also examines the impact of cropland and developed land on bird biodiversity. It finds that a 1-percent increase in cropland acreage in a county tends to reduce 1.4 percent to 3.5 percent of grassland bird populations and 1.7 percent to 3.2 percent of insectivorous bird population.

“The impact of cropland expansion on non-grassland birds and non-insectivorous birds is smaller,” Miao said. “Developed land is found to have negative impact on bird biodiversity as well, especially for non-grassland birds.”

The study, he said, offers empirical evidence that using a neonicotinoid leads to a decline in bird biodiversity, particularly for grassland birds, lending support to current effort of the U.S. Environmental Protection Agency to re-evaluate the uses of neonicotinoids.

“For agricultural chemical producers and farmers, this indicates that the search for more sustainable means of pest control is still unsettled,” Miao said. “About a quarter of a century ago, neonicotinoids emerged as a new class of pesticides with less toxicity to non-targeted species, and they replaced some of the earlier pesticides. One would expect that similar ‘creative destructive forces’ should lead to more sustainable means of insect control.”

Co-authors of the study are Yijia Li, graduate student in the University of Illinois at Urbana-Champaign Department of Agricultural and Consumer Economics, and Madhu Khanna, ACES Distinguished Professor in Environmental Economics and associate director of research at the Institute of Sustainability, Energy and Environment, also at the University of Illinois at Urbana-Champaign.

The USDA National Institute of Food and Agriculture and the Alabama Agricultural Experiment Station provided partial funding for the project. The research article can be found on the Nature website.

(Written by Paul Hollis)

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