June 27 (UPI) -- Researchers with the California Department of Fish and Wildlife have closed the case of the poisoned songbirds. Their conclusion: the birds died after eating elm tree seeds exposed to a pesticide synthetically derived from nicotine.
In March of 2017, the department received reports about dead birds in several front yards in a neighborhood in Modesto, California. The day before, the city had applied imidacloprid, a nicotine-derived pesticide, to the base of several trees in the neighborhood.
City officials acknowledged the use of imidacloprid, claiming the pesticide was mixed and applied in accordance with the package directions.
When researchers with the Wildlife Investigation Laboratory investigated, they determined the dead songbirds were American goldfinches. Though the city had only applied the pesticide to the base of the trees, researchers determined fallen elm tree seeds were contaminated during the spraying. When the finches ate the seeds, they were poisoned.
"Seeds were observed on the ground under the trees treated with imidacloprid at the location where the dead goldfinches were collected," researchers wrote in their report, published this week in the journal Environmental Toxicology and Chemistry.
"The mortality event investigated in the present study highlights a previously unidentified risk of drench application for imidacloprid," Wildlife Investigation Laboratory investigator Krysta Rogers said in a news release.
"The pesticide label states that the product be applied to the base of the tree and directly to the root zone. [However] Seeds, insects, or other invertebrates consumed by birds and other animals may be present within that zone. If these food items were contaminated during the drench application, they would be highly toxic to animals when ingested," Rogers said.
Imidacloprid, a neonicotinoid insecticide, was adopted in the 1990s, at least partially because of its lower toxicity to humans and vertebrates. The pesticide proved too much for the songbirds in Modesto. Several studies have identified neonicotinoid insecticides as the primary driver of honeybee colony collapse disorder.
Imidacloprid is the 20th most commonly applied pesticide by acreage in the United States.