LINCOLN, Neb. (DTN) -- A federal court declared unlawful a 2022 final biological opinion that led to national restrictions on the pesticide malathion, sending it back to the U.S. Fish and Wildlife Service on Wednesday, concluding it violated the Endangered Species Act and the Administrative Procedure Act.

The USFWS's 20,000-page biological opinion concluded that malathion would not jeopardize listed endangered species or their habitats.

The U.S. District Court for the District of Northern California sent the opinion back to the agency after ruling its jeopardy determinations and critical habitat categorization were arbitrary and capricious.

"The court agrees the biological opinion's 'usage' analysis is arbitrary because it relies on arbitrary, often inflated species' range estimates and does not offer a satisfactory explanation for its reliance on pesticide usage data," the court said.

"Given every 'no jeopardy' finding relies on the 'usage' analysis, the court finds the 'no jeopardy' findings are arbitrary, capricious and not in accordance with the law."

The original lawsuit was filed by environmental groups led by the Center for Biological Diversity. The parties in the case were ordered to come up with a remedy and file a joint statement with the court by June 5, 2026.

DTN reached out to the U.S. Fish and Wildlife Service for comment.

Malathion is used largely to control mosquitoes as well as aphids and other crop pests.

Lori Ann Burd, environmental health director at the Center for Biological Diversity, said in a statement the court's decision will allow the federal agency to change course.

"The court's decision is a much-needed course correction for the Fish and Wildlife Service, which submitted to the pesticide industry's demands and hung more than 1,500 endangered species out to dry," Burd said.

"This decision will force the service to figure out how to actually reduce harm to animals and plants from one of the worst neurotoxic pesticides on the market. That includes nearly every endangered butterfly, beetle and dragonfly we have."

The environmental groups' original lawsuit alleged the biological opinion contained "numerous analytical shortcuts and arbitrary policy choices."

The final biological opinion was based on "agreed-upon measures including no-spray zones, reductions in application rates and number of applications," as well as other label changes that, taken together, "avoid jeopardy and adverse modification of critical habitat," according to the U.S. Environmental Protection Agency.

The groups said in the lawsuit that the label restrictions may reduce harm to some listed species, but the most threatened and endangered species would not benefit.

They alleged USFWS policy choices conflict with USFWS' regulations and policies, as well as with the Endangered Species Act itself, and that it was arbitrary and capricious.

The action at issue was the EPA's national re-registration of pesticides containing malathion for all its label uses, which has more than 100 different registered uses throughout the country. The EPA determined that the chemical was "likely to adversely affect" 1,778 species and 784 critical habitats.

However, the court sided with the USFWS on the plaintiffs' allegation that the biological opinion failed to address species recovery in critical habitat determinations.

The court said federal regulations did not require a separate recovery analysis.

In August 2023, the EPA announced national restrictions on the use of malathion.

The EPA initiated an ESA consultation with the USFWS on the effects of malathion, chlorpyrifos and diazinon in January 2017.

In October 2017, the USFWS concluded in a draft biological opinion that registered uses of malathion products were likely to jeopardize 1,284 threatened and endangered species.

Read more on DTN:

"Lawsuit Targets Malathion Restrictions," https://www.dtnpf.com/…

"EPA Enacts ESA Limits on Malathion," https://www.dtnpf.com/…

"EPA Requires ESA Labels on Insecticides," https://www.dtnpf.com/…

Todd Neeley can be reached at todd.neeley@dtn.com

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