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Mosquito spraying caused a 'monarch massacre.' Can a repeat be avoided?

Inspectors for North Dakota and Minnesota's agriculture departments have determined in separate investigations that the aerial sprayer hired in Fargo-Moorhead committed no violations during the Aug. 26 spraying that was followed by a mass die-off of butterflies.

Dead Monarch Butterflies
Dead monarch butterflies were found across the Fargo area after aerial spraying in August 2020 to control mosquitoes. WDAY photo
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FARGO — Lawns and streets were littered with dead and dying monarch butterflies following aerial spraying to control mosquitos late last summer — mass deaths that occurred even though investigators found the sprayer acted properly.

Inspectors for the North Dakota Department of Agriculture and the Minnesota Department of Agriculture, which regulate pesticide applications, have since determined in separate investigations that the aerial sprayer hired by Cass County Vector Control committed no violations.

The dead monarchs were discovered around Fargo-Moorhead following aerial spraying for mosquitoes on the evening of Aug. 26.

One theory suggests the mass die-off occurred because spraying happened around the time the butterflies were migrating, and the insects had gathered in large numbers, making them susceptible to heavy casualties.

Fargo City Commissioner John Strand is asking city staff to come up with recommendations that can help avoid future mass deaths of butterflies — unintended casualties of more aggressive mosquito spraying to prevent the spread of West Nile Virus.


Numbers of monarch butterflies, an important pollinator species, have plunged throughout the United States in recent decades, with the eastern population down 80% by some estimates and the western population plunging by up to 99%.

“People care about butterflies and people don’t want to be a part of the decimation of a species,” Strand said. “I’m not looking to find blame. I’m not looking to point fingers.”

Strand has asked the city’s public works director to come up with recommendations, with a staff meeting scheduled for early February. Ultimately, he would like a public meeting to invite comments.

“I just don’t want to have it happen ever again,” he said, referring to the mass butterfly deaths.

Chad Peterson, chairman of the Cass County Commission, said he is open to a public discussion to explore ways of trying to prevent another mass die-off of butterflies from spraying to control mosquitoes.

“We’d welcome the conversation with the city of Fargo,” he said. “If there is still a concern to people, let’s talk.”

Last summer, wet conditions allowed mosquito populations to explode, producing record or near-record trap counts, said Ben Prather, director of Cass County Vector Control, which conducts aerial spraying to control mosquitoes over the Fargo-Moorhead metro area.

Mosquito control has become more aggressive since the arrival of West Nile Virus, which is carried by mosquitoes and can sicken or in rare cases kill susceptible people, including those who are elderly or have weak immune systems.


Also, many people don’t like to be bothered by biting mosquitoes while outdoors, especially in the evenings, when the insects are more active, Peterson said.

“Most people are far more concerned about comfort than deaths,” he said, referring to the butterfly casualties. Peterson said he gets many calls, emails and texts complaining about mosquitoes, but has not been contacted about the butterfly deaths.

“We spray every year, multiple times,” he said. “This has never happened before at this scale, to my knowledge.”

Still, Peterson said, he’s interested in trying to reduce deaths of beneficial insects from mosquito spraying, if possible.

“I think we can reach a happy balance,” he said. “No one is in this to kill insects we don’t want to kill.”

Monarch butterflies killed by mosquito spraying
Rebekah Haag of south Moorhead collected dead monarch butterflies on Aug. 27, 2020, and posted pictures on Facebook. She suggested that when spraying is done in the future to control mosquitoes that it not happen when butterflies are prevalent in the area. Special to The Forum

The aerial spraying that is widely believed to be the cause of the mass killing of the monarch butterflies was triggered by a 4-inch rainfall two weeks earlier, which set the stage for a massive outbreak of mosquitoes.


The Minnesota Department of Agriculture received an online complaint from a Moorhead resident soon after the spraying on the evening of Aug. 26, which covered an area of 100 square miles around Fargo-Moorhead.

“On the morning of August 27, I observed many, many dead and dying monarch butterflies on our walk with our dog,” wrote the complainant, whose name and identifying information was redacted by the agency.

The complainant noted the aerial spraying the night before the discovery of the dead butterflies, and asked for an investigation. “I am hoping you might be able to further investigate this so that it does not happen again,” the Moorhead resident wrote. “Many people are upset by what happened and answers as to why it happened are hazy at best. I think it’s very important that the true cause is determined so this never happens again.”

In response to complaints, the Minnesota Department of Agriculture investigated and determined that the spraying service hired by Cass County Vector Control, Airborne Custom Spraying of Halstad, Minn., followed state and federal laws.

A similar investigation by the North Dakota Department of Agriculture also found no state or federal violations. North Dakota’s pesticide investigation reports are confidential by law unless violations are found, so that report was unavailable.

The agriculture investigations center on whether the pesticide’s label specifications were followed, including the use of an authorized pesticide and adherence to label specifications, including application rate and allowable weather conditions.

In Minnesota, other agencies contributed comments. Christopher Smith, a wildlife ecologist with the Minnesota Department of Transportation, noted that insecticide spray used to kill mosquitoes, permethrin, is from a class of insecticides called pyrethroids that kills many insects, including non-targeted species.

“In this case monarchs were congregating ahead of migration, so these non-target impacts were very noticeable. Make no mistake, large numbers of non-target insects are impacted every time pyrethroids are sprayed in the environment regardless of time of day.”

A Minnesota DOT spokeswoman said Smith would not be available for an interview, since the matter was not under the agency’s jurisdiction.

Drums filled with permethrin, an insecticide used in aerial mosquito spraying, are seen Sept. 11, 2020, at Cass County Vector Control in West Fargo. Forum file photo

Prather, the Cass County Vector Control director, said permethrin is the least-toxic alternative that is effective in aerial spraying to kill mosquitoes, and is nontoxic to humans and other animals in the concentrations used to control mosquitoes.

Permethrin is widely used in agriculture for pest control of crops, over areas much more vast than residential mosquito control, and is used at much higher concentrations, he said.

Raj Mann, supervisor of the pesticide technical unit for the Minnesota Department of Agriculture, said the agency promotes best management practices for spraying, devised in partnership with the University of Minnesota, and also has programs to help pollinators.

Even so, spraying for mosquitoes “does have some unintended impacts,” including killing non-target insects.

Mosquito spraying is done during the evenings, when pollinators are not active, and aerial spraying for adult mosquitoes is the last line of defense against outbreaks, he said. Eighty percent of mosquito control, Prather said, involves killing larvae before they hatch.

Despite taking all the proper procedures, Prather concedes the aerial spraying on Aug. 26 is probably what caused the massive butterfly kill.

“It’s tough to argue this was not an effect,” he said. “It’s something that I think is rightly acknowledged on our behalf.”

Investigators have confirmed that Cass County Vector Control and its sprayer used best management practices, Prather said. “No one has called that into issue,” he said.

Although Prather doesn’t dispute that spraying is the likely cause of the butterfly deaths, he said monarch butterfly caterpillars were found in the Red River Valley and surrounding areas after the fatal spraying episode.

That raises questions, he said, about how great the impact was, since the dead butterflies already were near the end of their life cycle and already could have reproduced.

“That’s one I think is really puzzling to folks,” Prather said.

Monarch Butterfly
The monarch butterfly has the same pattern on both sides of its wings. iStock / Special to The Forum

One obvious way to try to prevent another mass killing of butterflies would be to monitor the migration and to avoid spraying, if possible, during that period.

That’s a strategy Strand is interested in exploring, and Peterson said he’s open to looking at the possibility.

But is it practical?

Prather has his doubts. Spraying logistics are cumbersome, requiring approval from state and local officials, as well as public notices, before each application, he said. “There’s so many moving parts that we have to account for.”

Waiting for the migration to pass in the midst of a bad mosquito outbreak like the one last August, Prather said, “is obviously very challenging and makes our folks who pay the bills very restless.”

Late last August, “Conditions were terrible,” he added. “We had a terrible situation.” Aerial spraying is “the last tool in the bag.”

“In this instance, that was the only real option.” Other methods, including encouraging populations of species that feed on mosquitoes, including bats, dragonflies and purple martins, haven’t been found to be effective, he said.

Every week since the monarch butterfly kill, Prather has been dealing with the aftermath, visiting with local, state and federal officials and other experts. So far, he said, he has yet to identify anything that was not done correctly.

“Everybody is, ‘Shucks, this just ended badly,’” he said.

aerial mosquito spray map.JPG
Map showing aerial spraying areas for mosquito control targeted by Cass County Vector Control. Special to The Forum

Patrick Springer first joined The Forum in 1985. He covers a wide range of subjects including health care, energy and population trends. Email address:
Phone: 701-367-5294
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