FARGO — North Dakota officials are investigating the deaths of monarch butterflies after mosquito spraying here last week that vector control officials say happened to coincide with the migration of the distinctive pollinator species.

Officials from the North Dakota Department of Environmental Quality and North Dakota Department of Agriculture said Wednesday, Sept. 2, they are investigating the mass deaths of monarch butterflies that sparked complaints and an online petition that has gathered more than 1,700 signatures urging the use of less toxic pesticides.

The North Dakota Department of Environmental Quality requires mosquito sprayers who apply pesticides over cities to file paperwork before the application to ensure safe chemicals and procedures are used, said Jim Semerad, the department’s air quality director.

That paperwork was submitted and approved before last week’s aerial application, after which residents in Fargo-Moorhead reported finding large numbers of dead monarchs and bees.

After learning of the dead monarchs, environmental quality officials went back to review the paperwork to verify that the proper chemicals and procedures were used according to label specifications, Semerad said.

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“So basically, from our standpoint the right procedures were followed,” he said. “But that didn’t explain the problem,” so his department contacted the North Dakota Department of Agriculture, which regulates pesticide applications, to investigate.

“This is unusual,” Semerad said, referring to the mass death of the monarch butterflies following the spraying. “Is it coincidental? Was it the time of year? Was it some other factor that caused it? We don’t want to see this happen again.”

Erik Delzer, pesticide and fertilizer program manager for the North Dakota Department of Agriculture, said his office started an investigation that will review documents and interview those who conducted the aerial spray for mosquitoes.

The focus of the review by agricultural officials is to determine whether proper procedures were followed or if there were any violations of federal or state laws regulating pesticides.

“We’re looking into it to make sure there was no violations of law, that they made a proper application,” Delzer said.

The investigation will examine application records, what pesticides were used and whether the applicators followed label specifications, including proper spray rates and concentrations.

If violations are found, civil penalties could include a fine of up to $5,000 per violation. More “blatant” violations, Delzer said, could result in license revocation or suspension.

“It’s already begun,” he said of the investigation. An investigation report typically concludes within 90 days, though he added, “Sometimes it’s a week or two, sometimes it’s a bit longer.”

Given the widespread concerns about the monarch deaths, results will be coming “sooner than later,” Delzer said.

If violations are found, officials will be able to see little more than enough to cite the violations because records of pesticide investigations are kept confidential by law, he said. “There’s very little we can say about it since those records are confidential.”

Environmental quality officials said the Minnesota Department of Agriculture — the spraying also occurred in Minnesota — was “aware” of the monarch deaths, but did not know if that agency is investigating. Efforts to reach Minnesota Department of Agriculture officials Wednesday afternoon were not successful.

Ben Prather, director of Cass County Vector Control, which conducts mosquito spraying, said sprayers used the least toxic pesticide in their arsenal. Last week, he told WDAY-TV that the killing of monarchs was an “unfortunate side effect.”

On Monday, Aug. 31, Cass County issued a statement, circulated on Twitter, defending Vector Control’s mosquito spraying program. The statement said the large number of deaths coincided with the monarch butterfly migration but also asserted it wasn’t clear whether spraying caused the deaths.

“The timing of the monarch migration is a sporadic event that unfortunately occurred during the latest adult mosquito control application, we are working with Federal and State resources to determine the cause of the loss of monarchs and if it was related to the application,” the statement said.

Last week’s aerial spraying for mosquitoes was warranted because of the presence of West Nile virus in the region, including in Grand Forks and Wahpeton, the statement from Cass County said.

“Again, it is very unfortunate that spraying for adult mosquitoes can result in the loss of other insects — we do not take this fact lightly, but there are considerable efforts underway to determine if there was a link.”

David Brown, a technical adviser of the American Mosquito Control Association, defended Cass County Vector Control’s spraying in a letter to the editor published in The Forum that was in response to a letter criticizing the spraying program.

“The products used by vector control agencies are formulated and dispersed at a dosage rate and in a manner to target adult mosquitoes while minimizing impacts to non-target organisms,” Brown wrote, adding that Cass County Vector Control followed “established best practices.”

Prather told The Forum Cass County is in regular contact with the Centers for Disease Control and Prevention.

“Clearly, this has gotten to be a very big issue,” he said.

The American Mosquito Control Association plans to meet later this month with the U.S. Fish and Wildlife Service to discuss the monarch butterfly — a discussion Prather said wasn’t prompted by the Cass County incident — because the monarch, which is considered a threatened species, is under consideration to be placed on the endangered species list.