Does mosquito spraying kill monarch butterflies?

September 10, 2024 1:19 PM

My area of the city has posters up everywhere warning about mosquito spraying tonight. This is a huge section of coastal NYC. Ive seen hundreds of monarch butterflies on my walk home today. I assume they’re migrating because Ive never seen them before. Is the spraying going to kill them? This seems like a nightmare scenario.
posted by octaviabutlerfan to Science & Nature (9 answers total)

Apparently they are spraying Anvil 10+10, Duet or MERUS 3. Duet claims that it needs to spray an insect in flight, and bees and butterflies shouldn't be active. The same claims are made for Merus and Anvil.

Do I believe that it won't harm butterflies and bees? I don't. But it's probably not as bad as you might imagine.
posted by advicepig at 2:22 PM on September 10

Even if that's true, even if it doesn't persist in the environment, there are plenty of beneficial insects that are active at night, including pollinator moths.

But of course the claims these pesticides are making are not actually true. Colin Purrington is an entomologist who does popular science writing about mosquitos and mosquito control. Here's what he has to say about pyrethroids, the active ingredients in all of those sprays.

Do pyrethroids kill other animals?

Yes.

For example, the spray kills monarch caterpillars, even weeks later later due to the presence of insecticide dried onto milkweed leaves (Oberhuaser et al. 2006).

And the spray can kill honey bees, even if honey bees are inside their hives when the pyrethroids are spayed (workers bring small amounts back to the hive the following day if they land on treated plants or if they find small puddles of water to drink). Sublethal amounts of pyrethroids can change honey bee behavior and make workers smaller. I would imagine the pyrethroids would end up in the honey, too.

He includes lots more details, with links to all of his sources.
posted by hydropsyche at 2:47 PM on September 10

I feel like I need to defend the New York City Department of Health and Mental Hygiene here.

First of all, I'd say that this is fairly narrowly targeted spraying (it is a lot of coast, but it's a small section of NYC; not sure if this is on the posters you're seeing), and targeted spraying like what's happening tonight is probably a direct response to trapping mosquitoes which tested positive for a pathogen like West Nile or Zika in this area (more information on the trapping program on page 20 of this PDF).

Also, I believe all of the pesticides being used in this program are all pyrethroids or pyrethrins, which are the type approved for use in organic agriculture. Not great, and I'm not disputing any of the information in hydropsyche's link, but they and are arguably worlds better than organophosphates and organochlorides, which are concerningly toxic to humans, or neonicotinoids, which are less toxic to humans but more persistent in the environment, water soluble, and very bad for beneficial insect populations.

New York is a densely populated urban area, even near the coast, and there are almost certainly a lot of potentially vulnerable people in this neighborhood. I'm not a fan of indiscriminate use of pesticides, and for myself alone I'd say "forgo the fogging, I'll wear repellent or protective clothing and take my chances with an arthropod-borne virus." This is what I do on my own property, but I'm a healthy, immunocompetent person in early middle age, and it's not a decision I can or should be making for others. These are serious diseases which kill and disable people, even in NYC, and I don't have a lot of appetite for pushing public health measures off onto the individual ("we're not spraying, it's up to you to protect yourself and your children from mosquito bites").
posted by pullayup at 3:02 PM on September 10

A quick note: pyrethrins are actually relatively dangerous for cats, because they do not produce liver enzymes which break them down. If you have outdoor cats, keep them inside, and if you have indoor cats, keep your windows closed.
posted by pullayup at 3:07 PM on September 10

Yep this is broad spectrum killing, including billions of innocent bystanders. I can't say it any better than hydropsyche and Purrington above, just commenting to emphasize.

It's probably less harmful to non-target wildlife than some other things they used to spray, but that's a really low bar.
posted by SaltySalticid at 3:16 PM on September 10

Thanks for the information all. On balance, I am as disturbed as I was before. Monarch butterflies are very important, and they are dying out. I’ve heard zero warnings about current virus risk with these mosquitos, and I live 10 feet from a mosquito-infested coastal marsh. I believe the timing should’ve been more carefully aligned to not overlap monarch migration and the proliferation of wildflowers and such that are everywhere around here right now. I am extremely pissed off.
posted by octaviabutlerfan at 3:41 PM on September 10

NYC Department of Public Health has a website dedicated to West Nile and mosquito control. They report and map mosquitos that have tested positive for West Nile.

They have a "Comprehensive" Mosquito Control plan (PDF) that on skimming does not appear to address risk to pollinators (though I just skimmed). I'm not sure how NYC grassroots politics works but the control plan lists who is in charge of this plan and that is who is going to be able to change that analysis.
posted by muddgirl at 4:04 PM on September 10

Should have added there's also a tab for human West Nile infections in NYC.
posted by muddgirl at 4:06 PM on September 10

Along with Zika and West Nile, I've seen some reports about a small surge in Eastern Equine Encephalitis on the east cost this year, mostly in Mass but with confirmed animal presence in New York and at least one human infection in New Jersey. EEE is rarely symptomatic, but symptomatic cases are incredibly dangerous.

In a city with a population like NYC, if even a minority of people are exposed to one of these viruses, and a small percent of those exposed go on to develop serious symptoms, that still could be potentially thousands of seriously ill or dead. Which is not to say that concern for beneficial insects is out of place, but that public health authorities are often thinking robustly about risk when making these kinds of decisions.
posted by radiogreentea at 9:14 AM on September 11