Karen Oberhauser: To spray or not to spray: Controlling mosquitoes comes with a cost

March 24, 2019 GMT

When I moved to Madison from St. Paul, Minnesota, in the late summer of 2017, I was thrilled to see fireflies in my neighborhood, lots of them.

There were yellow-green flashes from Photinus species, and greener flashes from Photuris species, and others that I didn’t recognize. I found females “calling” to the males by flashing from the ground and remembered that the world of fireflies is full of intrigue. Females of some species pretend to be from another species by mimicking their flashing patterns. When the unsuspecting males of the other species fly down to what they perceive as a mate, they can be eaten by this “femme fatale.”


Sadly, it had been a long time since fireflies were a part of my everyday life because so few of them were in St. Paul. We may be able to do something to protect fireflies and other insects we like by choosing not to spray our property with insecticides — including those used to kill mosquitoes.

Many theories try to explain why fireflies are disappearing. It could be too much light from vehicles, homes and streetlights. Light disrupts their ability to find each other. Increasing, urban areas are a problem, too. Fireflies can’t live in areas covered with paved surfaces and buildings. But my neighborhoods in St. Paul and Madison have similar amounts of light, housing and paved surfaces.

Another worrisome possibility is the increasing use of bug-killing treatments.

St. Paul has a fairly aggressive mosquito control program. When adult mosquitoes reach so-called “nuisance” levels, areas can be treated using either a barrier spray or an ultra low volume (ULV) spray. Barrier sprays stay on the foliage of trees, shrubs and other plants and kill mosquitoes when they land on the foliage. ULV sprays produce tiny droplets that stay aloft and kill mosquitoes on contact. In both cases, the sprays are usually part of a group of chemical insecticides called pyrethroids.

I would guess that most people, like me, enjoy seeing fireflies and butterflies in their yards and parks. While people might not be quite as excited about bees, they know that bees are important for pollinating wild flowers, garden flowers and much of the food we eat.

Here’s the thing to remember: The pyrethroids used to kill mosquitoes kill all insects. Fireflies, butterflies and bees are just as likely as mosquitoes to encounter fine drops of insecticide in the air or on foliage treated with barrier sprays. Treated foliage is toxic to adult insects that land on it and any insects that eat it. My own research documented high mortality of monarch caterpillars after they were fed milkweed leaves that had been treated with a pyrethroid up to three weeks ago.


Because I love to see fireflies, butterflies and bees (as well as beetles, true bugs, moths, dragonflies and other insects), I was unhappy to see billboards, trucks and fliers for private mosquito-control companies in my new hometown. A quick Google search showed one of these companies extending its reach as a coast-to-coast national chain. But you can buy the sprays online, too.

The risk of environmental impacts is often minimized by statements that the insecticides are similar to naturally-occurring substances found in chrysanthemums. But the synthetic pyrethroids used to control mosquitoes have been manufactured to be more toxic and to last longer in the environment.

Don’t get me wrong, I am not a fan of mosquitoes. Sure, I know they are important parts of food chains. The adults are meals for birds, bats and dragonflies. Many aquatic species, such as fish and other insects, eat the larvae. But when I’m hiking, sitting on my deck, or walking in my new Madison neighborhood, I’d rather not be slapping mosquitoes. So I can understand people’s attraction to services that promise to rid their yard of pesky mosquitoes, and let them enjoy being outside again.

But it is important to understand all of the consequences of killing them with chemicals. The barrier spray treatments are usually done in areas where mosquitoes spend the daytime — under leaves and in shady areas. Conscientious appliers avoid spraying flowering plants. But butterfly and moth caterpillars, and many other insects, eat the foliage of sprayed plants. Birds are not directly harmed by pyrethroids, but birds eat insects. Research also shows that areas with more insects, especially caterpillars, have more birds.

If you really can’t stand mosquitoes — and no one would blame you — there are alternatives to insecticides. Reduce the number of sites available to females for egg-laying — clogged gutters, old tires, plant holders, birdbaths and discarded containers. Use repellents that keep mosquitoes at bay. And when the mosquitoes get too bad, go inside.

We all have lots of decisions to make as consumers. For some people, a mosquito-free yard is worth the cost of some “by-kill.” But we should make the decision to spray our yard informed of the potential costs as well as benefits. Mosquito spraying is not the only reason insects such as fireflies, butterflies and bees are in trouble. But mosquito sprays can kill these species. Decreasing the amount of spraying is one thing we can do to help them.