The spray you use to keep bugs from biting can end up in local waterways. From there, the chemical can make its way to baby mosquitoes and the young salamanders that eat them. Insect repellant doesn’t do much to the larval mosquitoes. But it can harm salamanders, a new study shows.
People around the world use chemicals to protect themselves from insects and the diseases they can spread. The Centers for Disease Control and Prevention recommends using a bug spray with either of two chemicals. They are N,N-diethyl-meta-toluamide (or DEET) and picaridin. Each does a good job of keeping bugs away.
The chemicals don’t stay on skin, though. They wash off and can end up in local streams and rivers. That might happen directly when people swim outdoors. More often, the chemicals wash off in the sink or shower. Then they go down the drain and survive wastewater treatment.
“We see DEET everywhere that people show up,” says William Battaglin. “In national parks, remote locations in the atmosphere, it’s very persistent.” As a hydrologist, Battaglin studies water flow and the chemicals in water. He works for the U.S. Geological Survey in Blakewood, Colo., and was not involved in the study.
DEET and picaridin could affect animals living in polluted water. Mosquitoes, for instance, lay their eggs in water. Those eggs hatch and become worm-like larvae that dangle just below the water’s surface. Other creatures, including young salamanders, dine on those larvae.
Rafael Almeida is an ecologist, a scientist who studies how organisms and their environments interact. He works at Cornell University in Ithaca, N.Y. Almeida was curious about how water polluted with insect repellants might affect aquatic wildlife. Adult mosquitoes are easily driven off by insect repellant. But what about their babies? Would the spray repel or poison them? And what about other species — such as young salamanders — that dine on larval mosquitoes?
Spraying salamanders away
Almeida and his colleagues headed to a pond in New York. There, they scooped up the young spotted salamanders (Ambystoma maculatum) and larval mosquitoes.
In the lab, the scientists split the critters into groups. Each group had 9 salamanders and 30 mosquitoes. For 25 days, the researchers exposed the groups to four different amounts of bug spray with DEET or with picaridin. The amount of either repellant used was similar to what animals might experience in the wild.
None of the insect repellants affected the mosquito larvae. The bug spray with DEET didn’t harm young salamanders, either. But the repellent with picaridin was a different matter. The tails of salamanders exposed to that repellent began to twist and deform. This happened after only four days. After 25 days, the lowest dose of picaridin bug spray killed nearly half (45 percent) of the young salamanders. The highest dose killed 65 percent. A control group of salamanders was not exposed to either repellant. That group developed no tail deformities or deaths.
Nine salamanders per group is not very many. Studying more animals would give scientists a better idea of what’s happening. It’s possible that in a larger group of these amphibians, the scientists wouldn’t see these effects.
But if repellant chemicals are harming wild salamanders, it could be bad news. Populations of these amphibians have been declining around the world. Causes include pollution, a changing climate and humans taking over their habitats. Insect repellants could one source of that pollutant stress.
What’s more, fewer salamanders could ultimately mean more mosquitoes — and the diseases they carry. If baby salamanders aren’t around to eat baby mosquitoes, more skeeters might make it to adulthood. That means repellent water pollution might actually increase the number of these insects. Almeida and his colleagues published their work October 31 in the journal Biology Letters.
The researchers used the same bug sprays you can buy in stores today. “It’s interesting they used [them] instead of the ingredients,” Battaglin says. “It makes it hard to figure out what is causing the issue.” It could be the picaridin. Or it could be some other chemical in the spray.
Almeida agrees that other ingredients could be to blame. So the next step is to test those ingredients separately.
But this research is not an excuse to skip the spray when you’re outdoors in buggy areas, Almeida cautions. “Bug spray is important in avoiding mosquito-borne diseases,” he explains. “It’s too extreme to tell people to stop wearing bug spray.”
Instead, he says, researchers need to learn more about what DEET and picaridin might be doing in the environment. And they need to find ways to keep bug spray out of the water.
amphibians A group of animals that includes frogs, salamanders and caecilians. Amphibians have backbones and can breathe through their skin. Unlike reptiles, birds and mammals, unborn or unhatched amphibians do not develop in a special protective sac called an amniotic sac.
biology The study of living things. The scientists who study them are known as biologists.
bug The slang term for an insect.
Centers for Disease Control and Prevention (or CDC) An agency of the U.S. Department of Health and Human Services, CDC is charged with protecting public health and safety by working to control and prevent disease, injury and disabilities. It does this by investigating disease outbreaks, tracking exposures by Americans to infections and toxic chemicals, and regularly surveying diet and other habits among a representative cross-section of all Americans.
chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.
climate The weather conditions that typically exist in one area, in general, or over a long period.
colleague Someone who works with another; a co-worker or team member.
commercial (in research and economics) An adjective for something that is ready for sale or already being sold. Commercial goods are those caught or produced for others, and not solely for personal consumption.
control A part of an experiment where there is no change from normal conditions. The control is essential to scientific experiments. It shows that any new effect is likely due only to the part of the test that a researcher has altered. For example, if scientists were testing different types of fertilizer in a garden, they would want one section of it to remain unfertilized, as the control. Its area would show how plants in this garden grow under normal conditions. And that gives scientists something against which they can compare their experimental data.
diethyltoluamide (or N,N-diethyl-meta-toluamide) Usually abbreviated as DEET, it is one of the most common active ingredients in insect repellents.
ecology A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.
egg The unfertilized reproductive cell made by females.
environment The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of things in the vicinity of an item of interest).
habitat The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.
insect A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.
journal (in science) A publication in which scientists share their research findings with experts (and sometimes even the public). Some journals publish papers from all fields of science, technology, engineering and math, while others are specific to a single subject.
larva (plural: larvae) An immature life stage of an insect, which often has a distinctly different form as an adult. (Sometimes used to describe such a stage in the development of fish, frogs and other animals.)
organism Any living thing, from elephants and plants to bacteria and other types of single-celled life.
persistent An adjective for something that is long-lasting.
picaridin Also known as icaridin. This is a chemical commonly used as an insect repellant.
species A group of similar organisms capable of producing offspring that can survive and reproduce.
stress (in biology) A factor — such as unusual temperatures, movements, moisture or pollution — that affects the health of a species or ecosystem.
U.S. Geological Survey (or USGS) This is the largest nonmilitary U.S. agency charged with mapping water, Earth and biological resources. It collects information to help monitor the health of ecosystems, natural resources and natural hazards. It also studies the impacts of climate and land-use changes. A part of the U.S. Department of the Interior, USGS is headquartered in Reston, Va.
wastewater Any water that has been used for some purpose (such as cleaning) and no longer is clean or safe enough for use without some type of treatment. Examples include the water that goes down the kitchen sink or bathtub or water that has been used in manufacturing some product, such as a dyed fabric.
Journal: R.M. Almeida et al. High mortality in aquatic predators of mosquito larvae caused by exposure to insect repellant. Biology Letters. Vol. 14, published online October 1, 2018. doi: 10.1098/rsbl.2018.0526.