What to look for in mosquito repellents

There’s an old joke that mosquitoes are like family: they’re annoying, but they carry your blood.

Mosquito season is starting to get underway in much of the United States. And that means insect bites.

When a mosquito bites you, it pierces the skin using a mouthpiece called a proboscis to suck blood. As it feeds, it injects saliva into your skin, which can cause a reaction: a bump and itching. But the pests can also spread parasites such as malaria and viruses such as dengue, West Nile and Zika.

So you might want to pause your summer vacation planning and consider what to look for in repellents, which keep bugs away from you, and insecticides, which kill them.

WHICH MOSQUITO REPRESENTATIVES WORK BEST?

So says the Centers for Disease Control and Prevention that for hours of protection, people should look for products with these active ingredients: DEET, IR3535, picaridin, or lemon eucalyptus oil. Those ingredients are registered with the Environmental Protection Agency.

A note about lemon eucalyptus oil: Lemon eucalyptus essential oil has a similar name, but the agency does not recommend it because it has not been tested for safety and is not registered with EPA as an insect repellent.

Likewise, the CDC does not endorse other “natural” products that have not been evaluated.

WHAT OTHER STEPS CAN YOU TAKE TO PREVENT MOSQUITO BITES?

Repellents are one line of defense against insects, but there are others: Wear long sleeves and long pants. Avoid going outside at dusk and dawn, when some species of mosquitoes are most active.

Silvie Huijben, an evolutionary biologist at Arizona State University, helped develop an online game to help children understand how to protect themselves from mosquitoes, highlighting another prevention strategy:

“Mosquitoes need water to breed in,” so it’s important to make sure you get rid of any standing water, including buckets of water or kiddie pools left undisturbed in the yard for a week or more, she said. “Make sure you are not the one contributing to the local mosquito problem, and that you are not breeding mosquitoes on your property.”

You can also handle clothing and outdoor equipment with a pesticide called permethrin to repel mosquitoes and other unwanted pests.

HOW INSECTICIDES ARE TESTED

The Centers for Disease Control and Prevention operates a mosquito laboratory in Fort Collins, Colorado, where it evaluates insecticides but not repellents.

To test these types of products, researchers coat the inside of a bottle with a certain dose of insecticidal ingredient and then put mosquitoes in the bottle – usually about 25. They look at what percentage of insects die within two hours and compare that with a nearby uncoated bottle containing the same number of mosquitoes.

The test is widely used in the US and is also increasingly used around the world. It is considered simpler and cheaper than some more complicated alternatives, including a test that involves applying insecticide droplets directly to mosquitoes.

Scientists typically repeat the experiments each season to document changes in how the mosquitoes respond to insecticides, CDC officials say.

Huijben said repeating the test is important because it has limitations: the results can be thrown off by factors such as whether each bottle is coated with exactly the same amount of chemical.

“I think we see a lot of noise in the data,” which could lead to false initial conclusions, said Huijben, who has compared resistance testing approaches.

ARE MOSQUITOES BECOMING RESISTANT TO SOME CHEMICALS?

Just as bacteria can gradually develop the ability to shake off antibiotics, insects can develop resistance to some of the chemicals designed to kill and repel them.

Permethrin belongs to a class of insecticides called pyrethroids that have shown this type of resistance.

Pyrethroids became popular in the 1990s as a replacement for older pesticides and are often used to control adult insects. Community mosquito control programs and farmers use the chemicals, but homeowners can also find them on the shelves of hardware stores.

In laboratory experiments, resistance varies by product and dose, but in some tests, “none of them (mosquitoes) will die,” says Roxanne Connelly, a CDC insect scientist.

CDC officials are working with state and local officials to conduct more real-world field tests, including experiments that monitor mosquitoes placed in cages outdoors after a truck carrying insecticides drives by.

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