Which mosquito repellents work best?

FORT COLLINS, Colo. (AP) — There’s an old joke that mosquitoes are like family: They’re annoying, but they carry your blood.

Mosquito season is beginning to pick up across much of the United States. And that means insect bites.

When a mosquito bites you, it pierces the skin using a mouthpart 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 pests can also spread parasites like malaria and viruses like dengue, West Nile and Zika.

You may want to pause planning your summer vacation and think about what to look for in repellents, which keep bugs away from you, and insecticides, which kill them.

WHICH MOSQUITO REPELLENTS WORK BEST?

According to the Centers for Disease Control and Prevention that for protection that lasts for hours, people should look for ones with these active ingredients: DEET, IR3535, picaridin, or lemon eucalyptus oil. These ingredients are recorded with the Environmental Protection Agency.

A note on 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 the EPA as an insect repellent.

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

WHAT OTHER STEPS CAN YOU TAKE TO AVOID MOSQUITO BITES?

Repellents are one line of defense against insects, but there are others: wear long sleeves and long pants. Avoid going out at dusk and dawn, when certain types of mosquitoes tend to be more active.

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

“Mosquitoes need water to breed,” so it’s important to make sure to get rid of 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, that you are not breeding mosquitoes on your property.”

You can also treat outdoor clothing and 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 given dose of an insecticidal ingredient, then put mosquitoes in the bottle – usually around 25. They see what percentage of insects die in both hours and compare that with a nearby uncoated bottle containing the same number of mosquitoes.

The test is widely used in the United States and increasingly around the world. It is considered simpler and less expensive than some more complicated alternatives, including a test in which drops of insecticide are applied directly to mosquitoes.

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

Huijben said it’s important to repeat the test because it has limitations: Results can be skewed by factors such as whether each bottle was coated with exactly the same amount of chemical.

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

ARE MOSQUITOES BECOME RESISTANT TO CERTAIN CHEMICALS?

Just as bacteria can gradually develop the ability to ignore antibiotics, insects can develop resistance to certain chemicals developed to kill and repel them.

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

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

In laboratory experiments, resistance varies by product and dose, but in some tests, “none of them (mosquitoes) will die,” said Roxanne Connelly, an insect specialist at the CDC.

CDC officials are working with state and local authorities to conduct more field tests, including experiments in which mosquitoes placed in cages outside are monitored after an insecticide fogging truck passes by.

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Science and Education Media Group. The AP is solely responsible for all content.