What to look out for when choosing a mosquito repellent

Fort Collins, Colorado — There's an old joke about mosquitoes being like family: “They're a nuisance, but they carry blood.”

Mosquito season is in full swing in many parts of the U.S., which means more insect bites.

When a mosquito bites, it pierces the skin with its mouthparts, called a proboscis, to suck blood. While sucking blood, it injects saliva into the skin, causing reactions like swelling and itching. But these pests can also spread parasites like malaria, and viruses like dengue fever, West Nile and Zika.

So, it might be time to put your summer vacation plans on hold and consider what you should look for in a repellent to keep bugs away and an insecticide to kill them.

The Centers for Disease Control and Prevention says that for hours of effective protection, choose repellents that contain active ingredients like DEET, IR3535, picaridin, and lemon eucalyptus oil, which are registered with the Environmental Protection Agency.

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

Similarly, the CDC does not recommend other “natural” products that have not been evaluated.

Insect repellent is one defense against bugs, but there are other ways to protect yourself: Wear long sleeves and long pants Avoid going outside at dusk and dawn, when some types of mosquitoes tend to be most active.

Sylvie Huyben, an evolutionary biologist at Arizona State University, helped develop an online game to help children understand how to protect themselves from mosquitoes, which emphasizes a different prevention strategy.

“Mosquitoes need water to breed,” so it's important to make sure you remove any standing water, like buckets of water that have been left in your yard for more than a week or wading pools, she says. “Make sure you're not contributing to a local mosquito problem or allowing mosquitoes to breed on your property.”

You can also apply an insecticide called permethrin to your clothing and outdoor gear to ward off mosquitoes and other annoying pests.

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

To test these kinds of products, researchers paint the inside of a bottle with a set amount of the insecticide compound, then place mosquitoes (usually around 25 of them) inside the bottle and see what percentage of the insects die within two hours, compared to the same number of mosquitoes in an unpainted bottle nearby.

The test is widely used in the United States and increasingly around the world and is considered simpler and cheaper than more complicated alternatives, such as tests that involve applying insecticide drops directly to mosquitoes.

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

Huiben said repeating the tests is important because they have limitations: Results can vary depending on factors such as whether each bottle has the exact same amount of chemical applied.

“I think there's a lot of noise in the data that could lead to erroneous initial conclusions,” said Huiben, who has compared resistance-testing approaches.

Just as bacteria gradually develop the ability to dodge antibiotics, bugs can develop resistance to some of the chemicals developed to kill or repel them.

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

Pyrethroids became popular in the 1990s as an alternative to older insecticides and are often used to kill adult bugs. Local mosquito control programs and farmers use the chemicals, but you can also find them on the shelves of hardware stores.

In lab tests, resistance varies by product and dosage, but in some tests “it doesn't kill any mosquitoes,” said Roxanne Connelly, an entomologist at the CDC.

CDC officials are working with state and local officials to conduct more realistic field tests, such as testing mosquitoes held in cages outdoors after insecticide trucks pass by.

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