What are the best mosquito repellents to prevent being bitten?

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

Mosquito season is starting to ramp up in much of the United States. And that means bites.

When bitten by a mosquito, it pierces the skin with a mouthpart called a proboscis to suck blood. While feeding, it injects saliva into the skin which can cause a reaction: a bump and itching. But they can also transmit parasites such as malaria and viruses such as dengue, West Nile and Zika.

So you may 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.

What mosquito repellents work best?

The Centers for Disease Control and Prevention (CDC) says that for protection that lasts hours, look for those that contain the following active ingredients: DEET, IR3535, picaridin, or oil of lemon eucalyptus. These ingredients are registered with the Environmental Protection Agency (EPA).

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

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

What other measures can you take?

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

“Mosquitoes need water to breed,” so it's important to get rid of standing water, including buckets of water or paddling pools left in the yard for a week or more. “Make sure you're not contributing to the local mosquito problem, that you're not breeding them on your property,” he explains.

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

How insecticides are tested

The CDC has a mosquito lab in Fort Collins, Colorado, where it tests insecticides, but not repellents.

To test these types of products, researchers coat the inside of a jar with a certain dose of an insecticidal ingredient and then introduce mosquitoes into the jar (usually about 25). They look at what percentage of the insects die within two hours and compare it to a nearby uncoated bottle containing the same number of mosquitoes.

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

Scientists often repeat experiments each season to document changes in mosquitoes' response to insecticides, CDC officials explain.

Huijben says repeating the test is important because it has limitations: The results can be altered by factors such as the fact that each bottle is coated with exactly the same amount of chemical.

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

Are mosquitoes becoming resistant?

Just as bacteria can gradually develop the ability to evade antibiotics, insects can develop resistance to several of the 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 substitutes 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 depending on the product and dose, but in some tests “none of them (the mosquitoes) die,” says Roxanne Connelly, a CDC scientist.

CDC officials are working with state and local officials to conduct more real-world field tests, including experiments in which mosquitoes placed in outdoor cages are checked after an insecticide-diffusion truck passes by.