What to look for in mosquito repellent products

There’s an old joke that mosquitoes are like family: They’re a nuisance, but they take your blood.

Mosquito season is starting in most parts of the United States. And that means the biting insects are starting, too.

When a mosquito bites you, it sucks blood by piercing the skin with a mouth part called a proboscis. While feeding, it injects saliva into your skin, which can cause a reaction — a bump and itching. But these pests can also spread parasites like malaria and viruses like dengue, West Nile and Zika.

So you may want to put your summer vacation plans on hold to consider what to look for in pesticides that keep insects away from you and those that kill them.

Also read: National Dengue Day 2024: Effective tips to avoid mosquito bites

Which mosquito repellent products work best?

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

A note about oil of lemon eucalyptus: Lemon eucalyptus essential oil has a similar name, but the agency doesn’t recommend it because it hasn’t been tested for safety and isn’t registered with the 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 avoid mosquito bites?

Repellents are one form of protection against mosquitoes, but there are other measures to take: Wear long sleeves and long pants. Avoid going outside at dusk and dawn, when certain types of mosquitoes are most active.

Sylvie Huiben, an evolutionary biologist at Arizona State University, helped develop an online game to teach children how to avoid mosquitoes, which emphasizes another prevention strategy:

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

You can also use an insecticide called permethrin on clothing and outdoor clothing to keep mosquitoes and other unwanted insects away.

How are pesticides tested?

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

To test these types of products, researchers coat the inside of a bottle with a certain dose of an insecticide ingredient, then put mosquitoes in the bottle — usually about 25. They see what percentage of the insects die within two hours and compare it to a nearby uncoated bottle containing the same number of mosquitoes.

This test is widely used in the US and is growing in use around the world. It is considered simpler and less expensive than some more complex options, including a test in which insecticide drops are placed directly on mosquitoes.

CDC officials say scientists typically repeat the experiments each season to document changes in mosquitoes’ responses to insecticides.

It’s important to repeat the test, Huiben said, because it has its limitations: The results can be affected by whether each bottle was coated with the same amount of the chemical.

Huijben, who compares resistance testing methods, said “I think we’re seeing a lot of noise in the data” that could cause initial findings to be inaccurate.

Are mosquitoes becoming resistant to some chemicals?

Just as bacteria gradually develop the ability to reject antibiotics, insects also develop resistance to some of the chemicals developed to kill and repel them.

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

Pyrethroids became popular in the 1990s as an alternative to older insecticides 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 lab experiments, resistance varies by product and dose, but in some tests “none of them (mosquitoes) would die,” said Roxanne Connelly, a CDC entomologist.

C.D.C. officials are working with state and local officials to conduct more testing in the actual field, including experiments in which mosquitoes kept in cages outside are tested after a pesticide fogging truck passes by.