FORT COLLINS, Colorado – There’s an old joke that says mosquitoes are like family: they’re annoying, but they carry your blood.
Mosquito season is starting to accelerate across much of the United States. And that means bug 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 the skin which can cause a reaction – swelling and itching. But pests can also spread parasites like malaria and viruses like 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.
The Centers for Disease Control and Prevention says that for protection that lasts hours, people should look for those with these active ingredients: DEET, IR3535, picaridin or lemon eucalyptus oil. These 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 the EPA as an insect repellent.
Likewise, the CDC does not endorse other “natural” products that have not been evaluated.
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 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 against mosquitoes, which emphasizes another prevention strategy:
“Mosquitoes need water to breed,” so it’s important to eliminate standing water — including buckets of water or kiddie pools left undisturbed in the yard for a week or more, she said. “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 treat clothing and outdoor gear with a pesticide called permethrin to ward off mosquitoes and other unwanted pests.
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 an insecticidal ingredient and then place mosquitoes in the bottle – usually about 25. They see what percentage of insects die in two hours and compare 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 less expensive than some more complicated alternatives, including a test in which drops of insecticide are applied directly to mosquitoes.
Scientists typically repeat experiments each season to document changes in how mosquitoes respond to insecticides, CDC officials say.
Huijben said repeating the test is important because it has limitations: Results can be thrown off by factors such as whether each bottle was 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 stress testing approaches.
Just as bacteria can gradually develop the ability to ignore 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, which present this type of resistance.
Pyrethroids became popular in the 1990s as replacements for older pesticides and are commonly used to control adult insects. Community mosquito control programs and farmers use the 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 (the mosquitoes) will die,” said Roxanne Connelly, an insect scientist at the CDC.
CDC officials are working with state and local authorities to conduct more real-world field tests, including experiments in which mosquitoes placed in outdoor cages are checked after a fogging truck with insecticide passes by.
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