If you’ve ever had to prepare a meal for a picky eater, you know it’s not easy. They may not like certain flavors or textures, and there are often foods you have to avoid altogether.

And it turns out, humans aren’t the only species with picky eaters – mosquitoes can also be finicky.

We are biology researchers who study mosquito behavior, and we recently discovered that mosquitoes can be picky about their favorite food – that is, human blood.

Learning what attracts these mosquitoes could be useful to designing repellents that deter them from biting people and reducing the local populations of mosquitoes that spread disease. And understanding why the living community of microbes on our skin appeals to mosquitoes could help in the development of probiotic mosquito repellents.

Meet the mosquitoes

Our study focused on three mosquito species: Aedes aegypti, Aedes albopictus and Culex quinquefasciatus. All three are abundant where we live in South Florida, among many other places.

Of course, the average person won’t be able to tell which species of mosquito bit them, especially since they might not realize they’re being bitten in the moment. But we looked at each of these species, since they pose different health risks to humans.

Aedes aegypti live in urban environments, primarily in warm climates, and they prefer biting humans over animals. They are responsible for spreading diseases to humans like the Zika virus, which can cause birth defects, and dengue fever, which can cause a rash, vomiting and, in severe cases, hemorrhaging and dangerous drops in blood pressure.

Aedes albopictus, also known as the Asian tiger mosquito, are an invasive species in the U.S. They love biting humans, but these mosquitoes will venture out of urban environments in search of a good meal. They also spread dengue fever, as well as chikungunya, which causes fever and joint pain.

The final mosquito species, Culex quinquefasciatus, prefers to find a bite to eat in suburban areas in southern U.S. states. For these mosquitoes, also known as the southern house mosquito, their favorite meals are birds – but they also bite humans. This makes them an integral component of the bird-human disease cycle, spreading illnesses like West Nile virus, which can affect the nervous system and, in severe cases, lead to death.

No humans were bitten during this study

We recruited 119 participants and tested their attraction for each species using a device called a uniport olfactometer. It’s an acrylic container made up of three parts – a box where the human participant could place their arm, a tube into which we released the mosquitoes, and a smaller tube, called the attraction chamber, connecting them. A mesh screen divides the chamber from the participant’s arm so that the mosquitoes could get up close but couldn’t bite the participants.

We classified mosquitoes that traveled down the tube and into the attraction chamber as attracted to the participant. There is a funnel on the inside of the attraction chamber, so once the mosquitoes entered, it was difficult for them to go back. We calculated the rate of attraction for each participant by counting how many mosquitoes entered the attraction chamber.

For each participant, we released 30 mosquitoes at a time into the olfactometer. We tested each species three times, for a total of 90 mosquitoes tested per species and 270 mosquitoes per participant. We observed that each of the three species had their own preferences for which humans they wanted to bite.

Every person’s skin has its own unique microbiome – a community of microorganisms that live on the skin and keep it healthy. Typically, this includes a combination of bacteria species such as Staphylococcus, Corynebacterium and Cutibacterium.

Both the amounts and combinations of these microbes can change, depending on a person’s diet or overall health. In order to ensure the best possible sample from each person’s skin, we asked our study participants not to wear any perfumes or deodorants and to refrain from showering or bathing for at least 12 hours prior to coming into the lab.

Our team collected skin odors from each of the participants by having them put their arm inside a bag immediately after the attraction trials. Their arm was also swabbed for microbes so we could identify which bacterial species were present. This allowed our team to determine which odors and microbes each mosquito species was looking for.

What mosquitoes did – and didn’t – like

Each species showed clear preferences when it came to the odors and skin bacteria.

Aedes aegypti showed a slight preference for men over women, while the other two mosquito species did not discriminate on the basis of sex. The reasons for this are not clear, and the differences in odors and skin microbes between men and women were too great for us to tease out any individual factors driving mosquito attraction.

Aedes aegypti preferred people that had less of certain odors like cyclic alcohols, which are ingredients in some fragrances and cosmetic products. But they were attracted to people on whom Staphylococcus bacteria was more abundant. Meanwhile, lower attraction was associated with certain Pseudomonas bacteria.

Aedes albopictus was a little less picky, preferring people with plenty of odors. They particularly sought out people with high amounts of organic compounds called ketones. One of their favorite ketones was mesityl oxide, which has a sweet, honeylike odor. These mosquitoes also preferred people with more skin microbes, particularly when bacteria like Streptococcus intermedius and Anaercoccus octavius were in higher abundance.

But they also avoided certain bacteria, like the Mogibacterium genus. We suspect that these bacteria may produce odors that these mosquitoes don’t like.

Much like Aedes aegypti, Culex quinquefasciatus picked participants that had fewer odors overall. They were attracted to monoterpenes like alpha-pinene, which are organic compounds often found in essential oils.

Interestingly, there were no bacteria that were associated with high attraction for Culex quinquefasciatus, but there was a high abundance of the bacteria genus Actinomyces on the participants that were low attraction to these mosquitoes.

The results weren’t all differences: Some odors and bacteria were attractive across all three species. For example, all three were attracted to participants with the odor chemical ethyl 2-methylbutanoate, a fruity scent often found in fragrances. And all three species were less attracted to participants with lots of the odor 1-penten-3-ol, a chemical typically used in fresh- or crisp-scented products.

We also found that some skin bacteria were attractive across all three species. For example, people with more Corynebacterium kefirresidentii living on their skin were highly attractive to all the mosquitoes.

No person was attractive to all mosquito species

Scientists have known for decades that some people are more attractive to mosquitoes than others, and they have been studying these mosquito magnets since the 1960s.

We therefore expected that a few of our 119 participants would be mosquito magnets, and that these people would attract all three species. So we were surprised to find that no human was a universal mosquito magnet among these three species. The enticing aromas that allow each mosquito species to find us are just too different.

This tells us that mosquitoes can act as picky foodies with their own preferences for people and how they smell. Looking at the skin bacteria that help to make humans smell, there are clear differences and commonalities in terms of what attracts these mosquitoes. Our study suggests the secret of mosquito attraction is held inside the tiny microbes on our skin.

Matthew DeGennaro has received funding from the Defense Advanced Research Projects Agency (DARPA) Biological Technologies Office (BTO) and the Centers for Disease Control and Prevention to support this study.

Kaylee Marrero does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.