Does Washing with Soap Change the Odor Signals That Reach Mosquitoes?

Washing with soap changes the odors released from the skin. Beyond smelling clean to us, could those changes also affect the cues that attract mosquitoes?

A 2023 study found that mosquitoes changed their choices when presented with odors from arms washed with commercial body washes. However, the same product produced different results for different people. What mattered was not just the product’s fragrance, but its mixture with an individual’s body odor.

Connecting changes in odor after washing with mosquito behavior

A team at Virginia Tech investigated how soap changes the chemical composition of human odor and how those changes relate to mosquito choice. Using relationships between chemical analyses and behavior, they also tested whether they could create mixtures that mosquitoes would approach or avoid.

The main experiments involved four volunteers and four commercial body washes. These were specific products selected from the Dial, Dove, Simple Truth, and Native brands. The study did not evaluate the brands as a whole or other formulations sold later.

Both arms were first rinsed with water. Only one arm was then washed with body wash and rinsed again, while the other remained water-rinsed as a comparison. Nylon fabric was placed on each arm to collect odors for one hour.

Collecting samples from both arms of the same person on the same day was an important feature. It allowed washed and unwashed conditions to be compared while partly controlling for day-to-day variations in diet and activity. The experiments were repeated.

Participants did not offer their arms directly to mosquitoes

The behavioral tests used laboratory-reared female Aedes aegypti mosquitoes. Odor-bearing fabrics were placed in two containers, and researchers recorded which container mosquitoes landed on inside a cage over 20 minutes. Mesh covered the containers, preventing direct contact with the fabric.

The researchers distinguished the proportion of mosquitoes landing on either container from the choice made by those that landed. Many mosquitoes might respond but choose the two sides equally. Conversely, relatively few might respond while showing a preference for one side.

The experiment did not measure bites on human skin or the amount of blood taken. It also did not add carbon dioxide, an important host-finding cue, instead providing conditions to investigate differences between odor samples.

Washing changed both the amounts and combinations of chemicals

Analysis of chemicals evaporating from the fabric using GC-MS showed increases in the number of compounds and their total detected abundance after washing. This suggests that body-wash ingredients made a substantial addition to the original body odor.

In particular, the proportion of terpenes, including limonene, increased, while the proportions of groups such as aldehydes declined. A lower proportion did not necessarily mean a lower absolute amount, however. Aldehydes increased in amount even as their share of the total fell.

This distinction matters because washing should not be understood simply as removing body odor and replacing it with soap fragrance. What reaches a mosquito is a mixture of chemicals from the individual and the product, with altered amounts and proportions.

The same product affected different people differently

When unwashed odors were compared with unworn fabric, mosquitoes preferred samples from three of the four volunteers. For the remaining volunteer, overall landing increased, but there was no significant preference for the side carrying that person’s sample. Baseline odor also differed between individuals.

In comparisons between the two arms, samples from some volunteers became more attractive after washing with the selected Dove and Simple Truth products than samples from their unwashed arms. The change was not the same for everyone.

The selected Native product shifted choices toward avoidance of the washed side, with significant avoidance for the fourth volunteer. Again, this did not establish an equal effect in every individual.

The proportion of mosquitoes landing on either container also depended on the combination of person and product. A product that reduced landing for one person could increase it for another. Product names alone therefore cannot reliably determine whether mosquitoes will approach or avoid someone.

Designing new mixtures from combinations of ingredients

The study did not stop at comparing commercial products. The researchers combined chemical and behavioral data to investigate which combinations of compounds were associated with samples that mosquitoes preferred or avoided.

They then created a four-component “attractive” mixture containing ingredients including lilial and α-isomethyl ionone, and a three-component “repellent” mixture containing benzyl benzoate, γ-nonalactone, and benzaldehyde.

Next, they tested these mixtures using the odor of a fifth volunteer whose samples had not been used in designing them. Mixtures were added to fabric worn by that person and compared with fabric from the same person treated with solvent alone. Mosquitoes preferred the four-component mixture and avoided the three-component mixture, with significant differences in both cases.

This provided proof of concept that chemical information from real samples can be used to design mixtures that alter mosquito choice. However, the mixtures were presented at higher concentrations than in the original washed samples. They did not directly reproduce the concentrations or effects of using the commercial products.

Why a fragrance label cannot establish mosquito-repellent effects

All the tested products contained abundant limonene, yet mosquito responses differed. The authors discuss the importance not just of a single compound’s identity or amount, but of its ratio to other compounds and its combination with human odor.

The main comparisons involved four people, one mosquito species, and samples collected within an hour of washing. The experiment does not establish how long effects last, whether people are bitten more or less outdoors, or what happens with other mosquito species. Nor does it justify treating a coconut or vanilla fragrance label as evidence of repellency.

The starting point of this research was to investigate how everyday cleansers alter chemical cues reaching insects. Evaluating their effectiveness as mosquito repellents would require separate tests under actual use conditions.

The research was supported by a US Department of Agriculture grant, and the authors declared no conflicts of interest.

Summary

Washing with body wash changed the chemical composition of human odor and the landing choices of Aedes aegypti mosquitoes. The direction of the change depended on the combination of individual and product.

Mixtures designed from relationships between chemicals and behavior also shifted mosquito choices toward attraction or avoidance when added to another volunteer’s odor. Odor cannot be understood simply as the sum of individual ingredients. Considering body odor and product fragrance together offers a way to better understand our interactions with mosquitoes.

References

VanderGiessen M, et al. Soap application alters mosquito-host interactions. iScience. 2023;26:106667. doi:10.1016/j.isci.2023.106667