The beautiful smell of flowers is never a simple scent to mosquitoes. These tiny flying insects are attracted to certain flowers because they can detect the complex chemical components in floral scents and choose what they like. 

To study the nectar-feeding decisions of mosquitoes, UW researchers looked at the mutualistic relationship between Platanthera obtusata— also known as the blunt-leaf orchid — and Andes mosquitoes. The study was conducted by Chloé Lahondère who was a postdoctoral research associate in the department of biology. 

“One of the things that is very important for mosquitoes is the sources of sugar,” Jeff Riffell, a professor of biology at the UW and the senior author of the study, said. “Males need sugar. If they don’t have sugar, they can’t reproduce. The sources of sugar from flowers are really critical for their survival.” 

At the field sites in the Okanogan-Wenatchee National Forest, the research team verified that mosquitoes are attracted to blunt-leaf orchids. Interestingly, while pollinating and feeding on this species, mosquitoes show little interest in other orchids that grow in the same habitat, even if the orchids are close relatives to blunt-leaf orchids.

What leads mosquitoes to select one orchid species but not the other? Riffell’s team covered the orchids with bags and learned that even when mosquitoes can’t see, they could still land on the bagged flowers and seek food. It is the scent that drives these insects to the flowers.

The team wanted to learn about the substance orchids produce and the cues that attract mosquitoes. They bagged flowers and sucked the air around flowers through activated charcoal traps to collect the odor. The scent is absorbed and stored in the charcoal. Later, it was washed and collected. 

Riffell mentioned that some scents are very complex and might include 300 to 1,000 chemical components. With gas chromatography and mass spectrometry, researchers identify the chemicals that make up the floral scent. They separated the flower bouquet into individual chemicals and tested how mosquitoes responded. 

“We attached electrodes to the antennae,” Jeremy Chan, a graduate student in the department of biology and the co-author of the study, said. “The antennae is what the mosquitoes use to smell. [We want to] see which one of the chemical compounds did the antennae actually respond to. They only respond to some of them.” 

The team found that the blunt-leaf orchids release a large amount of nonanal and relatively small amounts of lilac aldehyde. Mosquitoes love the combination of the two chemicals when they are mixed in a certain ratio as presented in blunt-leaf orchids. Otherwise, they might be repelled even if both chemicals are present. The close-related orchids don’t have that certain ratio, so mosquitoes are not interested.

According to Riffell, as long as the chemicals are combined in the right ratio, mosquitoes can react to artificial scents just like they smell flowers. This might also explain why some people get more mosquito bites than others: Some chemicals in their blood might mix in a particular ratio that mosquitoes prefer. 

Further experiments suggest many other types of mosquitoes are attracted to this “recipe” produced by the orchids including one species from Africa. Riffell referred to the combination as a lure that could possibly turn into mosquito repellents. 

Researchers are interested in making traps with the lure out in the field and seeing whether the lure can attract other mosquitoes. If it succeeds, these traps can be utilized to control the high number of mosquitoes in our living environment and possibly prevent diseases that are transmitted by mosquitoes. 

Reach reporter Sunny Wang at science@dailyuw.com. Twitter: @sunnyqwang64

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