Study explains equal attraction of mosquitoes to flowers and humans

By Dr. Liji Thomas, MDJan 22 2020

A new study published online on December 23 in the Proceedings of the National Academy of Sciences reports the discovery of the basis on which mosquitoes are drawn towards a type of orchid of which they are inordinately fond, resulting in its pollination.

Mosquitoes are best known as blood-sucking insects. However, few people know that these insects depend on their sense of smell to find a warm-blooded host as well as suitable spots to lay eggs. This sense of smell helps them to find their chief source of nutrition as well, namely, nectar.

Scientists say that nectar is one of the crucial food sources for all species of mosquitoes. Indeed, for male mosquitoes, it is the only source of food, while female mosquitoes also feed exclusively on nectar for all but a few days of their life cycle when they are laying their eggs.

Even though it is known that mosquitoes are drawn by scent towards flowers and other sources of nectar, the nature of these scents is largely unknown. In fact, some flower scents repel mosquitoes instead of attracting them.

An Aedes mosquito with pollen sacs on its eyes feeding from Platanthera flowers. Credit: Kiley Riffell

The study

The blunt-leaf orchid, or Platanthera obtusata, on which the current study centered, grows in many northern parts of the northern hemisphere, in cool climates.

The researchers monitored 581 of these flowers, for 47 hours in all, at many observation points across the Okanogan-Wenatchee National Forest in Washington state and recorded or observed 57 mosquito feedings. They confirmed earlier findings in the same habitat that this species of orchid is pollinated by local mosquitoes, though many closely related species in the same location are not.

Next, they snuffed out the sensory stimulus by enveloping the flower in a bag. This meant the mosquito could no longer see the flower but could still land on the right spot. These mosquitoes tried their best to extract the nectar through the canvas bag.

From their observations on bagged flowers and mosquitoes, the researchers found that the insects were drawn by the scent of the flower. It was at this point that they began to analyze the scent itself. Describing the odor of this orchid as a grassy or musky odor, Riffell says its close cousins smell more fragrant.

As researcher Jeffrey Riffel explains, “We often describe 'scent' as if it's one thing -- like the scent of a flower, or the scent of a person. Scent is actually a complex combination of chemicals -- the scent of a rose consists of more than 300 -- and mosquitoes can detect the individual types of chemicals that make up a scent.”

The team used techniques like gas chromatography and mass spectroscopy to discover the numerous chemicals in this scent that stimulate the smell organs of mosquitoes. They found that it had higher concentrations of a chemical called nonanal but less of lilac aldehyde compared to other orchids in the same family.

When each of these chemicals is studied individually, it is found that some of them attract while others repress certain neurons in the mosquito brain. However, when they are combined in the right ratio, they form an irresistible smell that pulls any mosquito in the vicinity towards it. This chemical bouquet can be reproduced in the laboratory, the scientists found.

Simultaneous with the analysis, they also looked at how the mosquito antenna responded by electrical activity to scents. They also experimented with omitting lilac aldehyde altogether, or using more of it in the solution.

Finally, they examined the brain of the Aedes increpitus species, and that of an engineered strain of Aedes aegypti, which Riffell and co-author Omar Akbari had earlier developed. They used specialized microscopic techniques developed in the same laboratory to pick up calcium ion movement within the antenna lobe, where antenna signals are processed. Calcium ion movement signals nerve cell activation.

The findings

The researchers found that the resynthesized orchid scent attracts mosquitoes just like the flower does.

The nonanal and lilac aldehyde were found to attract not only native mosquitoes but also those from other regions such as Aedes aegypti, the carrier of dengue, Zika, yellow fever and other viral illnesses, and Anopheles stephensi, a malaria vector. All the mosquitoes showed the characteristic zig-zag flight that occurs when a mosquito catches a favorable scent.

When the mosquito behavior was analyzed, it became clear that all the mosquito species, whether native or not, showed stimulation of the antennas when exposed to these two chemicals in the same ratio as present in the scent of the blunt-leaf orchid.

Leaving out lilac aldehyde made the scent too bland to attract mosquitoes, apparently, but increasing it to match the odor of other more fragrant relatives of the blunt orchid left the mosquitoes cold or actually drove them away. This shows how precise the ratio of these chemicals must be to ensure the blunt-leaf flower is pollinated. The absence of this ratio means the mosquitoes don’t visit other close relatives of this orchid.

Brain imaging findings showed that different parts of the antenna lobe respond to nonanal and lilac aldehyde. The part of the lobe that responds to nonanal (abundant in P. obtusata) inhibits the part that is stimulated by lilac aldehydes (abundant in other Platanthera species). This kind of communication may vary with the amounts of nonanal and lilac aldehyde that is present in the scent, and results in the mosquito feeling either attraction or repulsion to the flower, depending on the ratio of these chemicals.
Moreover, the researchers also discovered that lilac aldehyde stimulated the same part of the antenna lobe that the well-known mosquito repellent called DEET does. They speculate that this region may be responsible for processing scents that drive mosquitoes away. This conclusion can only be arrived at following more experiments, however.

In short, says Riffell, “Mosquitoes are processing the ratio of chemicals, not just the presence or absence of them. This isn't just important for flower discrimination -- it's also important for how mosquitoes discern between you and I. Human scent is very complex, and what is probably important for attracting or repelling mosquitoes is the ratio of particular chemicals. We know that some people get bit more than others, and maybe a difference in ratio explains why.”

The implications

The study thus shows that mosquitoes are stimulated just as strongly by flower scents of the right type as by a warm-blooded animal. The environmental cues presented by the flower can either draw the mosquito towards the flower or cause it to fly away.

Knowing how and why mosquitoes are attracted by some flower scents could help in the development of improved mosquito repellents, or better mosquito traps. It could also help scientists to understand how the brain of a mosquito is stimulated by nervous signals in response to this type of stimulus to feed on nectar or sometimes, to bite a human. As Riffel says, the scientists will need to work more on declaring lilac aldehyde a mosquito repellent – but he would like it to be, because it smells very nice!