by Aedes aegypti Mosquitoes, one of the most common species in the US, love everything about people. They love our body heat and scent, which enables them to find us. They love to feed on our blood to make their eggs mature. They even love all the standing water we create. Uncovered containers, old tires and junk piles collect water and are perfect for breeding.
And with the arrival of warm weather across the southern US, mosquito breeding season is already underway.
Given all the options a Aedes in female urban areas, how do these cosmopolitan mosquitoes find the perfect location to lay their eggs? Scientists previously thought that this was a single act, but now research shows that it is a female Aedes aegypti mosquitoes – the main vector in the US for diseases like Zika, dengue, chikungunya and other viruses – can depend on each other for good reviews of breeding sites.
Our Tropical Genetics Laboratory at Florida International University discovered a new behavior in which these mosquitoes work together to find suitable egg sites. These results, recently published in Communications Biology, show that mosquitoes regulate their own population density at breeding sites—an insight that could inform future mosquito control efforts.
Where and why female mosquitoes cluster
Scientists know that female mosquitoes can be picky when it comes to where they lay their eggs. Aedes aegypti search for man-made breeding sites with relatively clean water, such as birdbaths, coins or even water-filled litter boxes. But given two equal options, you might expect them to spread evenly between the two.
Conversely, when we released females in a two-choice test where the two breeding site choices were identical, we repeatedly found more mosquitoes in one room than the other. Furthermore, this occurred regardless of where the chosen chamber was located, whether the mosquitoes could come into contact with water or whether mosquito eggs were already at the breeding sites.
Female mosquitoes were clearly following each other in small groups to one breeding site rather than another – a newly discovered behavior in Aedes aegypti which we call aggregation.
It is clear that the insects would prefer not to lay their eggs alone. When we tested 30 mosquitoes in our tests, they chose one site over another by a margin of 2 to 1. However, this changed as the test population increased beyond 30 mosquitoes. When we tested 60 or 90 females, the aggregation disappeared.
This suggests that females can regulate their own density at breeding sites – a response that is likely a mechanism to limit larval competition.
Mosquitoes are smelling each other
Mosquitoes feel the world mainly through smell, using three families of olfactory receptors. These receptors detect odors when females are choosing where to lay eggs. But how do females sense each other to control their density at breeding sites?
We explored this question by first placing 15 mosquitoes at one of our two test breeding sites. Other females looking for a place to give birth preferred the unoccupied site rather than the one that was already occupied, although we have already noted that the mosquitoes prefer not to lay their eggs alone. Something was leading him away from the occupied breeding site; we speculated that it might be carbon dioxide, which is important for mosquitoes in all stages of their life cycle.
When female mosquitoes are looking for a blood meal, they fly towards the smell of CO₂, which all vertebrates exhale and release through their skin. After feeding, they fly away, probably to avoid the risk of killing the host.
Mosquitoes also emit CO₂, and other mosquitoes can usually smell it, thanks to a receptor component called Gr3 in their olfactory organ. But when we released mutant females that lacked a functional Gr3 receptor to search for a place to lay eggs in our two-site test, we found that these insects, unable to detect CO₂, were willing to lay their eggs at predecessor breedings. This suggested that normal mosquitoes might be avoiding the pre-soaked breeding site because they could smell CO₂ emitted by mosquitoes that were already there.
To confirm this, we provided two unoccupied breeding sites to females seeking a breeding ground. However, we increased CO₂ levels at approximately one of the sites to between 600 and 750 parts per million, compared to the normal level of approximately 450 to 500 ppm at the other site. We got that Aedes aegypti females avoided the unoccupied sites with elevated CO₂. This behavior appears to be designed to keep occupied breeding grounds from becoming overcrowded.
Overall, we found that two families of receptors play a role in the interactions between Aedes aegypti females when they seek out breeding sites. Olfactory receptors detect an unknown odor, which attracts females towards a site; gustatory receptors detect CO₂, which deters females from breeding sites when the carbon dioxide level is high. The balance between these attractive and arousing odors will ultimately determine whether a female chooses or avoids a particular location.
Implications for mosquito control
Using biolarvicides – pesticides made from live bacteria that are toxic to mosquito larvae – is a key control strategy for mosquito populations in urban areas to limit the spread of deadly diseases such as West Nile virus and Zika virus. This is especially true for Aedes aegypti, which is the most common urban mosquito species that breeds in artificial breeding sites created by humans. Other control tactics, such as spraying pesticides over large areas, target beneficial insects as well as mosquitoes and can be controversial.
Knowing that female Aedes aegypti using social cues to select the best breeding sites for their young and moving on from a breeding site when it becomes overcrowded could lead to new control measures. Interrupting the reproductive cycle of a female mosquito would reduce the spread of mosquitoes and the spread of diseases carried by these insects.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. It was written by: Kaylee Marrero, Florida International University; Andre Luis da Costa da Silva, Florida International Universityand Matthew DeGennaro, Florida International University
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Kaylee Marrero receives funding from the National Institutes of Health.
Andre Luis Costa-da-Silva receives funding from the Centers for Disease Control and Prevention, the Southeast Center for Excellence in Vector-Borne Diseases and the National Institutes of Health. The opinions expressed in this article are his own.
Matthew DeGennaro receives funding from the Centers for Disease Control and Prevention (CDC), the Southeast Center for Excellence in Vector-Borne Diseases and the National Institutes of Health. The opinions expressed in this article are his own.
