What could radio-tracking mosquitoes tell us?

News that Australian scientists are about to start a study of bee behaviour using tiny sensors attached to the insects created headlines around the world. Why not, it is a neat story and there is no doubt a need to better understand bees and the threats they face from human activity, parasites, disease and a changing climate. The researchers have already flagged the potential for this methodology to be adapted to studying mosquitoes, what could we find out…if the technology works.

The research team at CSIRO will be attaching sensors (approximately 2.5×2.5mm) to the thorax of 5,000 honey bees in Tasmania. As described in the media release, these sensors act like the “e-tags” in our cars that are used to pay road tolls. An array of detectors will record the movement of those bees with sensors attached. The data could then be used to generate complex maps of bee movement throughout the environment.

Lead author on this “swarm sensing” study, Dr Paulo de Souza, has already flagged the possibility that this technology could be adapted to the study of mosquitoes. There is no doubt that the current size of the sensors would need to be reduced (and there are probably some practical issues to solve regarding the appropriateness of field based sensors when it comes to mosquitoes) but, if that can be done, what questions could we answer if we could microchip mozzies?

How far do mosquitoes fly?

An obvious question but one that has been thoroughly investigated already. Understanding how far mosquitoes disperse from local habitats has implications for nuisance biting and mosquito-borne disease management.

The best example of how an understanding of dispersal assists mosquito-borne disease management is found in Far North Queensland where QLD Health manages dengue outbreaks. When a case of dengue is identified by local authorities, mosquito control is undertaken within a 200m radius of the property. This ensures that any mosquitoes that may have bitten an infected person will be killed before they have the opportunity to infected more people. Understanding how far the “dengue” mosquito flies will assist in allowing authorities to target mosquito control activities.

We don’t need a high tech solution to find out this information. There is a long history of mark-release-recapture experiments measuring dispersalpopulation size and how mosquitoes interact with the environment. You don’t necessarily need to microchip the mozzies, all you need is a few grams of fluorescent powder.

I’ve be part of a couple of mark-release-recapture experiment. My favourite was an investigation into the dispersal activity of dengue mosquitoes in Cairns, Far North Queensland. We marked mosquitoes with a fluorescent powder, released them, and then set a network of traps. All specimens collected were then returned to the laboratory and scanned under a UV light to detect any marked mosquitoes. The results of this study not only assisted the development of strategies to manage dengue outbreaks but this information also assisted the development of a number of research projects (including the Eliminate Dengue program).

The major draw back in mark-release-recapture experiments is that once recaptured, that is the end of the individual mosquito’s contribution to the study. You are only provided with limited information. A simple snap shot. The information provided allows measurement of how far a marked mosquito has travelled from the release point but what if the “stops” along the journey are more important than the distance travelled?

The Saltmarsh Mosquito (Aedes vigilax) (Photo: Stephen Doggett)

The Saltmarsh Mosquito (Aedes vigilax) can travel up to 20km from larval habitats and mark-release-recapture experiments have recorded active movement within 3-5km for saltmarsh and mangrove habitats (Photo: Stephen Doggett)

Where do mosquitoes lay eggs?

In the study of mosquitoes that transmit dengue and chikungunya viruses, where eggs are laid can be critically important. Mosquitoes such as Aedes aegypti and Aedes albopictus lay eggs almost exclusively in artificial containers. Much work has gone into determining the preferred types of containers and the factors that drive egg laying behaviour but we can always learn more.

Mosquitoes “tagged” could be tracked between various types and locations of potential egg-laying sites. What types of habitats are mosquitoes most likely to visit? Is the location of these habitats important (e.g. are containers located in the shade more productive than those in full sunlight?). The better we understand these associations, the better we’re able to assess dengue risk.

With Aedes aegypti exhibiting “skip-oviposition” (i.e. not all eggs are laid in a single container), the ability to track multiple mosquitoes across a range of habitats could provide some valuable information. There has already been much research into this phenomenon using a range of techniques (including this recent study incorporating molecular analysis of specimens) but there are plenty of questions left unanswered. Traditional methods may help identify locations where eggs are laid but how many other sites are visited by mosquitoes where eggs are not laid? This information may be critically important.

Understanding how mosquitoes select the containers where eggs are laid is important for targeted mosquito control strategies (either treatment of containers or removal of containers) or to help in the development of more effective “oviposition traps”.  These traps can then be used as either surveillance or control devices. The technique may also help identify “cryptic breeding sites” that are difficult to identify using traditional surveillance methods.

Tracking movement of the malaria vector Anopheles gambiae in response to odors and heat. Could “microchipped” mosquitoes provide insights into this behaviour in the field? (PLOS)

How do mosquitoes hunt down blood?

The Great White Shark can purportedly detect a single drop of blood in an Olympic-size pool section of ocean. How sensitive is a mosquito to the carbon dioxide exhaled from a potential host? Do mosquitoes sit and wait for a plume of carbon dioxide to float by or do they fly around trying to detect a plume and then fly to the host?

It would be fascinating to see the long and short range flight patterns of “tagged” mosquitoes in response to potential blood meal sources. Do they react differently in response to the “smell” of humans or animals? How do they approach a host?

Researchers have already been working towards answers to these questions using various strategies. Mathematical models have been developed to determine how mosquitoes respond to hosts and cameras in wind tunnels have helped create 3D “maps” of mosquito movement.

Understanding how mosquitoes find and approach a host may help improve the design of mosquito traps. I often find that when collecting mosquito traps in the morning, some species are buzzing about and biting me but they’re not represented in the actual trap collections. Are they approaching the trap differently? In the last decade or so, there have been studies that demonstrate that sucking mosquitoes up into a trap, as opposed to blowing them down, can change the species composition collected. Understanding how mosquitoes approach, and are either collected or not collected in, these traps would be a great outcome of being able to “track” mosquito movement.

In which bedrooms do mosquitoes bite?

In the study of mosquito-disease outbreaks, particularly malaria, it would be useful to understand how many people are potentially bitten by individual mosquitoes. In particular, the spatial and temporal distribution of infective and uninfected mosquitoes and the environmental factors that influence that distribution. Models have been developed to investigate elements of these issues. With more detailed information on how mosquitoes may move around indoors, between rooms within a dwelling or between dwellings in small villages it may be possible to better understand how mosquito movement is influenced by bed nets or residual insecticides.

The great benefit in being able to “track” mosquitoes electronically is that it could be a way of collecting data passively. By that I mean you don’t need to lure, catch or kill specimens. As in the examples regarding egg-laying behaviour, being able to track the spatial and temporal activity of an individual mosquito over time would provide some valuable information. Variability in host-seeking behaviour in response to bed nets or other control strategies has been identified as a critical factor.  It may also provide some insight into host-seeking behaviour without relying on the need for human landing collections.

Some mark-release-recapture studies have provided insights into the movement of malaria vectors from the local environment into villages. The ability to track these movements more specifically, particularly in relation to the availability of hosts, vegetation and other physical/chemical factors will assist in public health risk assessment and the development of control strategies. Perhaps building design could be refined to reduce rates of malaria?

Where do mosquitoes take a nap?

This may sound silly but it could actually be incredibly useful in developing targeted residual insecticide treatments. The identification of resting sites has been the focus of many studies (here is one from South America) and is generally most commonly undertaken through “vacuum sampling” habitats (but a wide range of methods may be used). This technique will help identify locations where mosquitoes are currently resting but it is a little trickier to work out where they came from and where they’re going from those resting sites.

Take this example. That mosquito buzzing about your head at night keeping you awake (most probably the brown house mosquito Culex quinquefasciatus), did it enter your room that night, the previous night, or did it sneak in during the day and take refuge under your bed? While this mozzie may just be a mild nuisance, understanding the resting behaviour of mosquitoes that transmit pathogens will assist control activities. When and where would be the best place to use insecticides to ensure a good nights sleep?

With increasing scrutiny being placed on the use of insecticides in the environment, understanding where applications of residual insecticides are most effective will help balance mosquito control and environmental conservation objectives. For example, to get the most control with least insecticide application, should products be sprayed in the canopy of trees or in the leaf litter below? What surfaces on the outside or inside of homes are most likely to be favoured by mosquitoes? Is the resting behaviour of mosquitoes influenced by the presence of insecticides?

What do male mosquitoes do?

This may not have a great impact on mosquito-borne disease management but it could provide some idea of the ecological role of mosquitoes. What do male mosquitoes do? They’re generally not collected in the carbon dioxide baited traps commonly used to collect female mosquitoes. You can collect them with sweep nets or aspirators but that doesn’t really provide much insight into their spatial and temporal activity.

Wouldn’t it be fascinating to see what plants male mosquitoes are visiting? They don’t need blood but are they likely to seek out “plant juices”. Could male mosquitoes play a role in plant pollination? I’d love to know how male mosquitoes interact with the local environment. From some of our previous research with Culex molestus, we know that the diet of male mosquitoes can influence the fecundity of the females so perhaps understanding the activity of male mosquitoes could provide some novel control options?

Where to now?

Lots of ideas to consider once the technology is available. However, it is important that we don’t fall into the trap of repeating experiments and reexamining known behaviours just because we have some new technology. What excites me to most is the prospect of answering some questions we don’t actually know how to answer with current technologies. Watch this space!

The photo at the top this post is taken from the CSIRO media release.

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