Skip to 0 minutes and 13 seconds Up to now we’ve discussed why we need to trap mosquitoes, both in areas where Zika and other viruses are currently found, but also in areas where there is a risk that they might arise. The purpose of this step, therefore, is to provide an overview of adult mosquito trapping methods. Once we’ve trapped the mosquitoes we can then bring them back to the lab where we can identify them under the microscope and, if necessary, test them for the presence of viruses such as Zika. There are many different types of trap for adult mosquitoes, and the one that you choose will depend on which species you are targeting and what the purpose of your trapping programme is.
Skip to 0 minutes and 47 seconds With this in mind I will start by discussing the importance of understanding the basic biology and behaviour of the mosquito species that we want to collect. Next, using this information we’ll discuss how we can exploit these behaviours to actually collect the mosquito and provide a rundown of the most commonly used mosquito traps for Aedes mosquitoes. Finally, along the way I will also highlight some of the many challenges involved in actually conducting a mosquito trapping programme in the field. The first step in designing in particular method of trapping mosquito species is to understand the mosquitoes’behaviour. There are over 3500 species of mosquito worldwide, found on every continent except Antarctica.
Skip to 1 minute and 28 seconds Whilst each of these species follow the same four stage life cycle, consisting of egg, larva, pupa, and adult, there are many differences in the basic biology of each life stage between species which will influence how we are able to trap them. Here we’re focusing on adult behaviour in particular. There are three main adult mosquito behaviours that are frequently targeted to collect Aedes mosquitos these are host seeking behaviour, resting behaviour and oviposition or egg laying behaviour. In this step, we will mainly focus on host seeking and resting behaviour as oviposition behaviour is incorporated into the next step. We will also look at how we can design traps to target these particular subsections of the population.
Skip to 2 minutes and 9 seconds During host seeking behaviour female mosquitoes are actively trying to find a host, such as you or me, from which to take a blood meal. They do this by detecting the various chemical compounds that we and other animals naturally produce. These chemicals include carbon dioxide, which is a major component of our breath, as well as a large number of different compounds produced in our skin such as lactic acid. These skin derived compounds are very volatile, meaning that once they are produced on the skin, they rapidly evaporate and diffuse into the surrounding air and, together with carbon dioxide, produce a complex plume of odours which can be detected from some distance away by mosquitoes.
Skip to 2 minutes and 46 seconds Mosquitoes will then fly up the concentration gradient of the odour plume towards the source using shorter range visual cues, such as colour, heat, and movement, to land on the host and take a blood meal. Therefore, traps designed to collect host seeking mosquitoes are designed to mimic the characteristics of a real human or animal. This is done by providing a chemical attractant, often with carbon dioxide, and then replacing the actual host with a fan and a catch bag to collect the mosquitoes. I have in front of me two of the most commonly used traps for Aedes host-seeking, adult mosquitoes. These are the BG-Sentinels; this is the original version and this is the more recently released, updated version the BG-Sentinel 2.
Skip to 3 minutes and 28 seconds Now these traps work on the basis of creating a convection current mimicking the natural processes that release odours from a host. Here at the top of the trap we have a fan inside into which air is drawn.
Skip to 3 minutes and 44 seconds A chemical blend derived from human skin odours, such as lactic acid, is added the trap is a package or as a cartridge placed inside the trap. Now air drawn into the trap subsequently leaves through the outer edges of the trap, taking with it the chemical blend creating an odour plume which can be detected by the mosquitoes. Closer in, the mosquitoes are additionally attracted by the alternating dark and white colours of the trap. Now unfortunately it isn’t quite as easy as simply placing these traps out and waiting for the mosquitoes to be collected.
Skip to 4 minutes and 18 seconds It’s important to note that different mosquitoes may preferentially feed him some types of host over others, for example, humans or birds, and therefore, when designing your trap, you need to consider which attractive chemicals to include and other factors such as trap height. So for example, bird feeding species may require you to place a trap at a greater height to mimic a host in a tree. Aedes aegypti are mammalian feeders primarily and in urban areas feed largely on humans, therefore traps are frequently placed inside or close to houses. Most available traps rely on either battery or mains power to power the fan.
Skip to 4 minutes and 53 seconds Batteries will last for one to two days at most before they need replacing requiring collectors to visit each trap frequently with fresh batteries. During these visits, the trapped mosquitoes will also need to be collected. If they are left inside the trap for more than a couple of days, they will quickly dry out and make it very difficult to identify them. If you’re using a source of carbon dioxide as part of the attractant of the trap, you need to either carry large gas canisters around or regularly replace a source of the carbon dioxide such as dry ice.
Skip to 5 minutes and 22 seconds Finally, it is important to consider the weather on your trapping days; strong winds and heavy rain will stop mosquitoes from flying and can even damage the traps. It’s always a good idea to record these meteorological variables so that you can better explain variation in your trap collections.
Skip to 5 minutes and 39 seconds When not attempting to find a blood or sugar meal, adult mosquitoes can be found resting even in vegetation or, commonly in the case of Aedes aegypti, on the inside of houses. We can target these mosquitoes by using aspirators, such as this Prokopack aspirator here, which work like vacuum cleaners to hoover up mosquitoes into a collection pocket. Collectors will work primarily during daylight hours so they are able to collect mosquitoes resting on the inside of houses.
Skip to 6 minutes and 9 seconds Whilst relatively simple in principle, collecting mosquitoes from inside houses poses several challenges beyond simply gaining permission for entry. Mosquitoes may rest in areas not immediately obvious or accessible for collection, therefore collectors need to be well trained in finding areas where mosquitoes are most likely to be found. Just like the host seeking traps, aspirators also require a battery for power and with intensive use these batteries become rapidly drained. In my experience a regular 12-volt battery will become drained within an hour or two of intensive use; this means the collector will need to carry several heavy batteries with them in order to cover several houses.
Skip to 6 minutes and 47 seconds It’s not always something people think about when collecting mosquitoes, but carrying a pack of batteries around with you does get tiring, especially in the heat. Finally, the powerful vacuum generated by the backpack aspirators may damage your specimens, especially those containing blood which you may wish to analyse in future in the lab. Therefore, these specimens ideally need to be transferred as soon as possible to a storage pot where they can be kept until analysis. In summary, when designing a mosquito trapping programme, it is important to first understand the behaviour of the target mosquito you want to collect. And then which subsection of the population you need to collect for your purposes.
Skip to 7 minutes and 23 seconds The most commonly used adult traps for Aedes mosquitoes include the BG-Sentinel trap for host seeking mosquitoes and backpack aspirators for resting mosquitoes. All trapping methods have advantages and disadvantages which need to be considered alongside factors that you can’t control, such as the weather, when designing your collection programme. In this section, I’ve highlighted some of the most commonly used approaches for trapping adult Aedes mosquitoes, but there are many different trapping methods currently used for this species as well as for other important mosquito vectors. Many research groups are also developing and testing new, more efficient traps to improve our ability to collect mosquitoes in the field; you can read about these in the accompanying material.
Skip to 8 minutes and 3 seconds Trapping adult mosquitoes provides important information to help us understand the transmission patterns of mosquito-borne pathogens. This will ultimately help us better understand and control outbreaks of this type, including of the Zika virus.
Trapping and monitoring: adult mosquitoes
More than 3500 mosquito species have been recorded worldwide. Each of these species behaves differently, and therefore we need to understand the behaviour of our target mosquito in order to effectively trap it. In this step, Dr Victor Brugman describes the main types of adult mosquito behaviour that we can exploit to trap Aedes, and the challenges of collecting mosquitoes in a field situation.
In a practical demonstration, Dr Brugman describes a commonly used traps to collect Aedes aegypti, the BG-Sentinel, and explains how it exploits the host-seeking behaviours of the mosquito. He also demonstrates the use of a backpack aspirator to collect resting mosquitoes, and discusses the factors to consider when using this type of collection method.
The gold standard for measuring the densities of host-seeking vectors is the human landing catch method, where human volunteers catch mosquitoes that land on their exposed body parts. Unfortunately this approach exposes the volunteers to potentially infectious mosquitoes. Light traps placed indoors next to occupied bed nets are an invaluable and safe alternative for Anopheles mosquitoes, the vectors of malaria,1 but for Ae. aegypti they are ineffective as this species is an outdoor day biter.2 Larval sampling can be used, taking advantage of our understanding of Ae. aegypti to identify potential breeding habitats. Collected larvae and adults can then be identified by trained specialists.
Population size can be estimated through mark-release-recapture experiments, in which a portion of the mosquitoes in an area are captured and marked, then released. On a second occasion another portion is sampled and the number of marked individuals is counted. The number of marked individuals within the second sample should be proportional to the number of marked individuals in the whole population, so an estimate of the total population size can be obtained. Modelling can be done to improve the accuracy of the estimate, taking into consideration the distance between release points and traps, the time between release and recapture, and the loss of marked mosquitoes to death or dispersal.3
Before swatting them, have you ever considered how many different types of mosquito are found in your country or around your home? Have you used mosquito traps yourself? What methods of attracting mosquitoes did these traps use?
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