Trapping larval and adult mosquitoes

VICTOR BRUGMAN: Hello, everyone. And welcome to this step, trapping larval and adult mosquitoes. My name is Victor Brugman, and I lead research developing new technologies to control pest arthropods. In this step, we will cover the principles of mosquito trapping and the common methodologies used to do so. So why do we need to trap mosquitoes? Sampling mosquitoes enables researchers and public health officials to generate important data on mosquito populations to identify which species are found where, in which habitats, and at what density. We can additionally obtain information on what they are feeding on, and whether they’re carrying any important pathogens.

Collectively, these data help us to identify trends over space and time, and ultimately assist us in deciding how best to target our control efforts to where they will have the greatest effect. There are a wide range of tracking methodologies used across the world, each of which has its own benefits, challenges, and limitations depending on your purpose and resources. The aim of this section is for you to finish with an understanding of the principles of and commonly use techniques for collecting mosquitoes. I will principally focus on the key mosquito behaviours we are exploiting with different trap types. I will also highlight some of the more commonly used traps.

Along the way, I’ll discuss the different challenges that arise when designing and conducting mosquito trapping studies, and the ways in which we can overcome them. Throughout the step, I’d like you to have a think about what methods you might have seen in your local area, and I’d love to hear about them in the comment section. So let’s begin by recalling that the mosquito lifecycle consists of four distinct stages, egg, larva, a non-feeding pupae, and finally the adult. These life stages are either aquatic, terrestrial, or at the interface of both, presenting us with different challenges when designing ways to collect them.

Here, we will focus on the fully aquatic larval stage in the terrestrial adult, although larval collections can be broadly applied to pupil collections as well. However, because pupae lack many definitive morphological traits which we use to identify them to species level, larvae and adults are usually the target of sampling efforts. When collected, pupae are usually allowed to emerge and then can be identified as adults. So the first step is to identify where you’ll be sampling. This will be guided by a combination of epidemiological and entomological pointers such as disease instance and reports of nuisance fighting against a backdrop of logistical and resource constraints.

Once you have identified the general sampling area, you will want to hone on exactly where to sample. As water bodies in a given area vary greatly in size from the tiny to the very large and may be spread over a wide area, an important first step is to carry out desk-based mapping of your site. A simple yet powerful way to do this is with the publicly available satellite images. For example, on Google or Bing maps. Do note, however, that these may not allow you to see every water body, particularly small container habitats.

And secondly, they are but snapshot images reflecting the water bodies present at a given time, so are unlikely to pick up all habitats, especially those that are very transient. Before you implement your first round of larval sampling, it’s therefore a good idea to conduct a prelim visit to your site to understand the logistics of sampling there, and ensure you have the necessary permissions. Bear in mind that the most obvious sites are not always the ones supporting populations of your target species. Now, new, innovative methods such as drones have been trialled to overcome some of the challenges of mapping large sites, although these still require you to visit the site.

You can check out the references, for examples, of remote sensing and drone technologies used for habitat mapping. One of the most commonly used larval sampling methods is dipping. This involves carefully submerging a net or a pot into the water and seeing if any larvae are present when filled. One of the major challenges here is the evasiveness of mosquitoes. Larvae and pupae can move rapidly through the water column to escape if they’re disturbed.

We can increase the chances of collecting the larvae by minimising the shadow we cast over the water when we approach, by slowly submerging the dipper to reduce disturbance, and by waiting for a period of time in between each dip to allow the larvae to return to the surface. It’s also important to keep your dipper and the number of dips consistent if you are assessing population size so you can compare between areas. For smaller habitats such as treehouse or the artificial container habitats frequently encountered in urban environments, we might need to use a small pipette to access them, as shown in the image bottom right. Larval dipping targets mosquito genera whose larvae rest at the water’s surface.

We need to employ different methods to collect members of the genera coquillettidia and mansonia, which obtain oxygen by piercing plant stems. These require a more specialist approach, often involving the disturbance and/or remove of their preferred aquatic vegetation. Following disturbance, the water around the vegetation can be pumped through a sieve to isolate the larvae in pupae. Larval sampling data can tell us a lot about the structure of a site’s mosquito population helping us to better understand where, when, and how to implement our control programme to maximise its effectiveness or else minimising the negative effects on other species in the environment. Alongside the control measure itself, an effective campaign requires community engagement and a strong educational component.

For example, the image here shows a clear public campaign in Singapore, showing how urban aedes level habitats can be targeted with a few simple steps. Now, there are a wide variety of traps available for collecting adult mosquitoes. The trap of choice would depend on a combination of how effective it is and its logistical feasibility. But first and foremost, it needs to be guided by your research question or purpose. Our purpose will affect which subset of the mosquito population we need to collect, and therefore, which aspect of mosquito behaviour we need to exploit with our traps. It will also affect where we place the traps and how long we run them for.

And bear in mind that whichever trap we choose, we will introduce an element of bias into our data. So we need to be conscious of this when designing our study and interpreting the results. Now, some of the more commonly targeted mosquito behaviours are flight behaviour, resting behaviour, and egg-laying or oviposition behaviour, the three of which we will cover in the following slides. We will also cover both active and passive traps. Active traps use an electricity supply to power a fan which then draws mosquitoes in and prevents their escape. Passive traps rely on mosquitoes entering and remaining within the traps of their own accord.

Or they can sometimes use physical means such as a complex entrance way or baffle to prevent the mosquitoes from escaping. We can exploit various cues influencing mosquito flight behaviour to lure them into our traps. One of the most generalist cues suitable for mosquitoes active at dawn, dusk, or during the night is light. The gold standard trap for many species, including the anopheles vectors of malaria, is the CDC light trap pictured here. The white light bulb is situated beneath the rain shield, and mosquitoes approaching the light are actively drawn in via the fan and into the catch bag below. Both catch rates and species diversity can be improved by adding additional semiochemical attractants to bait the trap.

One of the most important of these is carbon dioxide from our breath. In the photo here, carbon dioxide is being produced by dry ice held within the red canister. To collect a wide range of both male and female mosquitoes, traps can also be better with other odours such as floral odours or those more broadly associated with sugar sources. Pathogens expectorated during sugar feeding can be detected then as part of surveillance activities. Check the references for more information on these. Now, although we have made significant progress in identifying specific attractive compounds and blends, some of the most representative data can be gathered by using the target host itself as bait for collections. This is especially important for generating biting rate data.

That is to say, the number of bites on a host per unit time. This is an important figure for modelling pathogen transmission dynamics. To assess biting on a human host, we can use the human landing catch. In this, we can expose a defined part of the body for a given amount of time, waiting for mosquitoes to land, and then collecting them using an aspirator or small pots, ideally before they bite. We can also collect directly from animals. Although to avoid the human interfering with the catch rates, we may place the host into enclosure with a baffle designed to prevent mosquito escape.

Although these host-baited methods can produce very valuable data, in disease endemic areas, in particular, they are accompanied by unacceptable infection risk. In addition, using animals compose considerable logistical and welfare challenges. Accordingly, considerable work has gone into formulating artificial blends and effective trap designs that can mimic the smell of the natural host with close to or equivalent collection efficacy. We can also exploit mosquito resting behaviour to collect them. When not blood or sugar feeding, mosquitoes will shelter and remain motionless in areas either indoors or outdoors. For species that display exophilic or outdoor resting behaviour, we can use a sweet net or high powered mechanical aspirator to collect them from vegetation.

Species that show endophilic or indoor resting behaviour can be collected from the inside of natural or man made structures such as caves or housing using aspirators. We can mimic these shelters on a smaller scale by using boxes of various size with resting boxes from which we can collect the mosquitoes. These are cheap and easy to deploy, but don’t actively trap mosquitoes inside them. And therefore, require us to go there and manually collect from them with aspirators. Other methods that can be used also include knock-down collections, which use an insecticide to kill indoor resting mosquitoes followed by a collection period of dead specimens a little while later.

The major challenge with collecting resting mosquitoes is that we don’t fully understand what attracts mosquitoes to a resting site. For exophilic species, especially, this can be challenging, as resting places may be spread over a large area, we can’t easily attract them to traps. For this reason, the blood feeding behaviour of many outdoor resting mosquitoes is less well understood than their indoor resting counterparts. Finally, we can exploit the oviposition or egg-laying behaviour of female mosquitoes. When mosquitoes have a developed egg batch, they’re referred to as being gravid. Once the egg batch is fully developed, females will search for a suitable site to lay the eggs.

We can therefore design traps to mimic a natural oviposition site using both visual and olfactory cues. The simplest are small black pots containing a wooden stick as an oviposition substrate for aedes species, which lay their eggs just above the waterline. These traps don’t capture adults unless they die upon oviposition, but they allow the eggs to be collected. These have been successfully used to identify invasive aedes species in several countries worldwide. And in combination with an insecticide, have been explored as a means of population control in so-called lethal oviposition traps. Other traps are designed to capture adult females as they approach the water body to lay their eggs.

These traps contain water and an attractant, such as a hay infusion, which mimics the natural decomposition odours of vegetation. As the larvae of most species feed on organic matter in the water column, the nutrient rich habitat is a preferential one. One of the key benefits of collecting gravid specimen is that any mosquito borne pathogen taken up in the blood meal will have had time to develop and disseminate throughout the mosquito. Therefore, gravid mosquitoes often provide a better measure of the presence of a transmissible pathogen in a collection site than unfed or recently blood fed females. In the preceding slides, we’ve touched on just a few of the major trap types frequently used.

There are many other trap types of no less importance, which exploit different aspects of mosquito behaviour. These include traps that collect mosquitoes as they enter into, exit from houses, or other structures. As well as those that intercept host-seeking mosquitoes mid-flight via electrocuting nets. These are frequently used for capturing tsetse flies, for example. These knock-down insects on the way to an attractant or host bait using an electric current passing through a grid with the insects collected into a trough at the base. More detail on these can be found in other modules and in the included references.

I hope you can now see that there are a number of ways in which we can collect mosquitoes. And I’m confident that some of you will come up with a novel, even more effective way of doing so in future. The final point before you start trapping is to be aware that each trap comes with logistical considerations, several of which are listed here. These include the weight of traps and their accessories, and the challenges of preserving genetic material in the field during long periods of high heat. If you can’t control something, such as meteorological variables, consider recording these so you can at least see the impact on your catches.

Finally, some of the most important factors and variables are those that you can’t control are the people in the natural world. Therefore, it’s always a good idea to bring spare parts.

This brings us to the end of this section. We have covered the main principles of larval and adult trapping, which can be summarised as follows. Researchers trap mosquitoes to increase our understanding of mosquito biology, behaviour, and distribution as well as to understand the dynamics of the pathogens they transmit. Mosquitoes can be collected at every life stage, but trapping studies are most commonly targeted towards larvae in adults. Adult trapping methods will further be targeted to trap a specific segment of the population, those in flight, those resting, and those that are gravid or egg-laying. Each trapping method differs in its effectiveness, and logistics play a significant role in influencing trap choice in field studies.

Prepare as best you can by mapping your site beforehand, and also by visiting a site, if possible, for a preliminary visit to identify any issues in advance. And finally, do you have any experience of collecting mosquitoes? Do you know what sort of traps are being used by public health professionals near you? I would love to hear about your experiences in the comments section below. Also if you have a 3D printer, why not try printing out a trap and seeing what you can collect? I’ve also included links to other useful resources if you’d like to find out more.