Mosquito Disease Transmission: Just the Facts, Ma’am.

Notorious disease vectors

Mosquitoes are some of the most notorious disease vectors in the world. Because of their worldwide distribution (except for Antarctica), proximity to humans, and inclination to feed off of humans, mosquitoes have been able to spread viral (e.g. West Nile, Zika, Chikungunya) and parasitic (e.g. Malaria) diseases to people throughout the world. Annually, over one million human deaths are attributed to mosquito-borne diseases.

But how do they do this?! It turns out female mosquitoes inject some of their own saliva into the host – humans, to name one – to stop the host’s blood from coagulating before the mosquito can retrieve the blood (Image 1). If that mosquito has previously fed on a human or other animal infected with certain diseases, those diseases may have been able to replicate within the mosquito without harming it.

Thus, when an infected mosquito injects saliva into a host, that host can in-turn become infected.


Spectrum of Viruses.

So, are all viruses and parasites able to be passed from mosquitoes to humans? The short answer is no. Now for the longer answer: some viruses and parasites cannot survive the mosquito’s gut (like HIV). Because of that inability, they’re unable to establish within the mosquito’s cells and replicate. Environmental conditions, predominantly temperature, can also affect how a capable a virus or parasite is at infecting and replicating within a mosquito.

Warmer temperatures generally mean that a pathogen is able to replicate at a higher rate within a vector. Finally, the amount of the virus or parasite ingested by the mosquito also determines the ability for the mosquito to transmit the pathogen – the vector competence. The greater the dose, the greater the vector competence (assuming the pathogen is able to infect and replicate within the mosquito).  he main diseases that can be transmitted by mosquitoes within Canada are the California serogroup viruses, eastern equine encephalitis, and West Nile virus (WNv).

WNv is the most commonly transmitted mosquito borne disease in Canada. In 2018, a total of 432 human cases of WNv were reported in Canada – the highest total since 2007 (Image 2). Large-scale, nation-wide surveillance efforts are conducted to keep track of WNv incidence in horses, birds, and humans. These data give program managers the ability to direct mosquito control efforts.



Integrated Mosquito Management Program

How do we Control Mosquitoes?

Morrow BioScience Ltd. uses an integrated pest management approach to controlling mosquitoes. The best time during the mosquito lifecycle to control them is when they are larvae. For this reason, most of the efforts are placed on finding larval development sites and treating the larva when they are found. Mapping and monitoring the sites assists in the continued improvement of information.

Mapping and Monitoring

For mosquito control to be effective from year to year the development sites must be mapped and entered into the database for reference.

During the fall and spring, technicians find new potential development sites to be monitored during the summer. When new sites are found, they are mapped using the GPS (Global Positioning System) and added to the database. During the spring and summer, known development sites are monitored by technicians for larvae. Most sites are checked at least once every week, more frequently as the weather warms. If sufficient larvae are found in the development sites, they are then treated using larvicides. If no larvae are found in the development sites, no treatment occurs and the sites are checked again within a week.

If there are any areas near your home which contain water (but cannot be mechanically controlled) they can be reported at any time by a call to your municipality (if they have a control program in place). They will send someone out to the area that will assess the sites to determine if they require further monitoring.


The use of larvicides such as B.t.i. has proven to be the most effective in decreasing mosquito populations. Larvicides are relatively environmentally friendly, target-specific, and very effective in killing larvae. They can be applied through the use of helicopters, boats, or by hand.
Whenever possible, our technicians hike into the mosquito development sites and apply the larvicide by hand. This is very effective for smaller channels, but not in larger areas when the water is high. Boats are used to access and treat areas when the water is too high.

Finally, a helicopter is used to treat large development sites that are inaccessible or too large to treat by hand. This includes islands on the river as well as other large sites that become flooded when the river gets above a certain height.

Typical mosquito lifecycle.

Biology of our main target mosquitoes

The primary mosquito targets are Aedes vexans and Aedes sticticus. Their biology is quite specific to floodwater habitats. Both are somewhat easily identifiable in the field as adults and, when combined with habitat, also easily identifiable in the field as larvae. Generally, if the mosquitoes are so numerous that you can’t help but breath them in, and if they are biting during the heat of the day, then you could very well have at least one of these mosquito types.

Adult mosquitoes of both species have pointed abdomens. One has cool white bands on its legs and the other has black legs. That’s good enough 9 times out of 10 (that is, if you’re near a flooded area and the mosquitoes are aggressive).

A. vexans tend to come on first as rising water levels flood low-level grasslands associated floodplains. The trigger for their hatching is a rise in water temperature (in northern climes this can be as early as 4C but typically when the water reaches 6-8C).

A. sticticus tend to come on later as water levels reach the wooded riparian zones of the floodplain. The trigger for their hatching includes a decreased DO (and can happen in 1- 2 hours from wetting).

Both species lay their eggs on soils prone to seasonal flooding. As water levels rise the eggs are wetted (covered) and in appropriate conditions are triggered to hatch, beginning the progression to annoying biting adult. In cooler waters this process can take 2-3 weeks, from egg to adult. That provides a 10- 14 day window for effective treatments (we try to target 3rd instar larvae).

As the weather, and water, warms up this timeline can be shortened considerably. From the time that eggs are first wetted until the end of the effective treatment window can be as little as 60-70 hours (both species can, under certain circumstances, skip the 4th instar and moult directly from 3rd to pupa).

Each time the water levels progress higher, or as sustained water levels increases seepage areas not contiguous with the main river body, new batches of mosquitoes are hatched.

A proportion of A. vexans eggs do not require diapause before they are able to hatch. The only requirement is that they are dry for a 3-5 day period. This means that, in suitable conditions, subsequent peaks in water levels that don’t reach the previous high can still produce mosquitoes.
These mosquitoes are extraordinarily successful. When field techs are looking for them it’s not uncommon to find them at densities of more than 1000/250ml of water. That can translate to roughly 80 million mosquitoes per hectare of water. Believe us, that’s a lot!! In our experience, if you aren’t finding 100+ larvae per dip then you aren’t in the right place!


MBL staff believe strongly in giving back to the communities not only in which we live, but in the communities that support us through our working relationships. We also believe in giving back to the land. As part of our Environmental Sustainability practices MBL works with communities on stewardship initiatives. MBL adopted a section of the Vedder River in 2014. (For more information on this program please visit “City of Chilliwack Adopt-a-River program”)

Watch for our crews throughout the season hosting cleanup events along this section of the river. Typically cleanups take place as part of larger initiatives held throughout the valley. However, on occasion we conduct cleanups as pseudo staff retreats. During the course of our work in the summer we are often working along this section of the river conducting abatement activities. If you see us down there come by – we’re always happy to chat and to talk about mosquitoes (and now our happy little section of clean river).

The Mosquito Crew