Could a “skeeternado” really happen?

skeeternadoWhile promoting the idea that the world needed a “Mosquito Week” to help raise awareness of mosquito-borne disease, Bill Gates also managed to plant the seed of an idea for Hollywood filmmakers. Forget Sharknado, how about Skeeternado?

While the idea may have been laughed off, there is a possibility that a “skeeternado” may actually occur. Perhaps not a “Hollywood” styled ento-storm event but we know mosquitoes swarm for various reasons and wind dispersal of mosquitoes could well be responsible for triggering outbreaks of mosquito-borne disease.

More like midges than mosquitoes

Is this a 1000 ft high swarm of mosquitoes in Leziria Grande at Vila Franca de Xira in Portugal? (Photo Ana Filipa Scarpa)

A great shot has recently emerged from Portugal of what is purported to be a “mosquito twister”, a swarm of hungry mosquitoes. Unsurprisingly, it created a bit of buzz on news websites! Unfortunately, it was probably more likely to be a swarm of midges than mosquitoes (I would love to know if it turned out to be mosquitoes). It is not unusual to get large swarms of non-biting Chironomid midges around wetlands, particularly in spring and autumn.

I’m often called out to investigate “mosquito problems” that actually turn out to be midge problems. Midges are a common component of Australia’s wetland ecosystem and although they don’t bite, they can be pests and there are even reports of sensitization and allergic reactions in humans exposed to the insects.

In some regions, chironomid midges can cause potentially significant nuisance problems and working groups on midge control have been established. This is particularly the case in new urban developments where wetlands provide habitats for the midges and the brightly lit, pale coloured buildings attract the insects at night.

Midges can often be seen in large numbers around lights and, in severe instances, dead insects can be a problem when they pile up against windows, doorways and on balconies. These problems are a particular concern of property developers trying to sell newly constructed dwellings! Some chironomid species have also been associated with crop damage in newly flooded rice paddies and control is often required.

Swarms of chironomid midges around freshwater wetlands at Sydney Olympic Park

Swarms of chironomid midges around freshwater wetlands at Sydney Olympic Park

The hunger swarms

Mosquitoes are known to swarm though. Hungry swarms. Huge numbers of mosquitoes have been documented in phenomenal numbers in Alaska. Many accounts from field stations, such as Toolik Field Station, have provided “nightmare” scenarios (for the record, I would LOVE to have the chance to get up there and do some mosquito research!).

While perhaps not as extreme as those mosquito populations in Alaska, there have been plenty of instances in coastal regions of Australia when huge numbers of saltmarsh mosquitoes chase me out of the wetlands. It does feel like you’re caught up in a swarm but I’m not aware of female mosquitoes hunting down a bloodmeal in a swarm!

Swarms of mosquitoes engulf a researcher at Toolik Field Station, Alaska (Photo: Alaska Dispatch)

Swarms of mosquitoes engulf a researcher at Toolik Field Station, Alaska (Photo: Alaska Dispatch)

The sexy swarms

It appears that sex is a primary motivator for mosquitoes to swarm. A wonderful account of swarming mosquitoes from 1969 documented male Culex pipiens swarming above vegetation with females darting into the swarm to find a mate. Similar observations have been made of Culex quinquefasciatus. Video analysis revealed that male and virgin female mosquitoes swarmed above vegetation (or some other physical structure that acted as a “swarm marker”) and that individual mosquitoes didn’t seem to have preferred positions within the swarm, but drifted at random until the male mosquitoes detected the sound of the female’s wing beats. Once they hear the female, the males slow their flight speed and alter their flight patterns until a female is found. Similarly, swarming behaviour of the malaria mosquito Anopheles gambiae has been documented and suggests there is even flight-path coordination between swarming male mosquitoes.

The pregnancy swarms

Swarming behaviour of saltmarsh mosquitoes in Florida has also be documented. I love this description from a 1953 paper, “Field Observations on the habits of Aedes taeniorhynchus“, published in Ecology describing the swarming behaviour of gravid female mosquitoes:

“They were flying around one another in a cloud from the ground up to a height of 7-8 m…The flight of the individual mosquito was slow and without sudden changes in direction,
graceful but with the same majestic calm which is seen in a female which has just finished a blood meal, and which in both cases is probably caused by the heavy weight of the abdomen.”

Interestingly, this paper did not offer an explanation as to why the gravid female mosquitoes were swarming in this manner. At first it was thought it was related to egg laying behaviour, perhaps assisting in finding suitable habitats, but when followed, no egg laying was observed. Why were they swarming?

The swimming swarms


Aquatic swarms of saltmarsh mosquito (Aedes vigilax) larvae in the estuarine wetlands near Newcastle

It isn’t just adult mosquitoes that swarm. The saltmarsh mosquito, Aedes vigilax, is famous for the high density aggregations of their immature stages that occur in the estuarine wetlands in coastal Australia. The behaviour is generally only observed in the late development stages just before pupation occurs. There can often be hundreds of thousands of larvae in these aggregations and the dark “swarms” of larvae can even be seen from the air (I haven’t check Google Earth but it wouldn’t surprise me). If you ever see something like this, best to head inland for the next week or so because these mosquitoes are serious nuisance-biting pests and can transmit Ross River virus.

The cyclonic swarms

One of the most interesting developments in insect-borne disease research recently has been the role of long distance wind-blown insect dispersal. Much of the recent research and modelling has been done with biting midges that transmit pathogens of veterinary importance (e.g. Bluetongue virus) and has shown that infected insects may introduce new pathogens into northern Australia from neighbouring regions or may distribute pathogens long distances within mainland Australia.

Similarly, windblown dispersal of infected mosquitoes associated with low pressure systems has been proposed as an explanation for the introduction of Japanese encephalitis virus into northern Australia in 1995 and 1998.

Kite traps” have been used, up to 310m, to trap windblown mosquitoes and to provide circumstantial evidence that aerial dispersal of mosquitoes from PNG to Cape York peninsula (about 200km) is possible. The results of these studies have been used as evidence to support the theory that major weather events could play a role in moving mosquitoes infected with Murray Valley encephalitis virus or Kunjin virus from endemic to non-endemic regions within Australia.

Were the meteorological events associated with the La Nina in 2010-2012 responsible for the 2011 outbreak of Kunjin virus amongst horses in southeast Australia?

I wonder how an increase in extreme tropical weather events in association with a changing climate influence outbreaks of mosquito-borne disease?

Perhaps a real life “skeeternado” is just around the corner…..

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