Mosquitoes on the move: part one

How has global trade helped one of our deadliest foes? (9 minute read)

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On a hot summer night, there is little more annoying than sharing a room with a hungry mosquito. I’ll be lying there, already too hot to sleep, when I hear a high buzz pass my ear. I wave at it, but a few seconds later, it’s back. I wave again, but I know my ineffectual swipes are making no difference. Unless I turn on the light and track it down, it will wait until I’m asleep and then bite me. And I’ll wake in the morning with a maddening itchy bite.

Here in New Zealand, a mosquito bite is nothing more than an annoyance. However, in many parts of the world, a mosquito bite is a deadly threat. Mosquitoes can transmit a number of nasty diseases – most notably malaria, but also dengue fever, West Nile virus, Zika, yellow fever and more. The most prevalent of these is malaria, which infected more than 200 million and killed more than 600,000 people in 2021. But the other diseases are a problem too. Each year, more than 100 million people suffer from dengue fever, with more than 20,000 dying. While other mosquito-borne diseases are rarer, there are still hundreds of thousands who suffer and tens of thousands who die.

In the animal kingdom, humans have no other adversary like the mosquito. Crocodiles, the deadliest wild predator, kill around 1000 people a year. Snakes, the deadliest of the venomous animals, kill around 50,000. Those are big numbers when measured in terms of human tragedy, but small in comparison to the deaths caused by diseases carried by mosquitoes. Estimates vary, depending on which years are included in the calculations, but in some years, a million or more deaths have resulted from malaria alone.

To most of us, one mosquito looks very much like another, but there are actually important differences. The deadliest of the mosquitoes, the ones that carry malaria, belong to a group known as Anopheles. There are hundreds of different species of Anopheles mosquitoes, of which around 70 can transmit malaria. The mosquitoes which transmit malaria in Africa are not the same as the mosquitoes which transmit malaria in Asia or the Pacific Region, and the species in the Americas are different again. There are also Anopheles species capable of transmitting malaria in the Middle East and Europe.

In contrast to the Anopheles mosquito, there are only a small number of Aedes mosquito species capable of transmitting diseases to humans. However, those few species transmit a number of diseases. The most important of these is dengue fever, which is transmitted by several different Aedes species. Before the development of a safe and effective vaccine, yellow fever, transmitted by the yellow fever mosquito, was a widespread and deadly threat. Aedes mosquitoes also transmit Zika virus, and a number of less well-known diseases such as Japanese encephalitis.

Aedes mosquito (image credit: Image by WikiImages from Pixabay)

One crucial difference between Anopheles and Aedes mosquitoes is their biting behaviour. Anopheles mosquitoes most often bite at night, which is why sleeping under a mosquito net, especially one treated with insecticide, is important in areas with malaria. Aedes mosquitoes most often bite in the morning and evening. As a result, we can’t protect ourselves by hiding under nets.

Anopheles and Aedes mosquitoes are the most important disease vectors, but there is one other group which can’t be forgotten. This is the Culex mosquito, the vector of West Nile virus, among others. In New Zealand, one of the most common mosquitoes belongs to this group, the southern house mosquito. Like Anopheles, Culex is a night-biting mosquito, and the type I was thinking of when I wrote the opening paragraph to this article.

In some parts of the world, the southern house mosquito is responsible for spreading important human diseases, but not so in New Zealand. Our native birds, however, haven’t been so lucky. The same mosquito also bites birds, and is responsible for carrying diseases like avian pox and avian malaria to endangered native species such as the black robin. I had never heard of avian pox until I looked into the story of the tūturuatu or shore plover a couple of years back, but it’s a real threat to a number of native birds.

Whether they bite by night or day, and whatever diseases they carry, there’s one important thing that all mosquitoes have in common – they all depend on water to breed. While the adults fly around, the juvenile stages live in bodies of still or slow-moving water. You’ve seen them, I’m sure, the wriggling creatures that appear in buckets of water if you leave them outside. The first time I ever saw them, I was fascinated. How did they get there?

The answer, of course, is that adult mosquitoes fly. Although some species typically travel only a few hundred metres, others can travel several kilometres in search of food and somewhere to lay their eggs. The eggs hatch in water into the familiar wriggling larvae – the equivalent to the caterpillar in a butterfly’s life cycle. A few days later, the larvae turn into pupae, which are the equivalent of the cocoons formed by butterflies. The pupae don’t feed, but unlike butterfly cocoons, the pupae are mobile – they are good swimmers (I’ve linked to a video which has both larvae and pupae swimming, you’ll see the difference). Finally, in as little as a week from when the eggs were first laid, adult mosquitoes emerge from the pupae to fly away and begin the life cycle again.

But here is where the story of mosquitoes gets complicated, and protecting ourselves becomes tricky. And so, to understand the challenge better, I contacted entomologist Mariana Musicante. She told me that the most fascinating thing about mosquitoes is that all the species have different strategies for survival, and that we have to understand these strategies to protect ourselves.

Musicante works for a company called Southern Monitoring Services, which helps protect New Zealand from the mosquitoes which turn up at the New Zealand border. At first glance, mosquitoes might seem to be unlikely travellers. The adults are delicate creatures and the larvae and pupae need to live in water. But travel they do, and every year dangerous exotic mosquitoes like the yellow fever mosquito and the Asian tiger mosquito are detected at our border. Without the work of Musicante and her colleagues, we would have a big problem.

It's no accident that the Asian tiger mosquito and yellow fever mosquito turn up so frequently, Musicante tells me. They are great travellers. Part of the reason is that they and other Aedes mosquitoes have a surprising strategy for laying their eggs. They don’t, as you might expect, lay their eggs in water. Instead, they lay their eggs near water, in a place which is likely to fill with water when it rains. And then the eggs wait. They can survive for months, completely dry. Then, when the water comes, they hatch and continue their life cycle.

Some Aedes mosquitoes lay their eggs on the edges of swamps, lagoons or ditches. But for the yellow fever and Asian tiger mosquitoes, a suitable habitat can be the smallest of containers. Naturally, they live in locations such as tree holes. These days, though, they are far more likely to be found living near human habitation, in all sorts of water containers – ponds, bird baths, roof gutters, the bucket of a digger, a pot plant saucer… if it holds water, these mosquitoes can breed in it. It’s their ability to survive for months as eggs, and then develop when they are submerged, which allows these two species to travel so well. Anything which has held water, and could hold water again, might have eggs on it.

There is one water-carrying item, more than any other, which has contributed to recent mosquito problems – tyres. An old tyre, left lying around, will hold a small amount of water, enough for a yellow fever or Asian tiger mosquito to breed. A mound of old tyres will have dozens of tiny pockets of water, like an apartment complex for mosquitoes. And, although many people might not realise it, there’s a global trade in used tyres, which has moved dangerous mosquitoes around the world.

I’m very familiar with the ways that mosquitoes have been spread around the world, since I spent many years working on biosecurity in New Zealand. However, one thing I learned from Musicante is that old tyres are not just part of the problem with mosquitoes – they are also part of the solution. Old tyres are so great for mosquitoes, Musicante says, that they are used to create mosquito traps.

Although New Zealand has a number of native and introduced mosquitoes, we don’t have either the yellow fever mosquito or the Asian tiger mosquito. Nor do we have the Culex species that is the most important vector of West Nile virus, or any of the malaria-carrying Anopheles mosquitoes. To help keep New Zealand free from these species, there are a series of traps around air and sea ports, which are regularly checked for these dangerous species.

An old tyre, says Musicante, is so appealing to a mosquito that nothing else needs to be done to make it attractive. Just put out the tyre and, after a week or so, drain out the water and see what species of mosquito are present. But there are other traps that she and her colleagues use as well. These have a number of tricks to trap mosquitoes. One trap uses lactic acid, a chemical found in sweat which is attractive to mosquitoes. Another trap, which targets night-biting mosquitoes, has a light and releases tiny amounts of carbon dioxide. It turns out that mosquitoes can detect the carbon dioxide on our exhaled breath, another way that they track us down.

New Zealand has been successful in keeping out the most dangerous species of mosquito but, on the whole, it is far from clear who is the winner in the battle of human versus mosquito. On the one hand, we have developed insecticides which can kill mosquitoes. On the other hand, mosquitoes are now becoming resistant to these insecticides. One of our successes has been reducing the suitable habitat for Anopheles mosquitoes in places like Europe, and getting malaria under control there. One of our failures has been the rapid global expansion of dengue, resulting from the continued spread of the Aedes species which spread it.

But there is another factor we need to consider when we think about mosquitoes – climate change. As a rule, the most dangerous mosquitoes are tropical species, which means a warming climate is cause for real concern. However, it’s no simple matter to understand the effect of climate change on a group of insects like the mosquito. It deserves a full article of its own, and so I will return to that topic in two weeks.

Mosquitoes on the move: part two

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