Flooding from Helene could cause a mosquito outbreak

By Jaymie Baxley

After Hurricane Florence caused widespread flooding across North Carolina in September 2018, several waterlogged communities were plagued by swarms of nickel-size mosquitoes. 

In addition to being bigger than ordinary mosquitoes, these biting bugs were noticeably more aggressive. They hatched en masse from eggs that had lain dormant in the soil — some for as long as two years — before being roused by the deluge.

The outbreak made national news, but floodwater mosquitoes are a common nuisance in low-lying areas throughout the state. Earlier this summer, Tropical Storm Debby triggered an emergence of the insects in Brunswick and New Hanover counties.

Hurricane Helene could cause another hatchfest. Current forecasts from the National Weather Service indicate that much of central and western North Carolina is at risk of “excessive rainfall leading to rapid-onset flooding” on Friday, creating ideal conditions for floodwater mosquitoes.

Few know this particular breed of bloodsucker better than Michael Reiskind. A swamp-loving native of flood-prone Florida, Reiskind wrote his master’s thesis on the spread of dengue, a mosquito-borne virus, in Peru. He is now an associate professor of entomology at N.C. State University with credits on several academic articles about the ecology and biology of mosquitoes.

Floodwater mosquitoes are generally “not a major threat to human health,” Reiskind says, but their “massive numbers” can make going outside a painful experience, especially for people in rural areas.

Reiskind recently answered some of our burning — and itching — questions about the pests. His comments have been edited for length and clarity. 

NC HEALTH NEWS: What are floodwater mosquitoes? How do they differ from other types of mosquitoes?

MICHAEL REISKIND: There are certain things that characterize floodwater mosquitoes, and one of them is rapid development.

Most of the mosquitoes that we consider floodwater mosquitoes have an egg stage that is resistant to drying out and dying. Their eggs can sit in the environment, sometimes for years, between flooding events. 

That creates a bank of eggs waiting to be used, which can lead to us having massive numbers of mosquitoes in a very short period of time after a flooding event.

NCHN: Can floodwater mosquitoes transmit diseases?

REISKIND: One of the positive sides, if you will, of floodwater mosquitoes is we generally don’t associate them with pathogens of people. 

But while they’re not a major threat to human health directly, they can be a threat to your dog. Some of these mosquito species are known to transmit heartworm disease to dogs, which is quite dangerous and can even be fatal.

Thankfully, it’s a threat that can be controlled. If you go to your vet and you ask for preventative medicine for your dog, you can prevent them from getting dog heartworm.

NCHN: What areas in North Carolina are most prone to floodwater mosquito outbreaks?

REISKIND: The coastal plain — where there is not a lot of topographic relief so water kind of stays where it lands — is generally where we see the highest risk of huge emergences of mosquitoes. 

But really anywhere in North Carolina where you can get a lot of rain, that’s going to stimulate some increase in mosquito populations. The flatlands east of Raleigh, for example, can be really bad after a major rain event. And it doesn’t have to be a hurricane — any major rain event can do the same thing to an area.

NCHN: How long does it take the mosquitoes to become active after a flooding event? 

REISKIND: Some of these floodwater species have very quick development. 

If it’s warm outside, they can develop in as little as four to five days after a major rainfall event or hurricane. Then they’ll probably continue to develop for as long as the floodwaters are there, which can be several weeks during hurricane season.

If we’re lucky, a cold front comes through — that can really be the best mosquito control there is. It essentially freezes them out. 

That being said, cool weather always seems further and further away these days.

NCHN: Are floodwater mosquitoes more aggressive or more likely to bite than other mosquitoes?

REISKIND: We can’t really say with confidence that floodwater mosquitoes are more aggressive than other species. It just hasn’t been studied. 

However, if you read the historical descriptions of the species that are floodwater mosquitoes, they are definitely aggressive. They have a pretty short adult lifespan, and they’ve got to get blood to produce the next batch of eggs. 

Almost all mosquitoes suck blood, and they all have to figure out a way to do it. Do they sneak up and feed on your ankle when you’re not looking? Or do they come straight at you and risk the swatter to get it done quickly? 

NCHN: What steps can people take to prevent or reduce the risk of floodwater mosquito bites after a flooding event?

REISKIND: It’s fairly unpredictable where we’re going to have a hurricane hit, so there’s really not a ton of easy answers when it comes to preventing an emergence of floodwater mosquitoes.

After the fact, it’s possible to knock down the adult mosquitoes and reduce the population through the aerial spraying of insecticides. Consumer-level repellents also work on floodwater mosquitoes, although I will say I have not tested that in a situation where a million mosquitoes are out swarming. 

Personal protection is another way to go. One example of this is wearing heavy clothing, although the idea of being outside with heavy clothing in a hot, humid environment after a hurricane is probably not much fun. 

There are also backyard mosquito-control solutions that are available through private industry. These usually work by spraying an insecticide that is stable in the environment that sticks to vegetation for about 21 days.

But heavy rainfall can essentially wash the insecticide away, making it not as effective, post-hurricane, as directly attacking those flying mosquito populations in a large area with an aerial spray from a large aircraft.

NCHN: What role does climate change play in the frequency or intensity of floodwater mosquito outbreaks?

REISKIND: One thing that is becoming fairly clear with climate change is that we are seeing more intense storms.

It’s not necessarily just hurricanes either. We’re seeing other weather systems that are also dumping large, hurricane-like amounts of rain at times.

I’m not a climate scientist, and I know that there’s still a lot of work being done on precipitation events and building accurate models to predict those in the future under climate change scenarios. But I do think that most people are in agreement that we will see more intense precipitation events going forward, and that will undoubtedly lead to more mosquitoes.

NCHN: What are some of the biggest challenges in controlling floodwater mosquito populations, especially in rural or less accessible areas?

REISKIND: With rural communities, more than anything, it’s just the sheer size of the area that’s involved.

For large areas, we don’t really have ways to predict where we’re going to see a lot of floodwater mosquitoes or very few mosquitoes. The science just isn’t available at this point.

I also think there are some cost-benefit analyses that probably come to bear in rural communities. If you’re talking about a very large, rural area with a low population density, is treating that area worth the financial cost and environmental impact? 

NCHN: Are there any new technologies or methods being developed to better manage or predict floodwater mosquito outbreaks?

REISKIND: In the mosquito world, we are developing genetic methods for controlling mosquitoes. 

However, floodwater mosquitoes are very difficult to work with by the nature of their biology. We do not, in general, have the ability to study them in the lab — and that makes doing something like a genetically modified mosquito almost impossible with these species. 

There are occasionally new chemical approaches that come out. But to be honest, these approaches usually are not revolutionary.

My guess is that floodwater mosquitoes will continue to be a problem in the foreseeable future. I don’t suspect there will be a major technological innovation that’ll be easy to apply in a post-flood setting. 

At the same time, I’m quite impressed with the work being done by some of my colleagues that study geography and landscapes. There is maybe some promise there for highly tailored control approaches, where you might have a drone using detailed maps and statistical models to apply larvicide in areas that could be hotspots for mosquito production. 

But I would guess that we need a good amount of research on how to build those models and deploy something like that. It’s a solution that, if we started working on in earnest today, could still take more than a decade to implement.