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Regional Report: Mosquitoes In The Northwest

By Jolene Hansen
female Culex tarsalis mosquito

Conditions in the Northwest region of the United States vary significantly from the Inland Northwest to the Pacific coast. Though the region isn't considered a major U.S. center for mosquito activity, areas of the Northwest have experienced significant outbreaks of mosquito-borne disease.

To help you learn more about mosquitoes and mosquito-borne disease in the Northwest, we spoke with mosquito expert and disease ecologist Dr. Krisztian Magori, assistant professor in the Department of Biology at Eastern Washington University. Dr. Magori leads the EWU Disease Ecology Lab, with the mission of protecting people, livestock and wildlife from the infectious diseases associated with mosquitoes and other vectors. Dr. Magori spoke with us in December 2017. The following interview is one of six regional mosquito reports available on the AMDRO.com website.

AMDRO: WHAT ARE THE LEADING CONCERNS IN THE NORTHWEST REGARDING MOSQUITOES AND MOSQUITO-BORNE DISEASES?

Magori: We're very fortunate in the Northwest when it comes to mosquitoes. They are not a big problem except in certain areas.

The biggest concern I have for the Northwest is West Nile virus. That's the most important mosquito-borne disease in the U.S. in general, and in this area. In the Northwest, we have the mosquito vectors that transmit that virus: Culex tarsalis and Culex pipiens. Culex pipiens are more in urban areas and tarsalis more in rural areas, but they also overlap. Both can transmit West Nile virus, and there are quite a number of them.

In the Northwest, our season is shorter because of the area's climate. We have some scant numbers of West Nile vectors starting in April, but we really don't get into the season before June. Then it extends into September.

So, West Nile virus and the mosquito vectors — those are my biggest concerns.

AMDRO: WHAT DIFFERENCES EXIST BETWEEN THE INLAND NORTHWEST AND THE PACIFIC NORTHWEST IN MOSQUITO ACTIVITY AND RELATED DISEASE?

Magori: West of the Cascades, in the Seattle area, for example, they really don't have a West Nile problem. It's east of the Cascades that we have West Nile. We do not have it as bad as in the Plains, such as North Dakota and South Dakota — that's where we have the highest number of human West Nile virus cases in the country. We don't have those numbers in the region, but West Nile virus is a problem.

In Washington state specifically, there was a study a few years back that looked at areas with most West Nile virus cases. They found it was associated with the Yakima Valley [in south-central Washington] in particular. There's a lot of agriculture going on there, especially apples and other orchards, so there's a lot of irrigation. That irrigation allows mosquitoes to breed, and so that's where they have the most West Nile virus.

West of the Cascades, the weather is very different. It is much more humid. You would think that's good for the mosquitoes, but it's good for other mosquitoes. It's not particularly good for the Culex mosquitoes that transmit West Nile virus. It is better for the pest mosquitoes or swampwater mosquitoes, like Aedes. So, they really don't have West Nile virus on the west side.

Two horses on a field.

Between 30 and 40 percent of U.S. horses infected with West Nile virus die.

AMDRO: THE DRY INLAND NORTHWEST HISTORICALLY HAS THE MOST WEST NILE CASES FOR THE REGION. IDAHO DECLARED A STATE OF EMERGENCY FOR WEST NILE LAST SUMMER. HOW DO THOSE DRY AREAS SUPPORT MOSQUITOES?

Magori: It has to do with the biology of the vector. Areas without any water at all won't have mosquitoes, but agriculture and irrigation create many temporary breeding sites. The mosquitoes that are the vectors for West Nile virus, such as Culex pipiens and Culex tarsalis, also like high nutrients in their breeding sites.

Culex pipiens is an urban mosquito, so you can be in an area where people just water their lawns, for example. This irrigation water accumulates, and there are high groundwater levels. Then you have a temporary breeding site. These are also associated with some sort of nutrient input. For example, livestock release a lot of feces. Swine are notorious for breeding mosquitoes — Culex mosquitoes particularly.

Not having as much rain in these areas is actually good for these vectors. As the summer months go on and it's dry and hot, people water their plants and add to natural breeding sites like ponds. These sites constantly provide nutrients for these mosquitoes. The water evaporates, but not the nitrogen or phosphorus or the feces. It just gets more fermented, and these mosquitoes like that.

Also, West Nile virus is really a bird virus; it's not a human virus in any sense. We get infected by mistake after birds transmit it to mosquitoes. Birds also look for some moisture in the landscape, and they tend to find the same places where mosquitoes are breeding. You have lots of birds congregating around these breeding sites, and that provides the right conditions for the virus to amplify in these populations, both the birds and the mosquitoes. Then it reaches a certain level where people start getting infected. That's when you have problems.

In terms of animals, horses are very susceptible to West Nile virus. This is also a major concern. There is not a human vaccine for West Nile virus, but there is a vaccine for horses. It's not expensive, but not everybody vaccinates their horses. When horses get infected, a large portion of these horses develop lameness. Then you have the horse lying down, and a horse lying down is not going to live long. There are ways to keep horses alive and they can recover, but that's very expensive and requires special conditions. A lot of the horses are very valuable, so in that sense it becomes an economical issue, too.

AMDRO: WASHINGTON STATE OFFICIALS EXPRESS CONCERN ABOUT EXOTIC MOSQUITOES, SUCH AS THOSE ASSOCIATED WITH ZIKA VIRUS. HOW PREVALENT ARE THOSE MOSQUITOES IN THE REGION?

Magori: I work a lot with the Washington State Department of Health. I'm one of their surveillance partners and I work a lot with the Zoonotic Disease Program. Traditionally, I would say that these mosquitoes, Aedes aegypti and Aedes albopictus, were not a concern because these are tropical mosquitoes that are unlikely to survive the winters we have here.

However, Aedes albopictus has been verified in the U.S., in the East and as far north as Chicago, so it is much more flexible in the areas where it can survive. That mosquito can lay eggs that can survive the winter — about six months without any water — and then it can still hatch. So that mosquito, Aedes albopictus, is a bigger concern.

Aedes aegypti is really still this tropical mosquito. We don't really have it in the U.S. except in pockets. We have some in Florida, some at the border of Mexico and New Mexico, in Texas and Arizona, and some isolated pockets in California. Also, there are some in Virginia and along the coast there. But you have to remember that the Northeast of the U.S. is so much more humid than the West Coast, even the Pacific Northwest. Even though it is the same latitude, it is not the same climate at all.

I believe Aedes aegypti is not suitable for our climate — both the harsh winters and the crushing summers. The humidity can be basically zero in summer. These mosquitoes are very sensitive to humidity, and so I don't believe Aedes aegypti would be capable of establishing in Washington state, for example.

Albopictus, that's a different question. I've seen some models predict that, with climate change, it could become established on the coast itself, but not inland. Aedes albopictus is also really sensitive to humidity, though less so than Aedes aegypti. So, I would be surprised, but these insects have given us some unpleasant surprises for a long time.

In terms of disease transmission, Zika and these other viruses, you have to remember that just having the mosquito vector being found in a location is not the same as finding disease transmission. That's very, very different.

View of a Yamika farm

Irrigation in Northwest orchards creates mosquito breeding sites.

AMDRO: THE CENTERS FOR DISEASE CONTROL AND PREVENTION (CDC) RECOMMEND THAT HOMEOWNERS WATCH FOR MOSQUITOES, REMOVE WATER SITES, AND TREAT SITES FOR BOTH MOSQUITO LARVAE AND ADULTS. WHAT ARE YOUR RECOMMENDATIONS FOR NORTHWEST HOMEOWNERS?

Magori: The Washington Department of Health's Zoonotic Disease Program has a surveillance program where they go out and collect mosquitoes to monitor for things that would be concerns. Their surveillance partners, like myself, trap mosquitoes and send them to the Department of Health for testing. There are also some mosquito control districts in different parts of the state, but there is not a lot of local mosquito control in the Northwest.

In terms of recommendations for homeowners, I always recommend source control. That means getting rid of any potential breeding sites in the backyard. If you have any kind of junk or garbage that is accumulating water, those are breeding sites. Get rid of the water that is in them, when that's possible.

If you have a backyard pond, obviously you don't get rid of that. For structures like that, you can use larvicides, which specifically target mosquito larvae. For spraying adult mosquitoes, keep in mind when they are active. The mosquitoes that transmit West Nile, the Culex, are not out in the day when you would normally be spraying. They are active at night.

People above the age of 65 should be particularly aware of mosquitoes and be more concerned. Limit your time spent outdoors at night during the mosquito season, even into late September. Putting on mosquito repellent helps. I don't recommend natural products. They don't tend to be very effective, and if they are effective, it's only for a very short time. Mosquito repellents that contain DEET seem to be very effective, lasting several hours. Those are the ones I use.

AMDRO: WHAT ADDITIONAL MOSQUITO ADVICE DO YOU HAVE FOR NORTHWEST RESIDENTS?

Magori: Climate change is particularly important for mosquitoes and mosquito-borne disease. I want to emphasize that. We already see our climate changing here in the Northwest. We have more drought in the summer; it's hotter. We have more rain in the winter and less snow. We actually have more winter precipitation than before, but the western region has a bigger drought in the summer.

This doesn't hurt the mosquitoes. It helps them, because both the mosquitoes and the virus can replicate faster. Lack of rain in summer is not an impediment to them. Even during the drought, they will find those breeding locations. So, the problems are not going to go away; they're going to stay around. West Nile virus and mosquitoes have somewhat intensified in the region.

One thing I would like to tell homeowners is they need to take West Nile virus seriously. There's a lot of complacency in the area. Because there are not that many mosquitoes here, people think they're not going to be the ones that get West Nile. But we have human cases of West Nile virus here.

Have a heightened sense of awareness, and be aware of your actual area. There may not be a problem in a larger area, but in your neighborhood – or even just on your property – there could be a problem.

Eighty percent of people who get infected with West Nile virus have no symptoms. Someone could be infected with the virus and not be aware. Only 20 percent of the people don't feel well, sort of like they have a cold. Only 1 percent get neurological disease. But I envision it like an accident, a car wreck. You're in the wrong place at the wrong time, and you can get bit by mosquitoes that have West Nile. Just like people are complacent about traffic accidents and don't wear seat belts or they drive too fast, there's the same sort of complacency about West Nile. Don't assume the mosquitoes around you aren't transmitting the disease. Don't think that you can't be the one.

CONCLUSION

Understanding mosquitoes and the risk of mosquito-borne disease in your area is an essential step in protecting your family against established and emerging mosquito threats. The people behind the Amdro® brand are working to bring you timely, expert information about mosquitoes, the diseases they transmit and effective mosquito control. You can count on Amdro® for the knowledge you need, when you need it, and for premium pest control products to help you prevent mosquitoes and protect your family.

This interview was edited for length and clarity.

Amdro is a registered trademark of Central Garden & Pet Company.

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