The South and Gulf Coast regions of the United States have historically been centers for heavy mosquito activity. This history takes on added significance as exotic mosquito species become established in these regions and the potential for mosquito-borne disease increases across the United States.
To help you learn more about mosquito issues faced by residents in the South and Gulf Coast regions, we spoke with mosquito expert Dr. Mike Merchant, AgriLife Extension urban entomologist at Texas A&M University in Dallas, Texas. Dr. Merchant's areas of expertise include mosquitoes, their impact on human health, and integrated mosquito control for both pest management professionals and homeowners in urban environments.
Dr. Merchant spoke with us in November 2017, approximately three months after Hurricane Harvey hit the region. The following interview is one of six regional mosquito expert reports available on the Amdro.com website.
Amdro: what are the leading mosquito concerns for texas and the greater south and gulf coast regions, and how did hurricane harvey impact those concerns?
Merchant: Our No. 1 concern in the South is West Nile virus, which is carried by the Southern house mosquito. This is an issue throughout the South. It's been especially bad in Texas the last few years, but it's a pretty widespread problem throughout the United States.
We have two basic kinds of mosquitoes in Texas that affect humans. The first kind are the mosquitoes that breed in standing water and containers. These container breeders include the Southern house mosquito [a species belonging to the Culex genus of mosquitoes] and also the Asian tiger and yellow fever mosquitoes [two species belonging to the Aedes genus], and they are the main vectors for human disease in Texas.
The Culex, the Southern house mosquito, is a vector for West Nile virus, and the Aedes mosquitoes, the Asian tiger mosquito and yellow fever mosquito, are both vectors for diseases that are not commonly [seen in this region]. Those include the Zika virus, dengue fever and chikungunya.
The Aedes mosquitoes are container breeders that live in urban settings, but right now they are primarily nuisance mosquitoes. They are the mosquitoes that bite during the day. The scary thing about these two is, if we ever were to get the diseases so the pathogens were actually being circulated in the human population here, they would be the ones already in place and ready to help spread those diseases. So, we don't take those two lightly. The worst season for those mosquitoes is in a hot, dry summer. It's kind of counterintuitive. You would think that when it's raining you would have bad mosquito problems, but these mosquitoes actually do best when there's not a lot of rain.
The second type of mosquitoes are called floodwater mosquitoes. This year, we had unusual conditions in much of Texas because of Hurricane Harvey. Harvey flooded a lot of areas that are not commonly flooded, and that type of weather brings out this second type of mosquitoes. Floodwater mosquitoes are generally larger. They're very good fliers, so they can fly long distances — five to 10 miles from their breeding site — and they're usually pretty painful, annoying biters. They come out during the day also.
With Harvey this year, we had massive infestations of these floodwater-type mosquitoes, especially along the Gulf Coast areas. We actually had to initiate a massive area-wide spraying for those mosquitoes, not because of disease, but simply because of the nuisance. There were so many of them, and people who were trying to get out and repair their homes and get back to reestablish the flooded areas were just being inundated with these mosquitoes, so we had a very large aerial spraying campaign for those.
We worry more about the container-breeding mosquitoes in urban areas for the disease perspective, but occasionally we do have these floodwater-type events. The floodwater mosquitoes are also the mosquitoes that are out first in the season. When you get the first heavy spring rains, those are nuisance floodwater mosquitoes, not so much the disease vectors.
Amdro: to date, the only local u.S. Transmissions of zika virus outside of Florida occurred in Texas. What is the current level of concern regarding zika in the region?
Merchant: Well, a lot of people felt like this year was going to be the make-or-break year — either Zika virus was going to be a thing or it wasn't — and Zika really has not appeared in any significant level this year at all in the United States. Last year in Texas, there was only one town that had Zika that was apparently locally transmitted. That was Brownsville, which is the very southern tip of Texas right next to Mexico. So, we are not highly concerned right now with Zika. There's still concern, especially for travelers to Zika-prone areas in South and Central America. But, as far as local transmission, it just didn't appear this year. So, probably the chances in the future of it occurring are lower than what we thought last year.
Amdro: what are the most important steps people can take to reduce mosquitoes on their property?
Merchant: Most people don't have any clue that they have mosquitoes breeding in their backyard. Some of these breeding sites can be very subtle. You're not going to go out in your backyard and see big puddles of water with mosquitoes breeding in them. Typically, it's in places that you don't expect, like the water dish under your flower pot on the patio, where mosquitoes are breeding in that little gap underneath the flower pot.
It can be a gardening container that's sitting outside that's collected water underneath a bush. It can be a French drain that is not draining properly in the backyard. It can even be a fence post that has lost the cap on top that's catching water, and mosquitoes will go into that and lay their eggs. It can be a lot of places that people don't necessarily think they have in their own backyard. So, people should understand where mosquitoes do breed. They breed in any kind of standing water, and any artificial container or structure outside is going to be capable of being a breeding site.
Everyone needs to learn to look at their backyard with a critical eye. Are there gutters that need to be cleaned out that might be holding water at the eaves of your house? Are there low spots in the yard that aren't draining properly? Anything like that can be mosquito breeding sites. So, No. 1 is don't be your own worst enemy. Make sure that in your own yard, you're not breeding mosquitoes.
Container gardens with standing water become mosquito breeding grounds.
AMDRO: WHAT SPECIFIC CONTROL MEASURES DO YOU RECOMMEND PEOPLE TAKE?
Merchant: There are several things that are fairly easy to do in your own backyard to be your own mosquito evictor. One of the things, especially during the peak of the summer when West Nile virus is at greatest risk, is spraying the doorways around your house with an insecticide to kill any mosquitoes that might be resting on the sides of your house, especially if you have a shaded entry way. Mosquitoes like shady areas during the day to hide, so that's actually one of the most powerful treatments you can do because it's going to prevent mosquitoes from possibly coming indoors and biting.
It's almost impossible to figure out where people that get sick with West Nile virus actually got bit. I have a personal suspicion that some of these bites occur indoors, just through mosquitoes that have been let in as we go in and out of the house throughout the summer. So that kind of treatment can be helpful.
Also, when mosquitoes are not actively searching for something to bite – a bird or a person or something – they like to hang out in vegetation, especially dense vegetation where there's shade and it's a little cooler. Those areas can be treated with insecticides designed for mosquito control. You can also use garden insecticides that include mosquitoes on the label to treat non-flowering plants and shrubs and grasses in the backyard. That can make a difference in your yard if they are sprayed thoroughly in the areas where the mosquitoes are hanging out. It will make a difference in the number of mosquitoes that you encounter when you go out on the back porch.
Treating plants in shady areas help reduce mosquito encounters.
Amdro: the centers for disease control and prevention (cdc) recommend treating the larval stage of mosquitoes in addition to treating adults. How can larvicides help?
Merchant: If you can find where mosquitoes are breeding in your backyard, most of the time you're going to be able to eliminate that breeding site by dumping it or draining it or filling it with gravel or sand or whatever. When you have a water-bearing site that you can't get rid of, like a creek or stream or a deep hole that can't easily be filled, you can apply some of these larvicide products in areas that are breeding mosquitoes.
The average home owner needs to make sure there is nothing that is holding water in their backyard. Eliminate the breeding sites. But, if you do have stagnant water somewhere that you can't get rid of, then larvicides are pretty effective.
Amdro: you specialize in urban entomology. What are the special challenges of urban environments versus rural areas?
Merchant: If there's a house or any kind of human habitation, it will have one of these three mosquitoes. It will have the Culex house mosquito or the Aedes mosquitoes, the yellow fever or Asian tiger mosquitoes, will be present. We've been doing some surveying recently and some of our surveys have been done out in the rural counties. We're finding these mosquitoes in both rural settings and urban settings. Surprisingly, some of these mosquitoes, especially the Aedes mosquitoes, love city-type environments because there's so much trash and so many places to breed in, but you can certainly find them out on the farm, too. They are not exclusive to the city.
As far as special things for urban dwellers, it's all kind of the same. We carry our baggage with us, whether we're in the city or the country. Mosquitoes will follow us. The Aedes mosquitoes have lived with people for so long, they are very well adapted to living around houses. That's why we call them container breeders — they love artificial containers that we provide.
Amdro: what more should people know in preparation for the next big mosquito season?
Merchant: There's a lot of products out there that don't work very well. Sound devices certainly are completely ineffective at repelling mosquitoes. Some of the traps that do collect mosquitoes are difficult sometimes to get to work in a backyard. They may not be powerful enough to draw mosquitoes in from a large enough area and you may need several of those vacuuming-type devices in the backyard to see any difference. Mosquito zappers, the bug zapper machines, do not help at all with mosquitoes. A lot of people buy them for that purpose, but they are totally ineffective. In fact, they probably kill more beneficial insects than they do bad insects, so we don't recommend those.
Citronella plants just do not put out enough citronella oils to be very repellent to insects. Don't count on planting citronella plants around your backyard to repel mosquitoes; they're not going to make a difference. And, unfortunately, bats and purple martins don't eat enough mosquitoes to keep mosquitoes out of your backyard. It's great to have a bat house or a swallow or purple martin house in your backyard — I don't want to discourage anybody from doing that — but if you're doing it for the sole purpose of getting rid of mosquitoes, you're going to be disappointed.
Arming yourself with timely, expert advice is the first step in protecting your family against established and emerging pest threats, including mosquitoes. The people behind the Amdro® brand are committed to providing you with authoritative information regarding mosquitoes, mosquito-borne diseases and effective mosquito control. You can count on Amdro® for the knowledge you need, when you need it, and for premium control products to help you kill mosquitoes and protect against the disruptions and threats they present.
This interview was edited for length and clarity.
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