Effect Measure

Mosquito spraying for West Nile Virus

In a post yesterday we talked about West Nile Virus. It causes a mosquito borne disease and most people will have mild or even asymptomatic infections. But you don’t want to the be the exception for this one. So what to do?

Here’s the typical response in many urban environments:

Yuba-Sutter’s annual battle against mosquitoes and West Nile virus will hit the streets tonight as spraying begins in residential areas of Robbins and Meridian.

The weekly spraying will begin tonight in Olivehurst, Live Oak, Linda, Plumas Lake, Marysville, Sutter, Tierra Buena and Wheatland, said Ron McBride, manager of the Sutter-Yuba Mosquito & Vector Control District.

Yuba City is not on the list because its mosquito count is not high enough, said McBride.

Spraying will begin at dusk – about 8:45 p.m. – and last about two hours. Residents are advised to avoid or minimize their exposure to the pesticide by staying indoors and closing windows. Residents who use swamp coolers instead of air conditioners should turn them off to avoid drawing spray into the house, said McBride.

As usual, the insecticide will be the chrysanthemum-based Pyrethrin.

Using truck-mounted foggers, workers will spray along every other street, letting the wind spread the insecticide to the other streets, said McBride.

California has had just one known human victim so far this year, a Kern County man who had to be hospitalized, he said.

Yuba-Sutter’s first West Nile victim of the year – a crow – was found early this month at Yuba College, said McBride. (Appeal-Democrat)

This spraying technicaue is called adulticiding. Biologically active chemicals are put in high volume into the human environment to kill adult mosquitoes. You are exposed along with the mosquitoes. With any luck, only the mosquitoes will be harmed. Like the virus itself the chemicals may be safe for most people. Risk – benefit trade-off, right? If we apply the same criteria for spraying as we would use for a vaccine against WNV, we would require it to be both safe and effective. But spraying has not been shown to be either, although it is assumed to be both. A vaccine could be voluntarily administered. Exposure to spray is not voluntary. It doesn’t matter what your desires or your risk factors are.

First, safety. Notice care is taken by public officials in the article to identify the pesticide as “the chrysanthemum-based Pyrethrin.” Pyrethrin does come from chrysanthemums, but most pyrethrins used as adulticides are synthetic versions like Resmethrin or Permethrin. They are chemicals, with no relationship to any botanical. Not that it matters. Botulism toxin is “natural” but is one of the deadliest toxins we know. Moreover, when permethrins are used for adulticides they are invariably used in combination with a knock-down agent, piperonyl butoxide (PBO). PBO inhibits an enzyme in the mosquito’s P450 system that detoxifies permethrin. This additive boosts the permethrin toxicity to the insect. But humans also have P450 systems. It is not known if exposure might, say, alter the drug metabolism of older subjects in ways that would have an adverse effect. Other possible side effects, like effects on the immune system, of children or asthmatics have yet to be thoroughly investigated in humans. Remember, these are biologically active compounds. If they weren’t they wouldn’t be useful. Probably these chemicals are relatively safe and non-toxic to most people. But we cause promiscuous exposure without a thorough check for safety.

For the sake of argument, though, let’s say the spray is perfectly safe or nearly so. It then might be justified if we knew adulticiding were effective in interrupting the transmission of disease from birds to humans. Unfortunately this is the biggest gap. There is no evidence that adulticiding has any discernible effect on interrupting the transmission of disease. This may seem counterintuitive. because if mosquitoes are the vector, killing mosquitoes would seem the obvious intervention. Here are some reasons why adulticiding in an urban area is problematic.

  • bridge vector mosquitoes (those that bite both birds and humans) prefer birds at rest. These are uncommon in the fronts of buildings and on the street, where truck-based spray is applied.
  • birds often roost above the level of spray.
  • the lateral spread of spray is restricted by buildings on the street, thus blocking backyard roosting areas.
  • most products used in sprays have relatively short duration of effectiveness.

The buildings that block the backyards also increase the likelihood of human and pet exposure. Mosquitoes breed best on the same warm summer nights that lead to open windows and ventilating fans. Keeping windows closed for the evening doesn’t seem reasonable and would depend on residents knowing the spray schedule as well. A CDC report, “Epidemic/Epizootic West Nile Virus in the United States: Revised Guidelines for Surveillance, Prevention, and Control (April, 2001)” essentially made the same points: “ground applications are prone to skips and patchy coverage in areas where road coverage is not adequate or in which the habitat contains significant barriers to spray dispersal and penetration.” Effectiveness of urban spraying will certainly be less, perhaps substantially so, than laboratory based benchmarks.

Spraying doesn’t kill all mosquitoes, either, but we don’t know the proportion that have to be affected before disease transmission is interrupted. Whatever that proportion is is quickly returned to the environment’s carrying capacity because mosquitoes reproduce very rapidly. Some of their predators, however, don’t breed as quickly and to the extent they are also affected by the spray, adulticiding may actually worsen the situation. Nor do we know that infected mosquitoes are equally susceptible. It is possible that they will reproduce more successfully than sensitive ones or be less sensitive to the spray. There are many unknowns and experience in areas where adulticiding is used intensively and continuously don’t seem to show reduced incidence of WNV infections. Whatever benefits there may be are certainly very short term, requiring repeated and frequent reapplication.

Where does that leave us? The current best advice for avoiding WNV is don’t get bitten by mosquitoes: wear long pants or long sleeves between dusk and dawn when you are out and about and use a mosquito repellent like DEET. Mosquito control in the spring with larvaciding is thought to be a good investment. It involves local treatments in speciified breeding grounds. Deny breeding areas by emptying standing water from cans, flowerpots, garbage cans and clogged gutters. Not very reassuring you say? Not as comforting as spraying mosquitoes?

Too bad. Get used to one more risk of modern life. As global warming induces climate changes and weather extremes, we will have more mosquito-borne diseases to face and in more places now unaccustomed to them.

Have a nice summer.


  1. #1 Marissa
    June 29, 2006

    Good post, Revere. We certainly need more evidence-base for spray control regarding WNV. What also concerns me about WNV is the large number of significant mutations that happened in 1999 linked with virulence and the neurological problems we see today. What next?

  2. #2 CCP
    June 29, 2006

    Just a heads-up that excellent, referenced information (though now about 1 year old) on all aspects of WNV is available at:


  3. #3 tan06
    June 29, 2006

    There is an invention, I don’t know the name, that white light weight matter usually pressed together for package of TV’s etc.; little balls (about 3 millimeters in diameter) that, covering the surface of water like snow in water containers etc. are preventing mosquitos from reproducing themselves. It was invented for tropical countries several years ago.
    It is cheap, easy to use and multi purpose.
    In ponds and rivers fishes could be used to eat the larvae.
    When I put fishes in my own pond they ate most larvae in about 1 hour. And kept the water clean from then.

  4. #4 Trina Bashore
    June 29, 2006

    If you are looking for a good illustration of something like this for kids look for; Charlotte Pomerantz’s “The day they Parachuted Cats on Borneo” ISBN 0201091666

    It covers how people in Borneo sprayed DDT in their huts to kill of malaria causing mosquitoes. (sort of like we did here in the states in the late 40’s early 50’s)

    Well the mosquitoes and some wasps die. Caterpillars, geckoes and roaches didn’t die but now harbour DDT in them.

    The geckoes eat caterpillars (which the wasps laided eggs in) and roaches, and slow down because the DDT makes the geckoes ill.

    The cats eat many geckoes. The cats die beacuse of the DDT in the geckoes, rats come. So does typhus and plague.

    The roofs fall in on all the thatched huts because the geckoes, predators of the caterpillars are not in the system.

    Now without cats the rats flourish so what do we do? Parachute in cats.

    Makes lots of sense. My daughters (4,6) saw how silly it was to think you know exactly what was going to happen when you try to solve an intricate ecological problem without awareness of possible repercussions.

    OK, they really didn’t understand all of that. I like to think something clicked, as they stopped asking me to spray the bees and wasps in our yard.

    Oh yes, the book is also very 70’s. The drawings are a hoot.

  5. #5 KR
    June 29, 2006

    EVERY SOLUTION HAS A PROBLEM – you can quote me on that!!!

  6. #6 cougar
    June 29, 2006

    Since I propagate water plants here on my farm, I have 20+ shallow tanks in which I grow my bogs and lilies. Into each new tank I throw 6 or 7 mosquito fish (Gambusia) which not only eat every mosquito larva but also breed like crazy ensuring an adequate supply of fish for future tanks. And talk about hardy: most fish would die immediately if more than 20% untreated city tap water were added to their environment, but not Gambusia. I’ve added over 75% tap water to their tanks at one time and still they survive. Don’t know about up north, but here in Texas this is the solution many pond owners use to control mosquito breeding in their backyard ponds and fountains.

  7. #7 Confounded
    June 29, 2006

    Tan06 and others, Does anyone know what kind of fish (or insect, etc) are best to introduce into a pond environment in Northeastern USA for mosquito control. A relative suggested only half in jest blowing up the pond that sits on another relative’s land b/c it is a mosquito attractor. We plan to use the pond as a water source in a pandemic (with appropriate purification, of course). Any suggestions or resources? Thanks.

  8. #8 caia
    June 29, 2006

    Very interesting. It seems that spraying in response to West Nile may be similar in a way to the Bush administration stockpiling Tamiflu: it may not work, we’d need a lot more than we think, and we don’t have ways to get it where it would need to go… but people want government to be responsive, to not only do something but be seen to be doing something.

    As for avoiding mosquito bites, my new favorite solution comes in the form of a garment, basically a mesh hoodie without a face. Slap some gloves on, and you’re mosquito-proof from the waist up. (You can also get pants and socks in this material.) After getting about eight mosquito bites in ten minutes in the garden (not at dusk or dawn, either), I was so relieved to have it. And my last long walk was so torturous, despite wearing bug repellant, that I’m considering wearing the mesh hoodie on the dog trails as well. I may get some weird looks, but it seems worth it.

    Trina – I recently saw a BBC News report about the controversy over spraying with DDT in Africa to combat malaria. After that awful walk, I remember thinking, a day of this, without netting or the ability to keep mosquitoes outside, and I’d be willing to spray virtually anything on my walls if it would stop the mosquitoes – and that’s without the immediate threat of a deadly disease that kills millions of children under five every year. I’m not saying it’s the right or wrong choice, but… other people’s burdens are always easy to bear. It’s not North American kids dying by the millions. Can you imagine what we’d do if they were? Look at what we’re doing to stop a disease that’s killed 800 people.

  9. #9 revere
    June 29, 2006

    tan06: The mosquitoes that are bridge vectors for WNV in the US east coast breed in puddles but gambusiae (the little fish) also eat larvae so they are a recognized control measure.

  10. #10 Peter
    June 29, 2006

    Agreed that spraying to kill adults is probably useless, but what are the feelings about the use of larvacides to reduce the populations?

    Public Health in our city are paying students to put methoprene and Vectolex WSP (Bacillus sphaericus) into catch basins. The larvicide Bacillus thuringiensis var. israeliensis will be applied to standing water known to contain mosquito larvae.

    At least we’re only going to kill the fish.

  11. #11 Brian
    June 29, 2006

    Isn’t permethrin pretty badly toxic (seizures, death) to cats? I hope that’s not what they’re spraying!

  12. #12 revere
    June 29, 2006

    Peter: Larvaciding is usually done in ths spring before the adults come out and I think is generally thought of as useful. I have mixed feelings about BT in the catchbasis but my reasons are probably not worth discussing here.

    Brian: Yes, concentrated permethrin (used for fleas on dogs I think) is a neurotoxin for cats. Not sure what the harm is from the much, much smaller amount in an ultralovolume spray application for mosquitoes, though. I think most of them use resmethrin now.

  13. #13 tardigrade
    June 29, 2006

    It is too bad that the larvacide was not used first in still water bodies, and a public education push to dump standing water around homes and such. But, I can tell you that even larvacide has its limitations – and mosquitoes produce several generations in a season, and…. mosquitoe’s generations – hence their numbers – can develop faster in polluted waters (like from urban runoff).

    I have been working with a friend on this issue and found that after a full course of insecticide (and other things that go into water bodies from urban areas) the standing water kills mosquito predators with the mosquitoes …. However, it does not just kill the mosquito fish, dragonflies and damselflies, but robberflies (Asilidae) which do well in drier environments (urban areas – ponds and lakes and streams are where Odonata must rear young and for larval development). The robberflies develop in the urban areas in soils and such, and they are just as susceptible to insecticides as mosquitoes…. So, spraying the buildings, whether above or below human heigth, will decrease the numbers of these fine predators (they catch the mosquitoes in flight) and while the next generation of mosquitoes emerge (the pyrethrin degrades – sometimes just adsorbed – in soils with water, in the water column taken up in the humic acids and bottom materials, and in sunlight) the numbers of mosquitoes increase…. So, trying to find which mosquitoes are better at spreading WNV becomes a mute point.

    After a kill, the second generation eggs were viable in less than a week – but the insecticide was no longer bioavailable.

    Another thing…. the mosquito that bites you just may have been lying in wait in your closet or elsewhere in your house.

    Deet is best. Vanilla does nothing. Wear long sleeve shirts that are loose and loose pants, too. Don’t eat bananas. And don’t wear perfumed anything.

  14. #14 Confounded
    June 29, 2006

    Tardigrade, Er…why no bananas?

  15. #15 Confounded
    June 29, 2006

    Revere, Are you saying that WNV mosquitos, if you will, breed in puddles but *not* small ponds and so my relative’s pond is not a concern? And if so, is that the case even if the pond has some still water with overgrowth of cattails, etc.?

  16. #16 tan06
    June 29, 2006

    Confounded: I use fishes that are resembling goldfishes, but they are quicker and can grow larger: Leuciscus Idus; maybe the best translation of ‘goudwinde’ is goldwind.
    That’s in the European situation. Does anyone know if these fishes are also imported in the US? They can be held in ponds with minimum depth of 80 centimeters (wintercold, freezing), eat mosquito larvae (a.o.)and can even jump 30 centimeters out of the pond to catch adult mosquito’s (fence your pond, or you’ll find a dead one lying on the shore one morning).
    The gambusiae may be more suitable to you. (Thanks Cougar and Revere, and others).
    Also preventive measures on being bitten by mosquitos in the house: screening of course, white walls and furniture. Beat them on the spot (first blow is the best) with wet cloth, the size of your hand, so they stick on it. Otherwise the insect can fall, get lost and recover. Dispose after destroying the insect very well. At night put a light on, they usually rest close to the head on the wall or furniture or at the edge between shadow and projected light ray on the wall. A glass of beer will attract them (they like the bubble gaz).

  17. #17 tan06
    June 29, 2006

    Confounded: I don’t know whether WNV mosquitos are different from other mosquitos, but I am finding larvae of the common sorts in EVERY plate and can left for one or two weeks in my garden with a tiny bit of water in it!
    Depending on the season (temperature).

  18. #18 tardigrade
    June 29, 2006

    My experience, having to go to places where I am not only a guest but the dinner, find that eating bananas (I do like them in the morning with my coffee) tends to get me bitten by more mosquitoes and by more determined mosquitoes, too.

    Deet should be sprayed on clothing over application directly on skin. I can tell you that even the smallest amount of Deet on my skin and I taste the stuff – BAD! Deet does not totally protect you either. So, for we poor souls who must venture into the world of wetlands, rivers, and bogs we wear loose clothing (sprayed with Deet just before entering), we wear hats with nets and many times garden gloves and rubber boots…. very attractive wear, yes?

    For my at home stuff I encourage dragon flies and damsels. I have trees that have bats. And…. there is this wonderful CHEAP thing you can build that I found on BoingBoing!

    Go to the site and then…. go to the homemade brewing machine…. So cool!


  19. #19 caia
    June 29, 2006

    Tardigrade – thanks for that mosquito trap link! It reminds me of a similar method you can use to catch fruit flies. For that, you make a cone out of paper, with the big end wider than the mouth of a plastic cup (or cut-away soda bottle, as in this method). Place an apple peel or banana peel or a grape in the cup. Place the cone, small end down, into the cup - it should be at least an inch from the bottom. Tape the cone to the cup where they meet to create a seal. It works on the same principle as this seems to – once the fruit flies fly down into the trap, they try to fly back up, and can’t. After that you can release them outside or kill them by putting the cup in the freezer.

    I think I shall try this mosquito trap, even though it doesn’t say how to kill them. Presumably leave them there long enough and they’ll die, I guess.

    Also… I heard recently about a new Deet-alternative bug spray, which is less toxic and non-smelly, but equally effective. Does anyone know the name of this?

  20. #20 tardigrade
    June 30, 2006

    The mozzies (and yellow jackets if you use old cat or dog food in some water at the bottom) tend to succumb to the heat and the humidity generated by the traps standing in hot corners…. or they die in the liquid at the bottom. The standard flying insect trap – Malaise trap – uses this method but some folks like to add some poison for a quick kill and to kill the captives that might eat the other specimens. I use soapy water with ethanol and check the traps frequently.

    This trap design is especially wonderful because it uses stuff that just about anyone can get….

  21. #21 Trina Bashore
    June 30, 2006


    Caia: I need to clarify.

    The people in Borneo had Malaria. The WHO (World Health Organization)in 1959 decided to fix the malaria problem with US manufactured DDT.
    It was the RAF of Great Britian that dropped the 14,000 fresh cats.

    I think that people were trying to help other people.

    In trying not to demonize the WHOs’ actions, I misspoke and said the people of Borneo sprayed the huts. Thus opening a can of worms that I was not trying to address.

  22. #22 traumatized
    June 30, 2006

    Thanks for the warm memories of working in a drosophila lab. The trap you described is pretty much a standard landmark in a fruitfly lab. Unfortunately, they don’t catch that many free flying fruit flies…but great for drowning large number of anaesthatized flies if you put a bit of EtOH in the bottom.

    BTW–freezing will kill anything w/ more than one cell. Fruitflies, mosquitos, etc… That is, if you can successfully trap them first.

  23. #23 caia
    June 30, 2006

    Trina, I didn’t think you were blaming the Borneoans (?), I was just pointing out that sometimes such disruptive actions are done under circumstances that seem to warrant it. To me, using DDT to try to stop malaria is understandable partly because the human cost of malaria is so great. (As opposed to, say, introducing cane toads to Australia to deal with cane beetles.)

    Traumatized, it makes sense that the trap doesn’t make a big dent in a lab that intentionally has hundreds or thousands of fruit flies, but it seemed to work decently once when we unintentionally had a few in the kitchen. (And my only fruit fly lab memory is of a friend who inadvertently got silly from working with Fly Nap. *g*)

  24. #24 SEL
    June 30, 2006

    Some years ago spraying for Mexican fruit fly was done by two or three helicopters flying side by side over the Echo Park and parts of Silver Lake area in Los Angeles.
    Since then, there have been no crickets, very few of the small blue/silver butterflies that appear in hot weather, and fewer insect-eating birds. Mosquitoes are not much of a problem in this dry climate, but lately there has been unusual weather, windy, damp and hot, unlike our usual baking heat. Public announcements of spraying and other measures are usually couched in nonthreatening language as you pointed out, poisons connected with a pretty flower. There are ambitious individuals a-plenty to profit from various civic abatements, and these custodians should be watched narrowly. Thanks

  25. #25 Dan R.
    July 1, 2006

    Question to the experts?

    Shouldn’t we be looking to state like Louisiana for population control? As a former resident, I can attest that New Orleans had a hell of a program.

    I don’t think I was ever bit while in the city — wearing shorts and avoiding repellent. OTOH, when venturing to the swamps, I actually resorted to stealing an errant can of insect repellent that someone left on a dock (I did return it) when I realized everything was closed.

    There were 2 cases of Malaria reported in LA in 2004, and none in 2005 (MMWR). This in an area with previous endemic infestation of the same.

  26. #26 revere
    July 1, 2006

    Dan: Malaria is not present in LA so I think all cases are imported. LA is the test case for another reason. They spray like crazy — very intensively — and had more WNV than anyone else a couple of years ago. It didn’t interrupt transmission. I can’t speak to mosquito control in NOLA, so I don’t know how they did it.

  27. #27 Dan R.
    July 2, 2006


    I agree that any Malaria currently found in LA is imported and relatively isolated

    What I was getting at is that Malaria (and Yellow Fever for that matter) once had significant purchase in the state. LA saw the first public Board of Health in the United States expressly for combatting Yellow Fever. Written accounts tell us that for many years, it wasn’t uncommon to hire 2 people for every job, and that significant amounts of cotton was left in the fields. Epidemics of yellow fever in the 1800’s claimed 20% of NOLA residents.

    The question in my mind is how did we get from there (say 1900) to here, where both yellow fever and malaria are essentially unheard of in the state. Obviously, their control campaigns (which started just after the discovery of mosquito’s as vectors in Panama), were effective. The question is are they still effective (they may not be), and is there anything which can be used in campaigns against West Nile, etc?

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