Effect Measure

I get misty

Here’s a great idea for your home. Install a sprinkler system that sprays pesticide mist twice a day, all summer long to control nuisance mosquitoes. The system uses quarter inch tubing a metal spray nozzles buried in the yard like a lawn sprinkler. A small tank and attached pump operate for 30 seconds at dawn and at dusk. The pesticides are pyrethroids, synthetic versions of a naturally occurring compound derived from chrysanthemums. The natural origin of pyrethrum is always touted as a sign of safety. If it grows in a chrysanthemum, it must be safe. Like botulinum toxin or the mushroom Amanita phalloides (the Death Cap)?

Well, never mind that. More importantly, pyrethroid pesticides used for killing adult mosquitoes don’t just have pyrethroids in them. If they did, they’d work poorly. The also contain a synergist, piperonyl butoxide (PBO). The role of PBO is to interfere with the insect’s ability to detoxify the pyrethrum and it does this by inhibiting a cytochrome P450 enzyme in the mosquito. Humans and many other animals also use the cytochrome system for phase I. detoxification and there is now concern that PBO from pesticide spraying may be potentiating toxicity of other agents in the environment against non-target organisms (i.e., non-mosquitoes). Pyrethroids are also toxic to fish and exquisitely toxic to bees. It would seem to be common sense not to put biologically active compounds continuously into your home environment at high volume designed to kill living things. Or so it would seem to us.

But not to everyone:

The sprinklers spray Ed and Fran Smith’s Burlington yard twice a day, all summer long, but it’s not water jetting from those nozzles. It’s pesticide.

The Smiths paid $2,500 last year for the system, which works on a timer and sprays a mist of mild pesticide in the mornings and evenings, when mosquitoes are most active.

“We love it. We’re in our yard all the time now,” Fran Smith said. “Once in a while you might see a mosquito, but I haven’t been bit since we got it.”

Their boys, ages 13 and 9, now swim in the backyard pool and play on the sand volleyball court without needing bug spray. Ed Smith even sets up his movie projector and a screen, to show films to his children in the yard.

“The kids get their little sleeping bags to lay on, and we watch movies,” Fran Smith said. (Boston Globe)

Fear of mosquito-borne diseases like Eastern Equine Encephalitis or West Nile Fever have become an added incentive for some to install these systems even though they haven’t been shown to interrupt the transmission of these arboviral diseases. Doing so involves delicate psychological gymnastics about competing risks.

Fran Smith said she much prefers to have pyrethroids sprayed along the perimeter of her lawn twice a day to frequently slathering her sons with bug sprays containing substances such as DEET and other potentially noxious chemicals.

It’s a good thing they won’t be exposed to any noxious chemicals by spraying pesticides in the air twice a day, every day all summer, so the kids can roll around in the pesticide laden grass.


  1. #1 tympanachus
    August 27, 2006

    Fran should take a short vacation to Chiapas and pick up a few Shell No Pest Strips.

    Speaking of Amanitas, we just visited Goose Creek in the Colorado Lost Creek Wilderness to forage for shrooms. The “edible” Amanitas, muscaria and pantherina, were plentiful.

  2. #2 Interrobang
    August 27, 2006

    I’m guessing that there haven’t been any really good studies on the long-term effects of pyrethrin exposure? (I’m sorry, I can’t be arsed to check at the moment, but very few chemicals have had in-depth studies done, so it seems a reasonable assumption.)

    I sincerely don’t understand this recent hysteria about mosquito-borne diseases. The one that seems to get the most press around here is West Nile, but all the fatalities I’ve heard of from West Nile have been people over the age of about 70, and I have to wonder about aggravating conditions.

    One minor note of hope: Systems like that would be illegal in certain jurisdictions, due to pesticide ban legislation. Much as I’m for the rights of people to do what they like with their own home, where that freedom extends to polluting the local ecosystem and the bodies of their neighbours and other passers-by (who may not even know the contaminants are there), I draw the line. If I lived in the Smiths’ area, I’d stay off their block.

  3. #3 revere
    August 27, 2006

    Interro: You are correct. Very few studies. And it is the additive, piperonyol butoxide which is even more troublesome.

    Regarding WNV, however, I consider it is nasty and dangerous disease, like EEE. My issue is that there is no good evidence that adulticiding interrupts the transmission of this disease to humans or other animals and there is an argument that can be made it might make it worse. It certainly has ecological downsides. It is a knee jerk response to a public health problem and is more public health theater, like our homeland security antics.

  4. #4 Julie Stahlhut
    August 27, 2006

    A quick literature search reveals reports of the evolution of pyrethroid resistance in populations of mosquitoes, corn earworm, head lice, citrus thrips, flour beetles, cattle ticks, plant bugs, and house flies. These folks are doing all of the following:

    * Exposing themselves and their kids to a stew of chemicals that are advertised as “low toxicity”, but have unknown long-term effects on humans.

    * Poisoning harmless or human-beneficial wildlife, such as birds and insect pollinators.

    * Selecting for resistant mosquitoes and Zarquon knows what else. (Pyrethroid resistance alleles have been found at multiple loci in house flies; piperonyl butoxide is not necessarily a panacea against pesticide resistance.)

    * Obtaining minimal, if any, protection against insect-vectored disease.

    Ugh. I’ve seen people in a state-park campsite spray a full can of pesticide over the grass at their site, as well as all over the tablecloth they were setting for dinner. Their kids then bounded out of their camper and began playing in the freshly sprayed grass. Look, Ma, no DEET. Of course, once they walked off the campsite to go to the restrooms, whatever “protection” the spray afforded was gone. And the saddest part was that this was on a cool night, when everyone could have comfortably covered up with jeans, socks, and sweatshirts.

  5. #5 mary in hawaii
    August 27, 2006

    When I moved to Hawaii (windward side, lots of rain…and mosquitoes) I was at first really plagued by them, but after a time I stopped reacting to the bites (look ma, no bumps!) I have no data, but it would seem my body decided to tolerate the irritant in the saliva. Also,when they are biting, I found that one particularly effective remedy was tea tree oil (available for about $5 in Wal mart and most large drug stores) mixed in with pure coconut oil (about 1 part TTO to 6 parts CO). (too much Tea Tree oil can irritate the skin and is a little pricey, but the watered down (oiled down) version works just as well to repel mosquitoes, is cost effective, doesn’t smell quite so strong, is soothing to the skin and …best of all…contains no harmful pesticides. Just thought I’d pass that on.

  6. #6 caia
    August 27, 2006

    I’m glad those people aren’t my neighbors. I’d have to talk them out of it (sounds unlikely), sue to make them stop (sounds costly and iffy), or move.

    Mary, thanks for the tip. I’ve tried a more natural repellant or two without much luck. I like my mesh suit for gardening, but I don’t wear it on my walks. Luckily the skeeters haven’t been so bad here since early July. (I think it’s unlikely that I’d become less sensitive to mosquito bites, my grandmother and father both got/are getting more sensitive as they got older.)

  7. #7 Ground Zero Homeboy
    August 27, 2006

    Exuding B vitamins makes them go away, or so I’m told. As does wearing a mesh screen burka. It’s the new street fashion.

  8. #8 bc
    August 28, 2006

    One problem with doing studies to measure adulticide efficacy is that it’s nearly impossible to design a controlled study to prove or disprove the effectiveness of community spraying for mosquito control in reducing WNV.

    There’s certainly evidence that spraying reduces the number of infectious mosquitoes, at least for the short term.

    There’s some evidence it reduces human cases. Aerial spraying in Mesa County, Colorado, in 2004 did seem to interrupt human transmission. (They used the organophosphate, Naled, rather than a pyrethroid with PBO; many would consider the former more risky than the latter–at least I would, but I have a cholinesterase deficiency.) Since control measures are implemented by local governments, rather than by academic researchers, most results aren’t written up for publication, but these were presented at the 2005 WNV conference (See http://www.cdc.gov/ncidod/dvbid/westnile/conf/2005ppt/Defeyter-session6.ppt

    In our county on the other side of the Rockies in 2003, an estimated 3-4% of our 275,000 population was infected with WNV (most asymptomatically) during a very severe season with about 550 lab-confirmed WN fever cases, 63 neuroinvasive cases, and 9 deaths).

    Our experience was that one city that used primarily adulticiding, fairly late in the outbreak, fared better in health outcomes than an adjacent community that had a long-established integrated pest management program (IPM)with extensive larviciding and selective adulticiding.

    Fewer people used repellent in the community with the IPM program and they spent more time outdoors, possibly because they had fewer mosquitoes in that community as one would expect with a long-term control program.

    What was NOT expected was that they experienced a neuroinvasive WNV disease rate that was 2.4 times higher (after age-adjusting) than the city that did little mosquito control except late adulticiding.

    One possible explanation is that a person’s mosquito avoidance actions are in part influenced by the “biting pressure” they experience when outdoors, and that when nuisance control programs reduce the mosquitoes all summer, as occurs with nuisance mosquito control programs, they may be reducing people’s motivation to take personal protective measures during an outbreak of a mosquito-borne disease.

    So even reducing the number of infected mosquitoes does not always equate to reducing disease, the ultimate goal.

    Neverthesless, public health mosquito control spraying for WNV probably contributes very little to overall human pesticide exposure, compared to what people get from eating fresh fruits and vegetables (non-organic), living in communities with nuisance mosquito control programs, exposures from home or business use of commercial pesticides, or living next door to folks like Ed and Fran.

    Just as I’m not ready to tell people to stop eating fruits and vegetables because of the risk of pesticides, I’m not ready to lump all public health adulticiding for WNV into the “knee jerk” category. It’s certainly possible the benefits exceed the risks, but it’s very difficult to prove. We may instead find that the risks exceed the benefits, but there’s even less evidence to assess that.

    Many who are concerned about pyrethroid adulticides are also concerned about applying DEET directly to their skin. The safety of long-term use of picaridin and plant oils is also not known completely. So what’s a person to do? Stay inside instead of going outdoors and getting exercise? Ah, but the extra 20 lbs you put on is dangerous to your health too……

  9. #9 g510
    August 28, 2006

    Seems to me the most significant public health issue here is the risk of insect vectors developing resistance to otherwise-useful pesticides. Resistant mosquitos, flies, head lice, etc., are not a good tradeoff for temporary convenience.

    Occasional adulticiding programs, including occasional spraying of yards, are probably OK so long as the spraying intervals are timed in a manner that is likely to minimize vector resistance hazards. But as for regular twice-daily dosing systems, those should be against the law.

    We didn’t learn our lessons from DDT becoming ineffective. We didn’t learn our lessons from antibiotics becoming ineffective. Now here we go for another round.

    Frankly I have to conclude that humans deserve what they’re going to get.

  10. #10 revere
    August 28, 2006

    bc: Many thanks for the interesting comment (and cite). I agree with you that it is an extremely difficult to demonstrate adulticiding interrupts transmission of the disease. Dick Levins has suggested it might even increase transmission, and given the strange and often counterintuitive dynamics of vector-borne disease transmission I would consider this a real possibility. That aside, however, let me make some defense of the “knee jerk” comment.

    In the years I have been involved in the discussions of this with state public health officials, my experience has been that they take it as “obvious” that if adult mosquitoes are infected with WNV or EEE, etc., that killing adult mosquitoes is the thing to do and they do it with broadcast application of biologically active chemicals put into the environment at high volume, often on short notice, and with little protection for the many people who will be involuntarily exposed. As I’ve said in other posts, if we were testing a WNV vaccine, which would involve voluntary exposure, we would require a valid demonstration of both safety and efficacy for protecting people against
    WNV. But for adulticiding, which is largely an involuntary exposure, we require neither.

    The fact that it is difficult to demonstrate efficacy means that we are doing it without knowing if it works, and in many instances the timing is such that it couldn’t work because it comes too late.

    I would say that most uses of adulticiding for WNV are indeed knee jerk. One might look more carefully at the evidence and decide to do it anyway (or not), but that’s not the reallity of the situation.

    In any event, many thanks for your comment.

  11. #11 cougar
    August 28, 2006

    MIH: It would appear I’ve become much more tolerant of mosquito bites as well through constant exposure. In the work I do, I receive 50+ bites per week and, although I feel them biting at the time, I rarely itch afterward. In heavily infested areas, I have been known to hang 2 or 3 small citronella-filled balls on my clothing (kinda like mini air-fresheners) as well as rub mint leaves on my skin, both with some success.

  12. #12 Science Teacher
    August 28, 2006

    This is completely off topic, but I don’t know where else to ask.

    Why haven’t you guys commented on the situation in Indonesia??? We know the situation is unclear and ongoing, but there is some verified information and so many of us are waiting anxiously for the Reveres to weigh in… even if you don’t have conclusions yet.


  13. #13 marquer
    August 28, 2006

    Bats can and do eat an amount of insects up to as much as their own body mass, nightly.

    And they are very receptive to inexpensive roosting boxes placed on tall poles.

    But that would be icky. Far better to walk around in (and inhale!) a cloud of pesticide vapors.

    I occasionally use products containing piperonyl butoxide, where there is no other insect control alternative. I place it as a liquid on sheltered surfaces only, and I wear impermeable gloves, overalls, goggles and a cartridge respirator while doing so.

  14. #14 revere
    August 28, 2006

    Science Teacher: We’ve been keeping an eye on it, but the Reveres are vacationing together in a fairly remote location (cell phones only work sporadically) and are working hard to keep the blog upright as it is. We also can’t return email, although by a quirk, our dialup can reach the blog. We return to our homebases next week.

    Having said that, so far we don’t know what to say about the Indon situation. We are cut off from our sources there and the national wire services provide little useful information. Henry, I’m sure, is keeping people up with his machine translations. EM has usually been ats its most useful via commentary and reflection rather than breaking news.

    We continue to watch. Our impression so far is that this is more of a bubbling pot, which is not to downgrade it. On the contrary, it is a reason to be concerned. So far we don’t see the pot bubbling over. But our information is limited.

  15. #15 Science Teacher
    August 29, 2006

    Revere, thank you very much.
    We will look forward to further comments. Your work is so apprecitated. You are a calm voice of sanity in this situation. So many of us come to you for your thoughts. Thanks again.

  16. #16 science writer
    August 29, 2006

    To add another anecdote regarding tolerance to mosquito bites: I have lived in Jamaica and Nepal, both humid countries with a lot of mosquitoes, and I noticed in both places that small children seemed to have many more visible bites than older children and adults had very few. At the time I thought that repeated exposure led to desensitization, although I have never bothered to find out whether that is actually true.

  17. #17 g510
    August 30, 2006

    Re. adult humans getting resistant to mosquito bites:

    There are two things going on here that should not be confused with each other.

    1) Mosquitos inject a dose of an anticoagulant into the humans and other animals they bite, to make the victim’s blood easier to suck out. This is where you get the swelling and itch of the typical mosquito bite. And it makes sense that some people could develop a relative immunity to this over time, so they don’t get the itch and swelling and so on.

    2) Mosquitos while in the process of doing (1) above, also inject viruses, bacteria, and so on, whatever disease they happen to be carrying at the moment. However, immunity to (item 1) above, is NOT the same thing as immunity to the diseases that mosquitos carry (item 2 here). Just because you don’t get itchy bumps from mosquito bites, does NOT mean you won’t catch whatever disease-du-jour the little bastards are spreading around when they bite you.

    Back to spraying: I wasn’t *endorsing* spraying, just saying if people wanted to spray they shouldn’t do it in a manner that has a high probability of breeding resistant bugs. Personally I tend to believe that insecticides, particularly adulticides, should be used as little as possible, for all the usual reasons of health risks to humans, ecosystem effects, and so on. But my conservatism about insecticides is not an ideological prejudice against them and should not be used as a basis to discount the concern about over-spraying leading to resistant bugs.

    Bats: Yes!, a hearty endorsement of bats here. However they are wild critters after all, and should be given the same respect as any other wild critter, i.e. they are not your pets, they have potential hazards of their own, and so on. For example if I’m not mistaken, some species of bats are susceptible to rabies. Teach the kids to stay the heck away from any bats they might see. When the bats are out doing their thing at night, it’s good to avoid getting in their way even if that means staying inside. (Put up the right kind of outdoor light and you might be able to watch them going after insects, like watching birds at a backyard feeder, eh?) And just because you have the Bat Patrol going out every night, does not mean you can assume there are no more mosquitos around your house. Just like with sex, “always play safe or don’t play.”

  18. #18 Anne Laurie
    August 31, 2006

    Eating brewers’ yeast does seem to deter both mosquitos and fleas, apparently by changing your body chemistry subtly so you don’t “smell” right to the little biters.
    Quite a few people feed supplements to their dogs & cats rather than using chemical sprays, so the effects can’t be entirely psychological. The real problem is that you have to dose yourself for about 21 days in advance for the ju-ju to work effectively, and who’s got that kind of patience when you’re getting bitten today?

    And G510, yes, bats are known to carry rabies — in some areas any over-personal bat contact (like rescuing one from an adventuresome housecat) gets you a mandatory round of prophylactic rabies shots, which are (I’m told) both expensive and unpleasant. I’m still grateful for bats but you are quite right that they should NOT be mistaken for pets, no matter how cute they are!

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