Berenbaum on DDT and malaria

Thanks for linking to that!

Seriously? Hecht? Sigh.
I may have to wade into that after all :(

May is absolutely the best person they could have interviewed. Brilliant, Member of the National Academy, author, well respected....you get the idea.

Larouche has...

*crickets*

Two questions for May:

- As far as I know, the inside spraying of DDT does work as repellent, not as direct insecticide, how should that lead to more resistence of the mosquito's?
- The price: the next cheapest alternative is mosquito nets sprayed with pyrethrum (derivatives), which have their own drawbacks (including growing resistence of the mosquito's), but that is about 4 times more expensive. How does one weight that against the lack of money in a lot of African countries?

Last but not least, the WHO actively supports the inside spraying of DDT for malaria control, because it still is the most (cost) effective way, with minimal harm for the environment:
http://www.npr.org/templates/story/story.php?storyId=6083944
and several environmental organisations support its use, only for this purpose.

If spraying DDT would eradicate endemic malaria, there wouldn't be endemic malaria. As Ferdinand pointed out, DDT is used (recommended even). Where it works best is spraying the inside surfaces (upper walls and underside of roof) where the most common vector species rest after feeding. Widespread spraying (which is what environmentalists really don't like) doesn't work all that well and is much much less cost effective (wastes lots of DDT basically).

Some potentially interesting developments though. Anthropophilic mosquito subspecies which rest outdoors (or in animal sheds, eaves, rubbish, ect) look like they are increasing in frequency in some places (preliminary data). It isn't as simple a behavioural change as you might expect, since the normal behaviour involves taking a blood meal so large that the mosquitoes really can't fly around (at least very far). Anyways, evolution is a relentless bitch when it comes to epidemiology and vector control.

Actually, the change in species composition has been well documented for quite a while.

You might want to look at:
MOSQUITO BEHAVIOR AND VECTOR CONTROL
Helen Pates, Christopher Curtis
Annual Review of Entomology, January 2005, Vol. 50, Pages 53-70
(doi: 10.1146/annurev.ento.50.071803.130439)

"The price: the next cheapest alternative is mosquito nets sprayed "

Wrong the treated nets are less expensive per life saved. You can find the post on this site (I am not your mother).

Jeez, elspi -- give Ferdinand a break. The post is here.

Though I do wonder why he is posting his two questions for May Berenbaum here, instead of at the forum where she is actually answering questions. G'wan, Ferdinand, don't be shy!

travc, ferdinand, jre, the problem with DDT was it was OVER-used. It was used as a generic insecticide and so critters it was not meant to target were targeted and the result was fairly predictable: resistance and the insects that fed on mosquitoes were removed, making mozzies more fit to grow in number.

jre,

Posted here, before I realised that May was answering somewhere else... But I posted there now, is awaiting moderation.

elspi,

Sri Lanka has a good health infrastructure, a lot of African countries have virtually none. DDT inside spraying needs to be done only a few times per year, pyrethrins many times to remain effective. Costs and lack of infrastructure is the main problem in Africa...

I have a question for you, Ferdinand:

Do you believe animal species evolve, or not? Tied to that is:

If you do believe animal species evolve, why are your questions coached as if animal species did not evolve, and cannot evolve, resistance?

Please actually research mosquito resistance to DDT and agricultural use of DDT. Until you've done at least a little research, you won't be able to ask an honest, intelligent question, even if you don't realize you can't.

No one is going to Africa and yanking DDT away from people trying to spray it as an insect repellant, not matter how hard you wish they were.

I posted this in the last malaria thread but that kinda got lost in Ray-related matters I think. In effect it seems to be possible to get the mosquitoes to carry pesticide to their breeding sites themselves - thus eliminating the need for widespread spraying 7 reducing 'by-catch'...

http://www.rothamsted.bbsrc.ac.uk/corporate/PressReleases/PressReleases…

Scientists turn deadly mosquitoes against their own offspring.

"A new field demonstration from Peru suggests that we might be able to co-opt adult mosquitoes into applying insecticides for us, and that they are far more efficient at doing this than humans are...

Scientists were able to achieve almost total coverage of the aquatic larval habitat by treating a small proportion of the area where adult mosquitoes rest with a safe, potent and persistent insecticide. This insecticide can be carried by adult mosquitoes but only kills juvenile stages. Amplification of the effect occurs because every adult mosquito completes several resting and egg-laying cycles during its lifetime. This results in multiple opportunities for contamination of the aquatic habitat.

The use of the adult mosquito as the transfer vehicle ensures that the larvicides are very accurately targeted: the more popular the breeding site, the greater the transfer of insecticide and the more effective the control. "

Marion,

I am aware of species evolution (including resistence against DDT and more modern pesticides). But that has nothing to do with its action as a repellent. With DDT sprayed at the inside of huts, that works for many months, while the alternatives like pyrethrum (derivatives) need far more frequent application.

Besides the extra costs, the lack of infrastructure in a lot of African countries needs the best (cost) effective solution. Which may be DDT or anything else...

Have you read the link in #3 about an article of the WHO supporting inside spraying of DDT? Here follows someone's comment:

Jay Feldman, executive director of a group called Beyond Pesticides, says using it is a war plan without an exit strategy.

and more of that.

My impression is that DDT is too symbolic for both sides of the "green" line, while it should be seen as one of the several possible routes which should be chosen -or not- based on local/regional (cost) effectiveness and drawbacks, the latter by far not comparable to its massive use in agriculture.

Chris S.,

Thanks for that link, sounds great, except for one point that they use a "persistent" pesticide. I hope not too persistent, which is one of the disadvantages of DDT... But it is a quite interesting strategy in addition to others.

A family member of my wife did work for the UNESCO (not sure of the right UN program), working on spreading petrol derivatives on breading grounds of the mosquito's. Was quite effective, but not that good for the environment...

Another strategy is fighting the malaria parasite itself. While a lot of drugs lost or are loosing their effectiveness, it seems that a herbicide (!) may be of help:
http://www.genomenewsnetwork.org/articles/03_03/weed.shtml

Ferdinand, from the link (emphasis mine): "⢠In the context of the environmental effects of larvicides, it is worth noting that THEY WILL BE APPLIED ANYWAY - whether conventionally disseminated or using this new method. The targeted water bodies are urban: from discarded tins to sewage seeps. These are not important ecologically and, in any case, the risk posed by the mosquitoes that breed there is far greater than any environmental hazard to non-target insects.
⢠Pyriproxyfen, the insecticide used, is a juvenile hormone analogue. It is already registered for public health use. It has drinking water approval from the WHO of 300 ppb which is 1000 times the dose needed to control Aedes aegypti."

hth

Chris,

I did lookup the material safety datasheet of pyriproxyfen, see: http://www.cdms.net/LDat/mp48S001.pdf

Indeed it is quite harmless for birds, bees and mammals. But it is (highly) toxic for fish species. Unfortunately that limits its use for large scale application in non-urban surroundings. But this strategy is quite promising and maybe some newer stuff without the fish toxicity may emerge.

The persistence of pyriproxyfen is not too bad, some remains on organic matter after a few months, but it is decaying in an exponential way, see:
http://www.ncbi.nlm.nih.gov/pubmed/1676671

BTW, that family member was working for UNICEF (the children's aid program), not for UNESCO (the cultural heritage program), all those abbreviations...

Fish toxicity may emerge with the new method but it should be bourne in mind that pyriproxyfen will be used in the area anyway - despite the problems outlined in the MSDS you link to - and a broad-scale spraying regime will likely affect more fish than leaving out small pots of it for mosquitoes to pick up & tranfer to their 'breeding grounds'.

As with any insecticide resistance will be an emerging problem, however if this new method is shown to work then research into other mosquito transmitted larvicides will be possible. As will application elsewhere in the world.