Carlos Trevino wrote:
> Nov. 9, 2004, 6:55AM
> 
> Mosquitoes could mutate beyond pesticides' reach County predicts most
>  potent weapon will be obsolete By ERIC BERGER Copyright 2004 Houston
>  Chronicle
> 
> Pesticides, long the weapon of choice against the nettlesome and 
> sometimes deadly mosquito, are losing their bite.
> 
> 
> Harris County's chief mosquito fighter has recommended rotating the 
> spraying of a handful of still-effective pesticides next year to 
> maximize their usefulness. But even with this plan, it's probably a 
> matter of when, not if, mosquitoes mutate beyond the control of 
> pesticides.
> 
My initial reaction to this was to be scpetical, but the explanation for
why this makes sense is below.

> Parsons and others also maintain that some companies, to save money, 
> dilute their pesticide, which not only doesn't kill mosquitoes, it 
> promotes resistance in the offspring of survivors.
> 
This may or may not be true.  For a counter-example, from my past life:

O'Hara R.B., Nielsen B.J., Østergård H., 2000. The effect of fungicide
dose on the composition of laboratory populations of barley powdery
mildew. Plant Pathology, 49: 558-566.

At about the same time, Mike Shaw was working on this:
Shaw, M.W. Models of effects of dose heterogeneity and escape on the
selection pressure for pesticide resistance. Phytopathology. 90,
2000,333-339.

 From which he concluded that a high dose heterogeneity can reduce the
rate of evolution of resistance.  In other words, using old, worn-out
maachinery might be better.

> "Oh, I have no question that this is happening," said Raleigh 
> Jenkins, owner of ABC Pest, Pool & Lawn Services, one of the largest
>  pesticide companies in Houston. Jenkins said his trucks spray at the
>  rate recommended by label pesticide labels.
> 
Wow!  Bright labels!  I wonder what criteria the labels use to get their
recommended rates.  Anyone know?

> Weekly spraying common Private spraying is common in Houston. Of the 
> more than 100 residential communities that CIA Services manages in 
> the greater Houston area, about 20 percent choose to regularly fog 
> for mosquitoes, said the company's president, Ralph Troiano.
> 
CIA Services!?


> Public and private sprayers have two types of pesticides at their 
> disposal that can safely be sprayed into residential communities: 
> organophosphates, a group of closely related pesticides that includes
>  malathion; and a synthetic form of pyrethrins, which are derived
> from chrysanthemum flowers.
> 
Organophosphates safe?  This must be some new meaning of the word "safe"
that has yet to cross over into British English.

For contrary views, see
http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5227a1.htm>
http://www.epa.gov/pesticides/factsheets/pyrethroids4mosquitos.htm>
http://news.bbc.co.uk/2/hi/health/1847580.stm>

So, The Lancet, the BBC, the CDC and the EPA all think that they are not
safe, or at least that they are dangerous in high doses.

> Scientists now believe most mosquitoes may have regained a 
> susceptibility to malathion. The reason, they say, is that it is 
> difficult for a mosquito to block more than one type of pesticide.
> 
> The trick is to try to nurture this susceptibility and breed the 
> vulnerability to at least one type of pesticide back into the 
> population.
> 
> So next year, in Harris County, Parsons said he has recommended that 
> the county rotate among spraying with malathion and two different 
> types of pyrethrins. This targeted spraying should extend the useful 
> lifetimes of the chemicals.
> 
> Scientists say an unfortunate byproduct of all spraying is that the 
> chemicals will eventually only kill those mosquitoes that have no 
> resistance at all. With no reproduction from this desirable group, 
> then, it will become increasingly difficult to breed susceptibility 
> back into the population.
> 
It is always nice to see that evolutionary ideas are being used for 
practical purposes: it makes me think that I might be able to do 
something useful.

Bob

-- 
Bob O'Hara
Department of Mathematics and Statistics
P.O. Box 68 (Gustaf Hällströmin katu 2b)
FIN-00014 University of Helsinki
Finland

Telephone: +358-9-191 51479
Mobile: +358 50 599 0540
Fax:  +358-9-191 51400
WWW:  http://www.RNI.Helsinki.FI/~boh/
Journal of Negative Results - EEB: www.jnr-eeb.org
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