> Can you confirm or correct my impression that... 
> something with half life of 12 months would be 
> reduced by half in a  year, by 3/4 in 2 years, by
> 7/8 in 3 years and so on ad infinitum.

Just because there are detectable residues after a period
of weeks or months doesn't mean the residues are
significantly harmful to insect reproduction and survival.

Example: It used to be a popular practice for wheat
farmers in the southern high Plains States to spray
Malathion or other insecticides on all the harvested
grain as it was augered into grain bins.  For a few 
months the Malathion would effectively suppress grain
moth and beetle reproduction and survival. Then in
subsequent months a point would be reached when the
moth and beetle populations would explode even though
Malathion residues could still be measured in parts per
million.

Another Example: In some restaurants battery operated
foggers are installed that squirt tiny amounts of pyrethrin 
insecticide into the air every 30 minutes to kill house flies.
If the foggers are removed, the pyrethrin residues can still be
detected on surfaces of the restaurant for many months, 
but these residues have no appreciable biological activity
against the flies or other insects in the restaurant.

Paul Cherubini
El Dorado, Calif.

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