Max Watkins wrote

"... the selection pressure is intense (eg same pesticide applied
over many generations) a population of insects/mites may develop a
method to
block that substance or detoxify it within a (relatively) short space of
time. That resultant population would be called resistant."

Max has written a very good piece on mode of resistance ,
detoxification & formation in a population!

However, the population response is a bit muddled.  The initial
population has individuals that are resistant to the introduced
chemical (more so than the average mite), these individuals don't die
and mate. The resistance does not show up because of the very few out
of the whole. After a treatment those that live produce progeny that
are potentially resistant, not all but those lucky enough to get the
genes conferring resistance. Over time more and more individuals become
resistant.  So basically it is a statistical population dynamic where
the harder you push the population eliminating susceptible individuals
the quicker you develop a resistant population because the only
successfully mating individuals are those that are resistant.

You will not eliminate all the susceptible genes so once you remove the
pressure the population will shift back, but the population will still
hold a larger number of individual with the "potential" to again become
resistant once the same chemical or a similar acting chemical is
reintroduced.  As an additional comment--similar classes of chemicals
can induce a population to more quickly become resistant.  So a
pyrethroid may make other pyrethroids less effective over the long run!

Cheers

Mike Griggs
Entomologist/ Support Scientist
Plant Protection Research Unit
USDA ARS, U.S. Plant, Soil & Nutrition Lab.
Tower Road, Ithaca, NY  14853
http://www.ppru.cornell.edu/PPRU.htm

phone:  607-255-1085
fax:        607-255-1132
email:    [log in to unmask]

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