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Section: Local & State

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
RESOURCES
Graphic: Pesticide-Resistant Mosquitoes
Graphic: Houston-Area Mosquitoes

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.

"I think, probably within 5 to 10 years, we will see resistance to
every pesticide," said Ray Parsons, director of Harris County's
Mosquito Control Division. "I'll say this: I'm glad I'm retiring
after the first of the year."

Much as bacterial infections have become more difficult to treat
because of the overuse and misuse of antibiotics, so too have
mosquitoes become resistant to insecticides.

Mosquito control officials note government programs to control
mosquitoes have contributed to this resistance, but Parsons said
private companies - spraying at the behest of community and civic
associations - also share a part of the blame.

Unlike the county, which sprays only at specific times to dampen
mosquito-borne viruses such as West Nile, private companies spray on
a more regular basis to control nuisance mosquitoes, which pose no
imminent health threat. And the more exposure mosquitoes endure, the
tougher they get.

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.

"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.

He says, however, that some of his competitors are not. Some private
bids to municipalities and communities are lower than the actual cost
of the pesticides, Jenkins said. To make money on these cheap bids,
he said, the product has to be diluted.

"I've heard rumors of this, but I have never heard of anyone being
found doing it," said Ken Myers, executive director of the Texas Pest
Control Association.

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.

Most communities spray once a week from April to October, he said.
Troiano said he asks spraying companies to spread pesticides at the
label-recommended rate, but admitted it is nearly impossible to
determine whether they comply.

"The most important thing is whether or not residents find it
effective," he said. "From the feedback we receive, residents can
tell when we're spraying, and when we're not. They think it's pretty
effective."

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.

"There are a lot of different kinds of pesticides, of course, but
there aren't a lot of pesticides that you can apply
indiscriminately," said Patricia Pietrantonio, an entomologist at
Texas A&M University.

Malathion came into widespread use nearly half a century ago. As
mosquitoes developed a resistance, communities turned to pyrethrins.
This year, Harris County sprayed 2 million acres with a pyrethrin
commercially sold as Scourge.

Jim Olson, another Texas A&M entomologist, said there are few
promising pesticides under development that could safely be widely
sprayed. That's why maximizing the effectiveness of the current
pesticides is critical, he said.

Nurturing a susceptibility
Pietrantonio and Olson have begun a study of mosquito resistance in
Harris County, collecting thousands of larvae from Culex mosquitoes,
the carrier of West Nile. They will raise the mosquitoes and then
test whether genetic mutations have made them resistant to the
pyrethrins used by the county.

About six years ago Pietrantonio led a similar study that determined
a significant number of the insects were no longer susceptible to
malathion.

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's pretty well proven that dead mosquitoes can't reproduce," Olson
said. "That's an Aggie paradigm right there."

eric.berger@chron.com

http://www.chron.com/cs/CDA/printstory.mpl/metropolitan/2890973