Insecticide-resistant mosquito population developing in Delta County

DELTA COUNTY — A population of mosquitoes in Delta’s North Fork area appear to be growing increasingly resistant to an insecticide commonly used to control their numbers, according to a recently concluded study by Colorado State University scientists.

The study conducted by Mike K. Bignall and William C. Black of CSU in the summer of 2007 found a two-fold increase in resistance of Culex tarsalis, a mosquito species, to the insecticide Malathion from early to late season. The mosquitoes also appeared to have grown in their ability to produce proteins known to enhance resistance to organophosphate pesticides — the family to which Malathion belongs.

Researchers tested a sample of mosquitoes collected near the towns of Paonia and Hotchkiss in June against a known susceptible strain of mosquitoes from California. In August, researchers captured another sample and tested for Malathion resistance; the results were compared to earlier samples.

As a result of these findings, which were released in December, the study’s researchers recommended the county explore another chemical form of mosquito control, or rotate pesticides regularly so the insects don’t become “overly resistant.” The larvicide Altosid, which contains an insect growth regulator, seemed particularly promising, the researchers stated. Minimizing adulticide spraying and population monitoring by trapping were also recommended.

For about five years, the Paonia Mosquito Control District Board sprayed Malathion regularly in the valley but last year switched to using Biomist in Paonia, said Ken Nordstrom, Delta County director of environmental health.

Biomist is likely to be used in the rest of the county within the next year, he added.

Residents who were concerned over the area’s high incidence of West Nile virus in the state during the summer of 2006 initiated the study, Nordstrom said. “The findings kind of verified (residents’) suspicions,” he said. “The West Nile virus has been the disease we were concerned about in the last five years.”

There are two mosquito control districts in Delta County: Delta and Paonia. The Paonia Mosquito Control District Board is pulling the use of Malathion in response to the study’s findings.

“We wanted to know as well as anyone (if the mosquitoes were becoming resistant),” said Bill Kolb, Board president. He pointed out that the board trapped larvae for the study. The district board disperses insecticide given the right weather conditions and if complaints or observations of mosquito breeding are received.

Spraying of Malathion spurred concerns about potential health risks. Malathion interferes with the normal way that the nerves and brain function, according to the Department of Health and Human Services. Exposure to very high levels of malathion for a short period in air, water, or food may cause difficulty breathing, chest tightness and other symptoms.

In late 2006, according to published reports, Paonia residents came to the Environmental Protection Agency with concerns, including that spray vehicles were being rinsed or washed out and that the insecticide was improperly stored.

A subsequent investigation and findings by the EPA led to an first-time violation warning being issued to the area’s Mosquito Control District Board. Kolb said the findings — particularly the one about the district’s foggers not being calibrated — were not validated. He said no new problems have arisen since the warning.

Contact Lisa Huynh via email at lisah@montrosepress.com