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Mixed-Brood Habitats Bring Challenges, But New Control Options Offer Solution

Set between San Francisco Bay to the west and the San Joaquin River delta to the north and east, Contra Costa County, California, includes densely populated cities, sprawling suburban areas and agricultural land.

Given the wide range of habitats in such a diverse area, it’s no surprise the county also is home to 23 different mosquito species. Some are simply nuisances, but others – such as Culex tarsalis and Aedes melanimon – are vectors of significant human diseases, including West Nile virus.

The Contra Costa Mosquito and Vector Control District (CCMVCD) and its 34 employees are responsible for protecting the health of more than 1 million citizens in an area covering more than 800 square miles. To accomplish that, the district operates a comprehensive disease surveillance program, which includes birds and rodents, as well as an integrated pest management program for mosquito control.

“We’ve had four to five human cases of West Nile virus each year since 2006, when two people died from the virus,” noted Steve Schutz, scientific program manager for the district. “Public health is a major concern for us, but we also deal with nuisance issues as well.”

Long History, Public Involvement

Before the Contra Costa district was established in 1927, salt marsh mosquitoes, such as Aedes dorsalis, caused school closures, the shutdown of waterfront industries and abandonment of recreational areas. With those problems long since mitigated, the district continues to serve and protect the public by monitoring and controlling vectors of disease in the county.

One effort that elevates the district’s public visibility is its popular mosquito-fish rearing and distribution program. “We raise mosquito fish in a greenhouse and give them to citizens for use in backyard ponds, abandoned swimming pools and other artificial habitats where they will not conflict with native species,” said Carlos Sanabria, operations manager. “By cultivating the fish, we have more of them available earlier in the season.”

VectoMax® Test

Though some mosquito species are active in the county year-round, Contra Costa mosquito control efforts typically begin by mid-February and continue through the

end of October. In 2010, the program had an early-season opportunity to try a new larvicide product, VectoMax® CG Biological Larvicide, which provides control of all mosquitoes through the actions of two naturally occurring microbial species, Bacillus thuringiensis subsp. israelensis (Bti) and Bacillus sphaericus (Bsph). Bti and Bsph are combined into every microparticle for ingestion by mosquito larvae.

The site was a 200-acre marsh with relatively deep water. “The marsh usually is pumped dry, but the pump broke and repairs took far longer than anticipated,” said Sanabria. “We were starting to see large numbers of Culiseta inornata and expected that Culex tarsalis would follow.”

The district used a helicopter to apply VectoMax® CG. Results were positive.

“We were interested in learning how long the treatment would last,” Sanabria explained. “We achieved 45 to 50 days of control by the time the pump was reinstalled. To us, it appeared the Bti provided rapid kill in the first five to seven days, and the Bsph kicked in after that. To get that level of control with a single application is important, especially since power lines in the area are an issue.”

Duck Pond Trial

The district conducted a second, more formal trial, in October 2010. Working with private duck hunting clubs, the CCMVCD selected two ponds in the eastern part of the county. Both were flooded at the same time, and both ponds attracted Ae. melanimon followed by Cx. tarsalis mosquitoes.

VectoMax® CG was applied to one pond by an all-terrain vehicle mounting a rotary seeder. Again, results were positive.

“We got a very rapid knockdown of the Ae. melanimon and had 95 percent control within 48 hours,” Schutz noted. “We delayed colonization by Cx. tarsalis by one to two weeks. And we still had 85 percent control 28 days later, when the trial ended. The product worked very well and gives us a much-needed control option in mixed-brood habitats.”