Aedes aegypti, a daytime-biting mosquito that predominantly feeds on humans, has spread to at least seven counties since June 2013, according to UC Davis medical entomologist Anthony Cornel of the UC Kearney Agricultural Research and Extension Center, Parlier, and the UC Davis Department of Entomology and Nematology.
“It’s an issue of great concern, especially as current control methods do not appear to be working well,” said Cornel, who does research on the mosquito in Clovis, Fresno County, where it was discovered in June 2013. Simultaneously, the insect was found in the cities of Madera and San Mateo.
“This ongoing widespread invasion and establishment proves that this is no longer a regional issue and has affected many cities and towns in California,” he wrote Feb. 8 in F1000 Research.
But Cornel is optimistic that the pest management intervention strategies and surveillance and control tactics now underway will help control its spread. Aedes aegypti can transmit dengue, yellow fever, Zika and chikungunya viruses.
Zika virus vector
The Zika virus, now spreading throughout the Western hemisphere, was first identified in Uganda in 1947 in rhesus monkeys, according to the World Health Organization. It was subsequently identified in humans in 1952 in Uganda and the United Republic of Tanzania. Outbreaks of Zika virus disease have been recorded in Africa, the Americas, Asia and the Pacific.
Despite the mosquito’s invasion in parts of the United States, there are no reported cases of locally transmitted Zika virus in California or in the contiguous United States, according to the Centers for Disease Control and Prevention. U.S. cases have all involved travelers returning home from countries plagued with disease outbreaks.
Cornel works with the Consolidated Mosquito Abatement District, based in Fresno County, to tackle the spread of the mosquito there. The district covers 1,058 square miles, including part of Kings County.
How far north in California will the mosquito spread?
“I don’t want to exclude the possibility that it may spread as far north as Sacramento,” said Cornel, who collects, rears and researches mosquitoes from all over the world, including the U.S., Mali, Cameroon, Comoros, Tanzania, South Africa and Brazil. “We need to see if it overwinters as eggs or adults or both.”
It’s troubling that the mosquito is becoming more and more resistant to pesticides, Cornel said.
“We have found that the Aedes aegypti have insecticide resistance genes which likely explains why their ultra-low volume and barrier spray applications have not worked as well as expected,” he said.
At Clovis, Cornel and his colleagues trap mosquitoes in gravid or ovitraps; study overwintering and flight dispersal; and employ mark-release-capture trials to estimate dispersal and population size, needed to plan biological (Wolbachia) and chemical auto-dissemination control strategies.
Their document, “Surveillance and Control of Aedes aegypti Mosquito in Clovis, Calif.,” published in F1000 Research details their research with text and maps. It is work of Cornel and Yoosook Lee of UC Davis; Stephen Dobson of the University of Kentucky; Corey Bansfield of MosqMate Inc. and Jodi Holeman, Mark Amireno, Charles Smith and Stephen Mulligan III of the Consolidated Mosquito Control District. In the document, Mulligan, director of the Consolidated Mosquito Control District, describes Aedes aegypti as “the rat of the mosquitoes.”
The California team works with University of Kentucky scientists to develop novel control strategies. One trial involves coating male mosquitoes with insect growth regulators, which are passed on to the females. Males are also infested with a biopesticide or “a good bacteria-like organism,” Wolbachia. “The male transfers it to the female, which affects the ovaries and negatively affects immature development,” Cornel explained. “It’s not new, but it’s not been employed in large trials.”
Cornel has found that males can fly well over 200 meters in one night from their breeding site. Aedes aegypti fly mostly during cooler periods of the day and stick to the shade when it’s hot, they Cornel said. People get bitten while walking around at dusk.
New yard drains a breeding site
The researchers target mosquito breeding sites, primarily yard drains. These drains installed in new home developments empty into the gutter or street and are places mosquitoes can breed out of sight – perhaps even underground.
“These drains are not easily accessible and we can’t see the mosquitoes,” Cornel said. He suggests that cities everywhere address this public safety issue and redesign the yard drains.
It’s crucial for the public to become involved, Cornel said. “We have to focus on public education. We have to get the message across to eliminate mosquito breeding sites. We can’t go to every house. We must rely on the public to eliminate the breeding sites.”