Male Aedes aegypti mosquitoes infected with a bacterium, Wolbachia pipientis, are being released in Clovis, Fresno County, where this mosquito was discovered in June 2013. Although this mosquito is now found in California, there has been no locally transmitted case of the Zika virus in the state.
The project, to determine dispersal and survival, began Monday, May 10.
“The daytime-biting mosquito, which feeds predominantly on humans, has spread to at least seven counties since its discovery in Clovis,” said Cornel, a mosquito researcher and faculty member with the UC Davis Department of Entomology and Nematology and the UC Kearney Agricultural Research and Extension Center, Parlier.
“The biting nuisance and potential of the mosquito Aedes aegypti to transmit Zika, Chikungunya and dengue viruses in California is cause for concern,” he said. “Efforts to curb its spread and reduce populations have not been very effective. Control efforts have included educating the public to remove standing water (source reduction) insecticide barrier sprays and bacterial larviciding.”
The collaborative effort involves Steve Mulligan, director of the Consolidated Mosquito Abatement District, based in Fresno County; Stephen Dobson of the University of Kentucky; MosquitoMate Inc, (mosquitomate.com); and UC Davis. The researchers will evaluate the population suppressive ability of the novel sterile insect technique, which is part of a comprehensive vector management approach.
Only Aedes aegypti is targeted. “When the Wolbachia-infected male mosquito mates with non-infected females, the result is “cytoplasmic incompatibility, which causes the female to lay infertile eggs that will not hatch,” Cornel explained.
“This approach requires the release of tens of thousands of Wolbachia-infected males into residential neighborhoods where this mosquito is a nuisance,” the medical entomologist said. “Releasing large numbers of males increases the chance that an introduced male will mate with the native females.”
Although residents will notice increased numbers of male mosquitoes, male mosquitoes do not bite and cannot transmit disease. Both U.S,federal and State regulatory agencies have approved the technique for evaluation of effectiveness.
“This sterile insect technique was evaluated in 2015 in Los Angeles to suppress another invasive mosquito, Aedes albopictus,” Cornel said, adding that the results from that trial look promising.
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 into new areas 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. The cases have all involved travelers returning home from countries plagued with disease outbreaks.
“We can't predict how far this mosquito will spread in California,” said Cornel, noting that its range has expanded “south of Fresno to San Diego. The farthest site north is Madera in the Central Valley, but it's also been found in the more coastal area of Menlo Park in San Mateo.”
It's troubling that the mosquito is becoming more and more resistant to pesticides, said Cornel, who collects, rears and researches mosquitoes from all over the world, including the United States, Mali, Cameroon, Comoros, Tanzania, South Africa and Brazil.
“We have found that Aedes aegypti have insecticide resistance genes which likely explains why the use of ultra-low volume and barrier spray applications for control have not worked as well as expected.”
The study, published in the Public Library of Science (PLOS), Neglected Tropical Diseases, contradicts the long-held assumption that once you're infected with a particular dengue serotype, you won't get it again.
“Our most significant result from this study is that immunity to dengue viruses does not always provide perfect protection from reinfection,” said principal investigator and medical entomologist Thomas Scott, distinguished professor and now emeritus, UC Davis Department of Entomology and Nematology. “The public health implications include evaluation of dengue vaccines, interpretation of a person's virus exposure history and susceptibility to new infections, and design of dengue surveillance programs.”
Dengue infects 400 million people worldwide each year, and 4 billion people or nearly half of the world's population are at risk for dengue,” said Scott, who has studied dengue more than 25 years and is recognized as a leading expert in the ecology and epidemiology of the disease. “There is no vaccine nor drug that is effective against this virus.”
“This finding could help explain results of dengue vaccine trials that showed poor efficacy against one of the four serotype,” Stoddard said. “It also has broad implications for vaccine development.”
The research team investigated the "validity of the fundamental assumption" by analyzing a large epidemic caused by a new strain of DENV-2 that invaded Iquitos, Peru, in 2010-2011, 15 years after the first outbreak of DENV-2 in the region.
"Our data indicates that protection from homologous DENV re-infection may be incomplete in some circumstances, which provides context for the limited vaccine efficacy against DENV-2 in recent trials," the research team wrote. "Further studies are warranted to confirm this phenomenon and to evaluate the potential role of incomplete homologous protection in DENV transmission dynamics."
Scott and Amy Morrison of the Scott lab and U.S. Naval Medical Research Unit, co-directed the project in Iquitos. The paper is also the work of Sandra Olkowski and Kanya Long of the Scott lab; Robert Reiner of Andrews University, Berrien Springs, Mich., and the Fogarty International Center; Brett Forshey, Angelica Espinoza, Stalin Vilcarromero, Tadeusz J. Kochel and Eric Halsey of the U.S. Naval Medical Research Unit; Helen Wearing, University of New Mexico, Alburquerque; and Wilma Casanova, Universidad Nacional de la Amazonía Peruana, Iquitos, Perú.
While vaccines are under development, it is not clear how they can be best applied when they are available, including in combination with other interventions like mosquito control, Scott said. “New disease prevention tools, in addition to vaccines, and an improved understanding of virus transmission dynamics, which will enhance surveillance and epidemic response, are needed to reduce the global burden of dengue.”
The paper, “Incomplete Protection against Dengue Virus Type 2 Re-infection in Peru,”
is online at
“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, http://f1000research.com/slides/5-149.
But Cornel is optimistic that the pest management intervention strategies and surveillance and control tactics now underway will help control its spread. Infected Aedes aegypti can transmit dengue, yellow fever, Zika and chikungunya viruses.
The Zika virus, now spreading throughout the Western hemisphere, is an emerging mosquito-borne virus that 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. The 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, commonly known as yellow fever 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 United States, 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.”
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. They also engage in “recruiting” mosquitoes to kill other mosquitoes. “We have mosquitoes spread insecticides for us, that is, we turn them into mosquito-control workers through the use of insect growth regulators and biopesticides.”
Their 27-slide 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.”
Regarding flight dispersal, Cornel has found that “males can fly well over 200 meters in one night from their breeding site. We previously thought it was no more than 60 to 100 meters.”
“The Aedes aegypti fly predominantly during cooler periods of the day,” the medical entomologist said. “When it's too hot, they hang around the shade.” When residents walk at dusk, both male and female mosquitoes can follow them. “Only the females bite but the males will hang around your ankle waiting for the females to arrive.”
The researchers target mosquito breeding sites, primarily yard drains. “Despite the drought and the elimination of visible bodies of water, such as bird baths, pet bowls and flower pots, there's a major issue: yard drains,” Cornel said. “Yard drains installed in new home developments empty into the gutter or street and are cryptic breeding sites for mosquitoes.” He speculates that these mosquitoes are breeding underground.
“These drains are not easily accessible and we can't see the mosquitoes,” Cornel pointed out. “We need to blow out the water and plug these yard drains to eliminate these breeding sites.” He suggests that cities everywhere address this public safety issue and “redesign the yard drains.”
Cornel works with the Consolidated Mosquito Abatement District in setting gravid or “killing traps” in the front yards of homes in Clovis. The traps, which look like five-gallon buckets, contain orchard grass and water. “When the orchard grass decomposes, it releases a plume of chemicals that attracts female Aedes aegypti to lay their eggs in,” Cornel said. A screen prevents them from reaching the water to lay their eggs. The insects adhere to black sticky paper.
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.”
It's possible—but he hopes not—that what is now a “mosquito nuisance” will result in a disease outbreak.
- Anthony Cornel: Mosquito Man UC Davis Department of Entomology and Nematology
- Anthony Cornel, Kearney Agricultural Research and Extension Center
- Yoosook Lee, UC Davis Department of Pathology, Microbiology and Immunology, School of Veterinary Medicine
- F1000 Research
- Traps Tested in Clovis (UC ANR)
- Aedes aegypti, California Department of Public Health, Aedes aegypti
- Consolidated Mosquito Control District, based in Fresno County
The seminar is from 12:10 to 1 p.m.
"Mosquitoes are well recognized as the most important arthropod vectors of disease-causing pathogens," Strand says in his abstract. "Interest in the gut microbiota of mosquitoes has risen recently as a potential tool for manipulating vector competency. In contrast, much less is known about the role of this community in mosquito growth, development and reproduction. In this talk I will discuss recent results from our lab group regarding the composition of the gut microbiome in different mosquito species and insights we have gained about the function of this community in mosquito biology and evolution."
Strand focuses his research in the areas of parasite-host interactions, virology, immunity and development. Current projects center on virus-host interactions, function of the insect immune system, and regulation of reproduction in mosquitoes and other insects.
Strand was nominated as a seminar speaker by Professor Shirley Luckhart, Department of Medical Microbiology and Immunology, UC Davis School of Medicine and a graduate student advisor, UC Davis Department of Entomology and Nematology; Associate Dean Ed Lewis of the College of Agricultural and Environmental Sciences and professor and vice chair of the UC Davis Department of Entomology and Nematology; and Distinguished Professor James R. Carey, UC Davis Department of Entomology and Nematology.
A native of Norfolk, Va., Strand grew up in Texas. He received both his bachelor's degree in entomology (1980) and his doctorate in entomology (1985) from Texas A&M University, College Station. He accepted a fellowship from the National Science Foundation-North Atlantic Treaty Organization (NATO) as a postdoctoral scholar at the Imperial College, University of London in 1986. He then accepted a position as an assistant professor at Clemson University. In 1987, joined the faculty of the University of Wisconsin-Madison where he advanced to the position of professor. In 2002, Strand moved to the University of Georgia where he holds appointments in the Department of Entomology, the Center for Tropical and Emerging Global Diseases, Faculty of Infectious Diseases, and Department of Genetics.
Strand is a fellow of the Entomological Society of America (selected in 2012) and the American Association for the Advancement of Science (2008). ESA presented him with the national recognition award in the Physiology, Biochemistry and Toxicology Section in 2009. He served as president of the section in 2013.
Plans call for his seminar to be video-recorded and then posted at a later date on UCTV. The departmental seminars are coordinated by Professor Steve Nadler.
Reiner, a RAPIDD (Research and Policy in Infectious Disease Dynamics) postdoctoral fellow, studies with UC Davis Professor Thomas Scott, a worldwide expert on the epidemiology and prevention of dengue. Scott chairs the mosquito-borne disease modelling group in the RAPIDD program of the Science and Technology Directory, Department of Homeland Security, Fogarty International Center, National Institutes of Health.
“Dengue takes an enormous toll on human health worldwide, with as many as 4 billion people at risk," said Scott.
Reiner noted that “Mathematical models for the transmission of mosquito-borne diseases often rely on very simple assumptions about the population dynamics of the mosquito vectors. Linking transmission models to real-world data on mosquito abundance requires a method that smooths over the discontinuous mosquito abundance data to yield complete time series, simultaneously accounting for the effects of covariates that also vary in space or time.”
“Generalized additive models (GAMs) offer a flexible way to disentangle the relative roles of seasonality, inter-annual variation, control, temperature, and land cover as predictors of mosquito abundance," Reiner said in an abstract of his talk. "Case studies on the abundance of vectors of different pathogens in two different locations are considered: dengue virus and Aedes aegypti in Iquitos, Peru and West Nile virus and Rift Valley fever virus and Culex tarsalis, the Culex pipiens complex, and Aedes melanimon in California.”
“Using over 150,000 entomological surveys conducted at the household level within Iquitos, Peru as well as spatio-temporally explicit control efforts of varying intensity, we identify locations within the city that systematically over or under produce Aedes aegypti as well as quantitatively assess the impact of various levels of control. Within California, using a spatially explicit surveillance data set (2003-2009, 102,188 trap-nights of 4,882,911 mosquitoes), we parse the relative contributions of seasonality, temperature and land-type on mosquito abundance, identifying significant interactions between seasonality and land-type. In both cases, GAMs produce simple, yet flexible products that can link real world vector abundance data to transmission models, increasing accuracy and utility to models used to inform both epidemiology and public policy.”
Reiner received his doctorate in statistics in 2010 from the University of Michigan, Ann Arbor. He began his academic studies in California, where he earned his bachelor’s degree in applied mathematics in 2002 from UC Berkeley. He went on to obtain his master’s degree in applied mathematics from California State University, Northrdige, in 2005; and his master’s in statistics from the University of Michigan in 2009.
Plans are to record the seminar for later posting on UCTV.
Dengue Higher Than Previously Estimated (Thomas Scott lab)
List of Upcoming Seminars Sponsored by the UC Davis Department of Entomology and Nematology