In a ground-breaking discovery encompassing six years of research, an international team of scientists led by UC Davis chemical ecologist Walter Leal announced they've identified the sex pheromone of the pest, which feeds on citrus and transmits the bacteria that causes the deadly citrus greening disease known as Huanglongbing (HLB).
Leal, a native of Brazil and a fellow of both the Entomological Society of America and the Entomological Society of Brazil, revealed the discovery during his presentation Dec. 5 at the 10th Annual Brazilian Meeting of Chemical Ecology in Sao Paulo. His team included scientists from UC Davis, University of Sao Paulo, and the Fund for Citrus Protection (FUNDECITRUS) from the state of Sao Paulo.
“Dr. Leal's discovery of the Asian citrus psyllid pheromone is a significant breakthrough in preventing the spread of this serious citrus insect, and may offer a less toxic method for its control,” said integrated pest management specialist Frank Zalom, distinguished professor with the UC Davis Department of Entomology and Nematology and a past president of the Entomological Society of America. He was not involved in the study.
“Having a lure to dramatically improve captures of this psyllid with the conventional sticky traps is a major progress toward integrated pest management,” said Professor Jose Robert Parra of the University of Sao Paulo.
Identifying the sex pheromone proved “complicated and quite a challenge” because of the insect's complex behavior and biology, said Leal, a UC Davis distinguished professor who has discovered the sex pheromones of moths, beetles, bugs, cockroaches, mites and other arthropods. A patent was filed Friday, Dec. 1, and journal publication is pending.
Citrus trees infected with HLB usually die within five years, according to the UC Statewide Integrated Pest Management Program. There is no known cure. “The only way to protect trees is to prevent spread of the HLB pathogen in the first place, by controlling psyllid populations and removing and destroying any infected trees,” UC IPM says on its website.
Native to Asia, the Asian citrus psyllid, Diaphorina ciri, was first detected in the United States in June 1998 in Palm Beach County, Florida, and in California in August 2008 in San Diego County. Scientists discovered HLB in Florida in August 2005, and in Los Angeles in March 2012. The mottled brown insect, about 3 to 4 millimeters long, or about the size of an aphid, is now widespread throughout Southern California and is now found in 26 of the state's 58 counties.
The Asian citrus psyllid, or ACP, feeds on new leaf growth of oranges, lemons, mandarins, grapefruit and other citrus, as well as some related plants. Infected psyllids can transmit the bacterium Candidatus Liberibacter asiaticus, which causes the fatal citrus disease. An early symptom of HLB in citrus is the yellowing of leaves on an individual limb or in a sector of a tree's canopy.
Currently growers are using yellow sticky traps to detect the insect and to monitor the population. “Efficient lures,” Leal said, “are sorely needed for sticky traps, particularly for early ACP detection. Otherwise, growers have to resort to regular sprays to avoid infection given that infected insects from gardens and noncommercial areas migrate to citrus farms.”
Pheromones and other semiochemicals are widely used in agriculture and medical entomology. “Growers use them as lures in trapping systems for monitoring and surveillance, as well as for strategies for controlling populations, such as mating disruption and attraction-and-kill systems,” Leal noted.
Although ACP is present in Arizona and California, the disease itself has not been established, Leal said. “The emphasis is on detection, eradication and limiting the spread of the disease. In Florida, where HLB is widespread, monitoring ACP populations is essential to avoid reinfection after eradication of infected plants.”
The detection of the pest has led to widespread eradication of citrus trees in China, Brazil and the United States. “In Brazil as many as 46.2 million citrus trees, representing 26 percent of the currently planted trees, have been eradicated since the detection of HLB in 2004,” Leal said. “In Florida, HLB has caused severe losses to the citrus industry. This year's production loss is estimated to be about 28 million fewer boxes of oranges than in 2014-2015.”
The announcement of the discovery coincides with the 40th anniversary celebration of FUNDECITRUS in Araraquara, Sao Paolo. “I am delighted that Walter Leal accepted our challenge to work on this project as the lead investigator,” said Juliano Ayres, FUNDECITRUS director. “The combination of his work ethics and qualifications are unparalleled. And, he loves challenges.”
In response to the ACP invasion in California, the California Department of Food and Agriculture (CDFA) has launched an extensive monitoring program to track the distribution of the insect and disease. They check yellow sticky traps in both residential areas and commercial citrus groves, and also test psyllids and leaf samples for the presence of the pathogen.
Survey methods for ACP include visual inspections, sweep netting, and placement of yellow sticky traps in trees in citrus nurseries, commercial citrus-producing areas and residential properties throughout the state, according to the CDFA. They also place sticky traps in California fruit packing houses, specialty markets, retail stores and airports that receive such produce from areas known to be infested with ACP.
Since August 2008, ACP has now been detected in 26 of California's 58 counties: Alameda, Contra Costa, Fresno, Imperial, Kern, Kings, Los Angeles, Madera, Merced, Monterey, Orange, Placer, Riverside, San Benito, San Bernardino, San Diego, San Joaquin, San Luis Obispo, San Mateo, Santa Barbara, Santa Clara, Solano, Stanislaus, Tulare, Ventura, and Yolo. “The ACP has the potential to establish itself throughout California wherever citrus is grown,” the CDFA says on its website.
CDFA has set up a hotline at 1-800-491-1899 for residents to report suspicious insects or disease symptoms in their citrus trees.
California Department of Food and Agriculture (CDFA)
Save Our Citrus: Hotline Information
UC Agriculture and Natural Resources (UC ANR)
That's what postdoctoral scholar Bodil Cass of the Jay Rosenheim lab, University of California, Davis, will discuss at her seminar from 4:10 to 5 p.m., Wednesday, Oct. 25 in 122 Briggs Hall, UC Davis campus. Admission is free, and the seminar is open to all interested persons.
"Citrus is a major agricultural industry in California with a established integrated pest management (IPM) program," she says. "However, the IPM guidelines for citrus are based on years of experience and careful field research in navel oranges, and have not been updated to accommodate the recent dramatic increase in mandarin acreage in the San Joaquin Valley. We know oranges and mandarins are very different plants, but not which practices need to be modified to effectively control pests in mandarins."
Cass says that updating the IPM guidelines for "a new citrus species is a substantial challenge, given the scale and pace of citrus production. We are using a combination of ecoinformatics--data mining of pest management records provided by cooperating citrus growers--and field experiments to expand our understanding of the arthropod pest complex in California citrus. Analyses of the historical commercial data indicate that fork-tailed bush katydids, Scudderia furcata, which are a key pest in oranges, very rarely damage some species of mandarin. We are using field experiments to test hypotheses to explain this intriguing observation, and to determine whether katydids are indeed a pest at all in mandarins."
Basically, she uses "ecoinformatics, field and laboratory experiments, and molecular methods to understand insect pest and natural enemy ecology in agricultural systems."
A native of the state of Queensland, Australia, Cass is an accomplished scholar who holds several degrees:
- A bachelor's degree (2005) from the University of Queensland, where she graduated with high distinction and a dean's commendation for high achievement. (She completed the Advanced Studies Program in Science in 2005, and the Enhanced Studies Program in Chemistry, 2012)
- Honors Integrative Biology (2006), University of Queensland, with high distinction and valedictorian
- Doctorate in Interdisciplinary Program in Entomology and Insect and minor in ecology and evolutionary biology (2015) from the University of Arizona (4.0 GPA)
Cass joined the lab of Jay Rosenheim, UC Davis professor of entomology, in 2016, and also serves as an associate in the Center for Population Biology at UC Davis. She has published her work in Oecologia, Biological Journal of the Linnean Society, Microbial Ecology, Research in Microbiology, Science, and PLoS Genetics, PLoS Biology and PLoS Pathogens, among other journals.
A member of the Entomological Society of America (ESA), Cass delivered oral presentations at the ESA annual conferences in 2011 and 2015, and also at the 2016 International Congress of Entomology (ICE), co-chaired by UC Davis chemical ecologist Walter Leal and held last September in Orlando, Fla. She is the co-principal investigator of a 2017 grant awarded by the Citrus Research Board.
Cass has also won a number of fellowships and awards, including national awards in the P.E.O. International Peace Scholarship Program in both 2008 and 2009.
It's not at all surprising that one of the many awards she won in the beginning of her career was the "Smart Women, Smart State Award" in the undergraduate category, statewide competition (Queensland) in 2005.
Chemical ecologist and mosquito researcher Walter Leal, distinguished professor in the UC Davis Department of Molecular and Cellular Biology, and his lab collaborated with scientists in Recife to ask “Does Zika Virus Infection Affect Mosquito Response to Repellents?”
The work, funded by the National Institutes of Health, was published Feb. 16 in Scientific Reports of the journal Nature. The researchers used mosquitoes originating from colonies reared by UC Davis medical entomologist Anthony Cornel and from colonies in Recife.
“We used assays mimicking the human arm to test the mosquitoes infected with the Zika virus,” Leal said, “and we asked whether the Zika infection affects mosquito response to repellents.” They tested DEET and Picaridin, considered the top two mosquito repellents. “We discovered that DEET works better than Picaridin against the southern house mosquito, Culex quinquefasciatus, and the yellow fever mosquito, Aedes aegypti, whether infected or not.”
The researchers also found that old mosquitoes that already had a blood meal “were less sensitive to repellents,” said Leal, adding “It was not clear whether this was due to the virus, but mostly likely because of age. ”
“Lower doses--normally used in commercial products--work well for young mosquitoes,” Leal said, “but the old ones are the dangerous ones because they may have had a blood meal infected with virus and there was enough time for the virus to replicate in the mosquito body.”
“The bottom line: to prevent bites of infected mosquitoes, higher doses of repellent are needed. The data suggest that 30 percent DEET should be used. Lower doses may repel nuisance young mosquitoes, but not the dangerous, infected, old females.”
Leal, a native of Brazil, collaborates with Rosangela Barbosa and Constancia Ayres of the Oswaldo Cruz Foundation (FIOCRUZ-PE), Recife, Brazil. The work with infected mosquitoes was conducted at FIOCRUZ-PE.
Leal, Barbosa and Ayres co-authored the paper with Fangfang Zeng and Kaiming Tan, both of the Leal lab; and Rosângela M. R. Barbosa, Gabriel B. Faierstein, Marcelo H. S. Paiva, Duschinka R. D. Guedes, and Mônica M. Crespo, all of Brazil.
The World Health Organization (WHO) recommends that people traveling to or living in areas with Zika virus (ZIKV) outbreaks or epidemics adopt preventive measures, including the use of insect repellents, to reduce or eliminate mosquito bites. Prior to the Feb. 16 published research, it was not known whether the most widely repellents are effective against ZIKV-infected mosquitoes, “in part because of the ethical concerns related to exposing a human subject's arm to infected mosquitoes in the standard arm-in-cage assay,” the researchers pointed out.
They used a previously developed, human subject-free behavioral assay, which mimics a human subject to evaluate the top two recommended insect repellents.
Scientists isolated the Zika virus (ZIKV) nearly seven decades ago from a sentinel rhesus monkey while they were trying to unravel the cycle of sylvan yellow fever virus in Uganda..
According to the Centers for Disease Control and Prevention:
- Zika is spread mostly by the bite of an infected Aedes species mosquito (Ae. aegypti and Ae. albopictus). These mosquitoes bite during the day and night. The virus can also be sexually transmitted.
- Zika can be passed from a pregnant woman to her fetus. Infection during pregnancy can cause certain birth defects.
- There is no vaccine or medicine for Zika.
- Local mosquito-borne Zika virus transmission has been reported in the continental United States.
That's what UC Davis chemical ecologist Walter Leal, co-chair of the International Congress of Entomology (ICE 2016) recently held in Orlando, Fla., wrote in his newly published opinion piece, titled "Zika Mosquito Vectors: the Jury Is Still Out" in F1000 Research.
Leal, a mosquito researcher and distinguished professor in the UC Davis Department of Molecular and Cellular Biology, co-chaired ICE 2016 with Alvin Simmons, research entomologist with the United States Department of Agriculture's Agricultural Research Service (USDA/ARS), U.S. Vegetable Laboratory in Charleston, S.C.
The conference, themed "Entomology Without Borders," drew 6,682 delegates from 102 countries. The last time ICE met in the United States was four decades ago. The venue then: Washington, D.C.
One of the symposia at the Orlando meeting was the Zika Symposium, "which covered multiple aspects of the Zika epidemic, including epidemiology, sexual transmission, genetic tools for reducing transmission, and particularly vector competence," Leal wrote. "While there was a consensus among participants that the yellow fever mosquito, Aedes aegypti, is a vector of the Zika virus, there is growing evidence indicating that the range of mosquito vectors might be wider than anticipated. In particular, three independent groups from Canada, China, and Brazil presented and discussed laboratory and field data strongly suggesting that the southern house mosquito, Culex quinquefasciatus, also known as the common mosquito, is highly likely to be a vector in certain environments."
Leal based his opinion piece mainly on the current literature and the Zika Symposium at ICE 2016, which was organized by Constância Ayres, Oswaldo Cruz Foundation (FIOCRUZ-PE), Recife, Brazil (he collaborates with Ayres) and Adriana Costero, National Institutes of Health, Bethesda, Md. The speakers included worldwide experts from the United States, Brazil, China and Canada.
Ironically, the Zika virus wasn't a household word or in most entomologists' vocabulary when Leal and Simmons began planning the ICE 2016 meeting several years ago.
Now scores of researchers are tackling the Zika virus (ZIKV), first isolated first from a febrile monkey and later from the mosquito Aedes africanus. "ZIKV was isolated from humans for the first time in 1954 during an outbreak of jaundice suspected of being yellow fever," Leal recounted.
"During the discussion at the end of the symposium, the forum was opened for questions and comments," Leal wrote. Scott Ritchie of James Cook University, Australia, asked “Is anyone looking for the virus in birds?”
"This question captures the sentiment that both questions were thought provoking, and we still do not have all or many answers when it comes to ZIKV. Hopefully, we will be better prepared when convening in Finland for ICE 2020. Wouldn't it be wonderful to report in Helsinki that mosquito vector populations have been reduced or eliminated, the Zika and other epidemics were contained, vaccines have been made available, and entomologists are ready to further improve the human condition by tackling other problems than the Zika epidemic?"
In a groundbreaking discovery, a scientific team of Brazilians and Brazilian-born chemical ecologist Walter Leal of the University of California, Davis, has announced that the Zika virus has been detected in wild-caught Culex quinquefasciatus mosquitoes in Recife, the epicenter of the Zika epidemic.
Scientists from the Fiocruz Institute, Pernambuco, confirmed the discovery July 21. The detection could have widespread repercussions, as the Culex mosquitoes are more common and widespread than the yellow fever mosquito, Aedes aegypti, known as the primary carrier of the Zika virus.
Leal, who collaborates with Fiocruz Institute researcher Constancia Ayres in a National Institutes of Health-sponsored project on the investigation of Zika in the C. quinquefasciatus, said that the Brazilian lab earlier discovered that Culex had the capability of transmitting the virus. Although the scientists were able to infect the lab mosquitoes with the virus, they had not found the virus in wild-caught mosquitoes—until now.
“This could have major repercussions here in the United States and in other parts of the world,” said Leal, a distinguished professor in the UC Davis Department of Molecular and Cellular Biology who is co-chairing the International Congress of Entomology meeting Sept. 25-30 in Orlando, Fla. The conference is expected to draw some 7000 entomologists throughout the world.
Leal said more work needs to be done to see if Culex mosquitoes are playing a role in the current epidemic. In an interview July 21 with health reporter Jennifer Yang of the Toronto Star, Canada's largest daily, he commented: “It looks like there were more vectors than we thought, and this is one of them. We don't have to panic, but we have to know. And now that we know, we have to take care of the Culex.”
C. quinquefasciatus, known as the southern house mosquito and the vector of West Nile virus, thrives in temperate climates, including areas of California and the southern United States. In Brazil, it may be 20 times more common than A. aegypti, a day-biting mosquito that feeds primarily on humans. The mosquitoes differ widely: the yellow fever mosquito prefers urban environments and breeds in clean water, while the C. quinquefasciatus bites at night, breeds in dirty water and is commonly found in rural environments.
A. aegypti is already established in California; it has spread to at least seven counties since its discovery in Clovis, Fresno County, in June 2013, according to medical entomologist Anthony Cornel of the UC Davis Department of Entomology and Nematology and the UC Kearney Agricultural Research and Extension Center, Parlier.
The Zika virus, which can result in birth defects in pregnancy, can be transmitted through exposure to infected blood or sexual contact. The Centers for Disease Control estimates that between 400,000 and 1.3 million cases have been discovered across South, Central, and North America, where the disease was previously unknown.
Leal and a group of 18 students just hosted a Zika Public Awareness Symposium on May 26 at Giedt Hall, UC Davis campus. The podcast can be accessed at https://video.ucdavis.edu/media/Zika+Virus+Public+Awareness+Symposium/0_n3aupf5c