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)
Think ACP. Think HLB. Think ASAP.
ACP spreads the killer huanglongbing (HLB) disease, also called "citrus greening," which is threatening California's citrus industry and residential landscapes. Some 60 percent of California homeowners have at least one citrus tree.
The brown-mottled, aphid-sized pest (Diaphorina citri), a native of Pakistan, was first detected in California in 2008. It earlier wreaked havoc in Florida's citrus industry. Due to HLB and citrus canker, the Florida citrus industry has lost nearly 50 percent of its citrus production in the past 10 years, according to the national Citrus Research Board.
California has more to lose. The Golden State is the No. 1 economic citrus state in the nation, ranking first in the U.S. in terms of economic value and second (after Florida) in terms of production, says the national Citrus Research Board. "California produces approximately 80 percent of the nation's fresh fruit citrus and is the country's main source (80 percent) of fresh-market oranges (Florida grows oranges mainly for juice)."
It's crucial to check for signs of this pest now--right now--because of the new leaf growth (flush). The young, tender leaves are perfect for psyllids. Tell-tale signs of psyllid presence include distorted new leaves and stems, waxy deposits, honeydew and sooty molds.
Checking for psyllids is our first line of defense.
“We encourage home citrus growers and farmers to go out with a magnifying glass or hand lens and look closely at the new growth,” said Beth Grafton-Cardwell, UC Agriculture and Natural Resources (UC ANR) citrus entomologist. “Look for the various stages of the psyllid – small yellow eggs, sesame-seed sized yellow ACP young with curly white tubules, or aphid-like adults that perch with their hind quarters angled up.”
Photos of the Asian citrus psyllids and the life stages are posted on the UC ANR website at http://ucanr.edu/acp. If you find signs of this insect, you're urged to telephone the California Food and Agriculture (CDFA) Exotic Pest Hotline at (800) 491-1899.
Yellow mottling on the leaves may be the first sign your citrus tree is infected with the HLB. Other indications are sour misshapen fruit. If your tree has HLB, that's a death sentence. It will die. You cannot save it.
CDFA officials recently removed a few HLB-infected trees in urban Los Angeles County.
“In California, we are working hard to keep the population of ACP as low as possible until researchers can find a cure for the disease,” Grafton-Cardwell said. “We need the help of citrus farmers and home gardeners.”
A new UC ANR ACP website for citrus growers and homeowners provides help in finding the pest and what to do next. The site, spearheaded by Grafton-Cardwell, includes an interactive map so viewers can locate where the psyllid is established, and areas being targeted.
The website outlines biological control efforts that are underway, and directions for insecticidal control, if it is needed. An online calculator allows farmers and homeowners to determine their potential costs for using insecticides.
Additional measures and precautions are advised:
- When planting new citrus trees, purchase the trees only from reputable nurseries. Do not accept tree cuttings or budwood from neighbors, friends or relatives. HLB can be spread by grafting.
- After pruning or cutting down a citrus tree, dry out the green waste or double bag it to ensure that live psyllids won't hitch a ride to another region and spread HLB from tree to tree.
- Control ants in and near citrus trees with bait stations. Scientists have released natural enemies of ACP in Southern California to help keep the pest in check. However, ants will protect the psyllids from the natural enemies. Ants feed on honeydew.
- Learn more about the Asian citrus psyllid and huanglongbing disease on UC ANR's Statewide Integrated Pest Management website.
- Assist in the control of ACP by supporting CDFA insecticide treatments on your citrus or treating the citrus yourself when psyllids are present.
- Support the removal of HLB-infected trees.
HLB has seriously impacted citrus production in Brazil, India, Asia, the Arabian Peninsula, Africa and now Florida. We don't want California added to that list.
The public, says entomologist Kris Godfrey, needs to become more aware of the threat of invasive species.
And, she adds, we need to educate people and organizations about the incoming pests and pests that are already here.
With that in mind, she's coordinating an all-day conference, themed “Educating the Public about New Invasive Species Threatening California’s Plant Ecosystems,” set Tuesday, April 24 in the UC Davis Conference Center.
The goal? "To bring together biologists, social scientists, and communication experts to discuss how to educate all segments of society about the threat of invasive species and how to assist in their exclusion and detection,” said Godfrey, an associate project scientist with the Contained Research Facility at UC Davis.
Godfrey, formerly with the California Department of Food and Agriculture, says conference attendees will learn about developing and delivering effective and consistent messages about invasive species to a variety of audiences. They also will learn how to access the resources available to conduct effective outreach programs on invasive species.
Among the pests to be discussed: the golden spotted oak borer, Asian citrus psyllid, European grapevine moth, Japanese dodder, sudden oak death, and zebra and quagga mussels.
Some of the speakers:
- "Predicting the Next Pest Invaders and How To Prevent Their Introduction," Joseph DiTomaso, UC Davis Department of Plant Sciences
- “New Pest Plants,” Doug Johnson, California Invasive Plant Council, Berkeley
- “New Arthropod Pests,” Kevin Hoffman, California Department of Food and Agriculture, Sacramento
- ”New Plant Pathogens,” Richard Bostock, UC Davis Department of Plant Pathology
- ”Zebra and Quagga Mussels,” Ted Grosholz, UC Davis Department of Environmental Science and Policy
- “European Grapevine Moth,” Lucia Varela, UC Cooperative Extension, Sonoma County
- ”Asian Citrus Psyllid/Huanglongbing,” Beth Grafton-Cardwell, UC Riverside Department of Entomology
- “Sudden Oak Death and Buy-Where-You-Burn Campaigns,” Janice Alexander, UC Cooperative Extension, Marin County, Novato.
- “Japanese Dodder, “Ramona Saunders, Sacramento County Agricultural Commissioner’s Office
- “Newspaper Perspective,” Matt Weiser, Sacramento Bee.
More information, including the full list of speakers, is available at http://crf.ucdavis.edu. Conference registration is taking place online at https://registration.ucdavis.edu. Bin at right shows Huanglongbing (HLB) symptoms caused by Asian citrus psyllid. At left: normal fruit.(Photo by S. Halbert, Florida Department of Agriculture and Consumer Services). For additional information, contact Kris Godfrey at email@example.com or (530) 754 2104.
The conference, supported with a grant from the UC Davis College of Agricultural and Environmental Sciences’ 2011 Spring Programmatic Initiative, is a cooperative project of the UC Davis College of Agricultural and Environmental Sciences, the UC Davis departments of Plant Pathology, Entomology, Plant Sciences, and Food Science and Technology, the California Center for Urban Horticulture at UC Davis, the UC Statewide Integrated Pest Management Program, and the UC Riverside Department of Entomology.
A side note: take a look at the oranges below and see the damage that the Asian citrus psyllid causes. The bin on the right shows small lopsided fruit, typical of HLB infection. The bin on the left shows normal-sized fruit. These photos were taken in Florida.