If you like or grow citrus, you ought to be worried about the worldwide threat of the deadly citrus greening disease (Huanglongbing or HLB) caused by infected Asian citrus psyllids (ACP). The global economic toll is already estimated in the billions. 

The insect, a native of Asia, was first detected in the United States (Palm Beach County, Florida) in 1998, and California (San Diego County) in 2008. HLB, the bacterial disease that it transmits, was first detected in Florida in 2005, and in California (Hacienda Heights, Los Angeles County), 2012. 

 “The ACP has the potential to establish itself throughout California wherever citrus is grown,” according to the California Department of Food and Agriculture (CDFA).

Now an international team of scientists, led by chemical ecologist Walter Leal of the University of California, Davis, has discovered a new lure or male attractant that's more efficient and effective than the current trap surveillance system of yellow sticky traps. 

“Our newly discovered lure captures an average of three times more ACP than the current trapping system, the sticky yellow traps,” said Leal, a distinguished professor in the Department of Molecular and Cell Biology and a former chair of the Department of Entomology and Nematology. “And it works in areas with low population densities. This is particularly important in California, because HLB is already established in urban areas and, therefore, trap surveillance systems are at a premium.”

The research,  “Laboratory and Field Evaluation of Acetic Acid-Based Lures for Male Asian Citrus Psyllid, Diaphorina citri,  is published today (Sept. 9) in Scientific Reports. It can be accessed free online at https://www.nature.com/articles/s41598-019-49469-3.

The 16-member team, which includes scientists from Brazil and Costa Rica and  Benjamin Lehan of  California State Polytechnic University, Pomona, wrote in their abstract: “Lures are much needed for improving ACP trapping systems for monitoring populations and surveillance. Previously, we have identified acetic acid as a putative sex pheromone and measured formic acid- and propionic acid-elicited robust electroantennographic responses. We have now thoroughly examined in indoor behavioral assays (4-way olfactometer) and field tests the feasibility of these three semiochemicals as potential lures for trapping ACP. Formic acid, acetic acid, and propionic acid at appropriate doses are male-specific attractants and suitable lures for ACP traps, but they do not act synergistically.”

“An acetic acid-based homemade lure, prepared by impregnating the attractant in a polymer, was active for a day,” they wrote. “A newly developed slow-release formulation had equal performance but lasted longer, thus leading to an important improvement in ACP trap capture at low population densities.”

Native to Asia, ACP is a mottled brown insect about the size of an aphid. It 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 is the yellowing of leaves on an individual limb or in a sector of a tree's canopy. Diseasesymptoms also include twig dieback, yellow leaf veins, and green misshapen fruit. 

“The disease has ravaged the citrus industry in China and Brazil,” said Leal, a native of Brazil. “In Brazil, about one-fourth of the citrus trees in the state of São Paulo has been eradicated since 2004 as part of an HLB control strategy. Florida has also sustained severe losses. 

Now the disease threatens California's citrus growers “Of note, 1,100 findings of HLB in urban, but not in commercial orchards, suggest that the disease is already established in urban areas in California,” the scientists wrote. “Monitoring ACP populations is essential for integrated vector management and, more importantly, for surveillance. One of the challenges for abatement personnel in areas of low ACP densities is to capture the vector to determine infection status so that control strategies can be implemented before HLB is spread. Therefore, the development of trapping systems is at a premium, particularly the discovery of lures for enhancing ACP captures in areas of low populations.”

Earlier, an international team of scientists led by Leal identified the sex pheromone of the Asian citrus psyllid. Leal, a fellow of the Entomological Society of America and the Entomological Society of Brazil, announced the discovery, encompassing six years of research, at the 10th Annual Brazilian Meeting of Chemical Ecology in Sao Paulo in December 2017.  (See Bug Squad blog)

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.

In response to the ACP invasion in California, the 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.

CDFA has set up a hotline at (1-800-491-1899) for residents to report suspicious insects or disease symptoms in their citrus trees.

Resources:

• Sex Pheromone Discovery: Walter Leal-Led Team
• UC IPM: Website on Asian Citrus Psyllid
• CDFA: Pest Profile of Asian Citrus Psyllid
• UC ANR: Asian Citrus Psyllid

A major citrus pest may experience a “Bah, Humbug!” kind of year.

If all goes as planned, UC Davis chemical ecologist Walter Leal's discovery of the sex pheromone of the Asian citrus psyllid--which spreads the deadly citrus greening disease, Huanglongbing (HLB)--may result in the insect version of “The Fatal Attraction.”

“We are now working on a formulation to be used in traps,” Walter Leal said this week. “This might take a year, but hopefully will be ready before the flight season in California.”

Leal, who led an international team of scientists in the six-year research project, announced the discovery Dec. 5 at the 10th Annual Brazilian Meeting of Chemical Ecology in Sao Paulo. Leal is a native of Brazil and a fellow of both the Entomological Society of America and the Entomological Society of Brazil.

“The Asian citrus psyllid (ACP) is a major threat to the multibillion dollar citrus industry in the United States,” said UC Cooperative Extension advisor Surendra Dara of San Luis Obispo and Santa Barbara counties. “When an insect pest vectors a deadly disease, the threat is more serious and ACP being an invasive pest made its management even more challenging.  Discovery of a sex pheromone by Dr. Leal's team is a major breakthrough not just for managing a dangerous invasive pest, but also a significant contribution to environmental sustainability.  I envision this pheromone becoming a clean, green, mean weapon in the IPM arsenal against ACP.”

The citrus greening disease, which has wreaked havoc in Florida, now threatens the California citrus industry. The mottled brown insect, Diaphorina citri, is now found in 26 of the state's 58 counties, but the disease itself has not been established.

Joel Nelsen, president of the California Citrus Mutual, called the discovery “exciting news” and a “first step toward protecting the citrus industry. We're hoping that the next steps come in time to protect thousands of citrus growers around the country.”

“Let's move forward fast and furious,” he said, noting that the citrus industry spends millions to support the research community.

The Leal-led research team was funded solely by Fund for Citrus Protection (FUNDECITRUS).

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, hailed the discovery as a “significant breakthrough in preventing the spread of this serious citrus insect, and may offer a less toxic method for its control.” He was not involved in the study. 

Kris Godfrey, associate project scientist at the UC Davis Contained Research Facility and formerly with the California Department of Food and Agriculture, agrees that better detection traps are needed. “I hope that Dr. Leal's discovery of an Asian citrus psyllid pheromone will provide the improvement in detection trapping that is needed by anyone trying to manage this insect and slow the spread of huanglongbing, a devastating disease of citrus vectored by the Asian citrus psyllid.”

Leal's Brazilian liaison, Haroldo Xavier Linhares Volpe of the Fund for Citrus Protection (FUNDECITRUS) from the state of Sao Paulo, pointed out that the discovery could “increase the ACP catches using lures with the attractive compound, leading to a more assertive, precise monitoring and could promote an early detection of ACP.”

“With a more accurate detection, we can adopt ACP integrated pest management (IPM) tools as soon as possible, avoiding or minimizing HLB spread.” He added that the lures should help decrease the population.

“However, researchers need to test all the management strategies before they are adopted,” Volpe said. “Formulation techniques need to be investigated to determine lures that release the compounds for a long time and at doses that attract the insect.”

Although ACP is present in California, the disease itself has not been established, Leal emphasized. “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.” California now leads the nation in citrus production, surpassing Florida, for the first time in 70 years.

Currently growers are using yellow sticky traps to detect the insect and to monitor the population. Said Leal: “Efficient lures 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.

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.

Native to Asia, ACP 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. 

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.

(Editor's Note: The research was published in the Jan. 11 edition of the journal Scientific Reports.)