May 3, 2013
A major U.S. agricultural research collaboration is under way, funded in part through a $9 million USDA grant, which has the Asian citrus psyllid (ACP) pest directly in the crosshairs.
Researchers are exploring ways to physically alter the psyllid as one way to rid the U.S citrus industry from the psyllid-transmitted pathogen Candidatus Liberibacter asiaticus which causes the disease Huanglongbing (HLB), also known as citrus greening.
Currently, the psyllid takes up Liberibacter bacteria into its body during feeding, where it reproduces and transfers the bacteria to other trees as the insect feeds on leaves and branches.
The end result is an HLB-infected tree. Every tree infected by HLB worldwide has died.
HLB has devastated commercial and residential citrus trees in Florida. The psyllid appears first before the disease is found.
Multi-state and institution research efforts funded by the USDA grant hope to uncover multiple ways to prevent the insect - less than one-tenth-inch long in size – from transferring bacteria from an infected tree to a non-infected tree.
One of the research groups involved in this cooperative effort is located at the University of Arizona (UA) in Tucson.
Perched atop the College of Agriculture’s fourth floor in Forbes Hall, UA virologist and vector biologist Judith Brown and her research team utilize complex technology to determine ways to prevent establishment, multiplication, and transmission processes which lead to psyllid transmission of the bacteria.
“The ultimate goal is to eliminate the psyllid’s ability to harbor the bacterium and transmit it to the tree,” Brown said.
Brown’s team is drawing on six years of research on the related potato psyllid, plus more than three years of research already conducted on the citrus psyllid.
The project’s plan of attack is to genetically alter the insect to produce small interfering ribonucleic acids(RNAs) which attack genes crucial to bacterial survival in the psyllid.
Going for the gut
“We are working to develop a modified Asian citrus psyllid with biological changes to the insect’s gut and salivary glands to halt bacterial uptake,” Brown explained.
The Brown team has learned that large numbers of Liberibacter bacteria pass into the insect’s alimentary canal (gut). The bacteria cause lesions in the gut. This could be tied to an enzyme or toxin generated by the bacteria or the psyllid.
It appears that as the gut wall breaks down, bacteria enter the blood stream and move to the salivary glands. As an infected psyllid feeds on the citrus tree, the bacteria are released causing Liberibacter infection.
Several years can pass after the initial infection before the tree shows visual symptoms, which include mottled leaves and tree yellowing. Fruit from diseased trees is pithy, tastes bitter, and is unmarketable.
Brown is confident her research team will be successful.
“We know which organs to target and how to feed the RNA inside the psyllid. I believe we will find answers to stop the problem at the psyllid level.”
Brown hopes to have an early modified insect to test in the next four or five years.
Brown says gains in past research projects have formed the foundation for current research work. The knowledge gained in this current wave of psyllid research by her and other researchers can be extrapolated for future vector management issues.
“We are hoping this new generation of knowledge and applications will open the door for important improvements in how we study and manage pest complex problems in all crops,” Brown said.
HLB was first found in the U.S. in 2005 in Florida which has led to millions of dollars in economic losses. Two cases of HLB were found in Texas citrus groves last year.
A single case of HLB was found in California in Spring 2012 in a residential citrus tree near Los Angeles. The tree was quickly removed and destroyed. The Liberibacter pathogen and the disease it causes have not been detected in California commercial citrus.
Likewise, the psyllid has been found in several southern Arizona locations. HLB has not been found.
The western U.S. is the last citrus-growing area in the world to get HLB. Some California citrus leaders contend a severe outbreak of HLB in California groves could wipe out the state’s $2 billion commercial citrus industry. The same could be the case for Arizona’s $37 million citrus industry.
Thousands of psyllids are in the Los Angeles area and moving into commercial citrus. Every psyllid found in commercial citrus has tested negative for HLB.
Until now, ACP-HLB research has focused on a variety of possible HLB solutions, including the psyllid itself, the development of HLB-resistant tree varieties, parasitic wasps, and other methods. The USDA funded research strictly targets the psyllid.
The grant-funded research is underway in Arizona, California, Florida, and Texas. The UA team, working collaboratively with the David Gang lab at Washington State University, will receive about $870,000 in grant funds.
MaryLou Polek, vice-president of science and technology with the California Citrus Research Board, is very involved in the fight against the psyllid and disease.
Polek helped write the exhaustive 900-page USDA grant application, along with project director Tom Turpen of Florida, and other researchers.
“I am confident that though cooperation and collaborative research that viable solutions will be found within the next 3-5 years,” Polek said.
Other research scientists searching for psyllid answers include:
Bruce Hay - California Technical Institute: Chromosomal driver system;
Bryce Falk - University of California, Davis: Viral driver system;
Kirsten Peltz-Stelinski - University of Florida: Bacterial driver system;
Robert Shatters – USDA-ARS: Effector mechanism - Genetic sequence library approach;
Joseph Patt, USDA-ARS: Rear, release, and monitor;
Neil McRoberts, University of California, Davis: Socio-economics/modeling;
Elizabeth Grafton Cardwell – University of California, Riverside: Public outreach/Extension.
In addition to saving the citrus industry, another reason to solve the psyllid-disease issue is to reduce the use of pesticides currently applied to citrus trees for psyllid control.
In Brazil, citrus growers can spray trees every two weeks for psyllid control. In Florida, 10 or more sprays annually in some mature groves are common to gain control.
In California and Arizona, insecticidal applications have focused on psyllids found primarily in residential citrus trees.
“The solution to the ACP-HLB problem will not be one silver bullet but rather a combination of strategies and methods,” Polek said.
The results of this cooperative project will be one aspect of the biological control strategy. Psyllids which cannot transmit the lethal bacteria will be released in urban areas along with parasitic wasps which will prevent the spread of the disease.
People will still be able to grow citrus in yards without the continuous application of conventional pesticides, Polek says.
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