As tiny as the point of a pencil, the Asian citrus psyllid is creating havoc among American citrus growers.
You’d never guess this common-looking insect, with mottled gray-brown wings, would be responsible for loss of one-third of Florida’s citrus trees and quarantines of citrus in eleven states and several U.S. territories.
Scientists have not found a “silver bullet” to stop Asian citrus psyllids from spreading huanglongbing disease (HLB), commonly called citrus greening. Once a tree is infected, it eventually dies.
The Center for Invasive Species, UC Davis, lists three forms of citrus greening. The African form produces symptoms only under cool conditions and is transmitted by the African citrus psyllid, Trioza erytrea. The Asian form prefers warmer conditions and is transmitted by the Asian citrus psyllid, Diaphorina citri. A third American form transmitted by the Asian citrus psyllid has been discovered in Brazil.
HLB disease causes misshapen citrus fruit with mottled yellow and green coloring. The Center for Invasive Species, UC Davis, describes psyllids as “flying syringes carrying HLB in their needle-like mouthparts.”
When Asian citrus psyllids feed on young leaves and stems of an infected tree, they ingest HLB bacteria. When they feed on a healthy tree, HLB bacteria are transmitted into the healthy tree.
The Asian citrus psyllid originated in southern Asia and by the 1940s had traveled to South America. Over the years, it steadily moved northward and was first detected in Florida in 1998, with HLB disease found in 2005. By 2008, the psyllid had reached California, with HLB disease found in 2012. In 2009, a psyllid was found in San Luis, Ariz., causing portions of Yuma County to be quarantined. In 2013, psyllids were found in Lake Havasu City, Ariz., causing more land to be quarantined.
The U.S. Census Bureau’s statistics for 2012 list the major producers of citrus in the U.S. as Florida, California, Texas, and Arizona; with California and Arizona ranking No. 1 and No. 2 for lemon production. All four states have found psyllids on citrus trees, with Texas, Florida and California also finding HLB disease.
When asked about quarantine locations in Arizona, Dr. Glen Wright, citrus specialist at the University of Arizona Agriculture Research Center in Yuma, said, “At present, parts of Yuma County, La Paz County and all of Mohave County are under quarantine. Citrus trees in quarantined areas must have a tag certifying they are disease free before they can be moved. Since commercial growers are the only ones who can have trees certified, the general public cannot move trees or fruit out of quarantined areas. The good news is that, so far, we have been able to keep HLB disease out of Arizona.”
Both private and university research groups are working diligently to characterize the HLB bacteria in order to develop detection methods and control of the disease and the psyllid.
“The use of a tiny, parasitic wasp native to India (Tamarixia radiata) is being tried in Lake Havasu City. The wasps are released, and females lay single eggs on psyllid nymphs. When the eggs hatch, the larvae feed on the nymphs. Adult wasps also feed on psyllid nymphs, with each wasp killing as many as 500. This method of biological control is not as effective as insecticides on major infestations and normally cannot be used in conjunction with insecticides,” John Caravetta, associate director of Plant Services, Arizona Department of Agriculture, stated.
One research study found nutritional deficiencies in citrus make them more susceptible to HLB. Trees with HLB had 65 percent lower amounts of phosphorus than healthy trees.
“Some citrus growers are using foliar applications of micronutrients, along with liquid fertilizers containing nitrogen, potassium and phosphorus, to improve their trees’ resistance to HLB,” Caravetta said.
Several Agriscience companies have developed, or are developing, foliar-applied products to control psyllids. The problem with insecticides is that over time, if the same products are used continuously, the psyllids seem to build up a resistance. Therefore, use of a variety of insecticides is necessary, but costly.
“California researchers are working on a PCR test, a method of cellular testing, to identify infected trees by RNA (ribonucleic acid) markers associated with HLB disease. Because it takes time for HLB to spread throughout a tree, this type of testing may miss some infected trees. Infected trees typically die within 5-8 years, but disease symptoms may take as long as 2 years to become obvious,” Dr. Wright said.
Carolyn Slupsky and her colleagues at the Department of Nutrition, Food Science and Technology, UC Davis, tried a novel approach to detect HLB disease. They studied the amino acid composition of orange juice from healthy trees, those with HLB but not showing symptoms and from infected trees with visible symptoms. Amino acid compositions were strikingly different for each type of juice tested. This may be the best answer yet for early detection of HLB-infected trees.
Slupsky explained that without the necessary amino acids, infected trees cannot defend themselves against infections, such as HLB. Perhaps this is the first step in finding the “silver bullet” everyone is looking for.
The seriousness of HLB is shown by statistics from the Center for Invasive Species, UC Davis. Between 2005, when HLB disease entered Florida, and 2009, there was a 10% loss of citrus trees (60,000 acres), a loss of 6,600 jobs, $1.3 billion in lost revenues to growers and $3.6 billion in lost economic activity associated with the citrus industry.
Since 2009, HLB disease has continued to ravage Florida’s citrus groves. Arizona does not want to follow in Florida’s footsteps.
“We continually monitor for psyllids in Arizona and are using stringent eradication measures when any are found. We are hoping our diligence in staying on top of the situation will keep Arizona free of HLB disease,” Caravetta said.
******************************************
No Silver Bullet to Prevent Spread of Asian Citrus Psyllid,
