The Western citrus industry could be saved from its greatest challenges, Huanglongbing (HLB) disease and the Asian citrus psyllid (ACP), due to a small group of people who provide strong leadership, knowledge, and the passion to effectively manage these menaces.
One such person is University of Florida entomologist Michael Rogers based at the Citrus Research and Education Center in Lake Alfred, Fla.
Rogers lives more than 2,600 miles from the heart of the Western citrus industry in California yet his contributions to help California and Arizona citrus growers prepare for the pest-disease combo are multitudinous. Rogers is a highly sought-after speaker on the subject.
Florida, which boasts the nation’s largest citrus industry, first found the psyllid and HLB in 1998 and 2005, respectively. Rogers has openly shared the details in Florida’s fight against the insect and disease with the western citrus industry to help the West prepare for the battle against these citrus nemeses.
The first psyllid was found in California in 2008. The first and only identified case of HLB in the state was in a residential pummelo-lemon hybrid tree found in the Hacienda Heights area of Los Angeles in March. Tens of thousands of psyllids are currently in the L.A. Basin and are slowly spreading into some commercial citrus groves.
Psyllids at much lower numbers have been trapped in Arizona. HLB has not been found.
The psyllid is the primary vector of the bacterium Liberibacter which causes HLB, a global citrus disease. Every citrus tree infected with HLB around the world has died. Tree death has resulted in the removal of thousands of acres of citrus worldwide.
California citrus leaders concur that the psyllid and disease could wipe out the state’s citrus industry which could have happened also in Florida.
Rogers conducts research to better understand psyllid feeding behavior. His goal is to alter the feeding cycle to reduce or eliminate the insect’s ability to vector the bacterium to the tree.
In the West, Rogers is a knight in shining armor of sorts for his willingness to share the Sunshine State’s psyllid-HLB control successes and failures. The entomologist has spoken at numerous California citrus meetings.
Rogers shared his latest findings during Bayer CropScience’s Vegetable and Citrus Consultants Meeting held in San Diego in late July.
HIV of citrus world
“HLB is the HIV of the citrus world,” Rogers stated in his opening remarks. “HLB is considered one of the most significant citrus diseases worldwide due to difficulties in disease management,” Rogers said.
The ACP primarily feeds on new citrus tree flush for reproduction. The adult female psyllid lays eggs on new tree flush. The eggs hatch in three-to-five days. The psyllid life span is 30-to-60 days.
Rogers says HLB-related tree death is usually not caused by the disease alone. HLB disrupts the phloem system of citrus trees which causes a decline in tree health. This makes the trees more susceptible to other diseases, including phytophthora root rot and blight, which combined with HLB greatly speed up the demise of HLB-infected plants.
“The combination of HLB and phytophthora can take out a tree almost overnight,” Rogers said. “Young infected trees decline quickly. A tree can have a full canopy and several days later the leaves have all dropped and the tree is dead.”
HLB infection tree symptoms – yellowed leaves plus multi-colored smaller, off-sized, sour-tasting, unmarketable fruit – may first appear several years after the initial infection.
HLB-disease symptoms can mirror symptoms caused by nutrient deficiency, including low levels of manganese, zinc, and boron.
On average, Rogers says Florida citrus growers apply 8 to 12 insecticidal treatments annually in a standard, mature grove.
Why so many treatments? Foliar-applied insecticides have a short residual period and the insects constantly move, Rogers says. Psyllids generally fly until a suitable host is found. Laboratory windmill studies in Florida suggest the psyllid can sustain flight for more than an hour and fly more than one-and-a-half miles.
“These little guys can disperse over fairly long distances despite their very small, 3-to-4 millimeter adult body size.”
In Brazil, the pest and disease have caused substantial losses in commercial citrus. Producers apply insecticides up to 26 times annually for psyllid control.
Rogers also discussed UF psyllid research insecticide use and other methods to limit psyllid numbers and HLB transmission. In a 2006 UF research trial, psyllid nymphs were targeted during flush with diflubenzuron, fenpyroximate, petroleum oil, abamectin, and other products.
“This was our first mistake,” Rogers reflected. “We wanted to break the psyllid’s life cycle by knocking out all nymphs and allowing the adults to die of old age…When you get nymphs on new flush, even with the best coverage, you get 85 to 90 percent nymph control but you still miss some.”
Florida has placed a strong emphasis on the protection of young citrus trees. In a UF trial, three soil-applied systemic insecticides – Admire Pro, Platinum 75 SG, and Belay 50 WDG – were applied every six weeks to young trees. Belay can only be used in non-bearing citrus in Florida, Rogers says.
The trial results showed long-lasting systemic protection of the entire tree, control of the psyllid and leafminer pests, and the disruption of psyllid feeding which reduces HLB pathogen transmission.
Other areas of Rogers’ work include the application of integrated pest management (IPM) for psyllid control where natural insect predators kill the psyllid. The parasitoid Tamixaria radiatawas first introduced in Florida in 1999 after importation from Taiwan and Vietnam. The Tamixaria population was tracked from 2006 and 2007 to determine actual population numbers.
The Tamixaria level never reached above 20 percent except for a brief period at 50 percent. Overall, Rogers says the parasitoid failed to provide suitable biological control for effective psyllid control.
“There have not been many promising solutions in biological control when dealing with an insect-vectored pathogen; especially in citrus since it’s a perennial crop,” Rogers explained.
“We have trees in the field year-after-year which are constantly exposed to potential HLB infection by the psyllid,” the entomologist said. “The real threshold for psyllids is zero. Bio-control is better suited in general where some level of damage can be tolerated. We cannot tolerate any psyllids in citrus.”
Other Rogers trials suggest broad-spectrum insecticides applied to foliage are effective in adult psyllid control, Rogers says. These applications target adults before the flush cycle begins which eliminates reproduction. It is far easier, he says, to control a small population versus expanding numbers.
“We gain nearly a 100-percent death rate in adults with foliar applications with pyrethroid products,” Rogers said.
The complete list of the tested pyrethroid products and results is available in the Florida Citrus Pest Management Guide located on the Citrus Research Education website at www.crec.ifas.ufl.edu/extension/pest/.
Rogers also discussed Florida’s Citrus Health Management Areas (CHMA) established early last year. A CHMA (CHEE-MUH) is an area where citrus producers voluntarily coordinate psyllid control efforts to achieve higher levels of control. This is better than a grower can gain individually and also prevent insecticide resistance through the coordinated rotation of pesticide modes of action.
“Psyllid populations in CHMAs have decreased where coordinated insecticide applications have been implemented,” Rogers said. “Florida’s citrus industry has benefitted from everyone working together.”
Statewide, 38 CHMAs cover more than 486,000 acres of commercial citrus. Federal and state citrus experts scout about 6,000 blocks over a three-week period; equivalent to 106,000 acres per scouting cycle.
Additional CHMA information is available at www.flchma.org.