Finding an effective way to rid Florida of the Asian citrus psyllid—the vector for citrus greening or huang longbing—won’t be easy, but researchers are working overtime to find a solution.
“We simply don’t have a magic bullet or unconventional breakthrough right now,” says Lukasz Stelinski, assistant professor of entomology at the University of Florida’s Citrus Research and Education Center in Lake Alfred.
There are, however, a number of possibilities that researchers and growers are testing that seem promising. Some growers already have launched programs involving nutritional sprays, and they seem the most effective course of action to date. Other methods still under study include wasps, fungi, repellants and antifeedants.
“There’s a whole lot more that we don’t know than what we do know at this point about this disease,” says Bob Ebel, citrus physiologist at Institute of Food and Agricultural Sciences’s Southwest Florida Research and Education Center in Immokalee.
Infected psyllids that penetrate the circulatory system of a tree cause greening when they attempt to feed on the tree’s sugars, he says. In so doing, they inject bacteria that spread throughout the plant. Symptoms may not show up for 18 months.
Since bacteria disrupt the nutritional status of the tree, one of the most effective means of controlling citrus greening so far is by attacking the disease through nutritional sprays, he says.
“Nutritional sprays do reverse the symptoms of the disease,” Ebel says, but they don’t kill the bacteria.
Nutritional sprays delay the death of the trees, but no one knows for how long because growers and researchers just started the practice a few years ago.
Grower Maury Boyd, president of family-owned McKinnon Corp. in Winter Garden and president of South Lake Apopka Citrus Growers Association, has been using nutritional sprays for about four years.
“Basically, we’re putting a full range of nutritionals on when the trees are flushing—or growing,” he says.
Boyd makes a ground application three or four times a season and covers about 100,000 acres using a more dilute aerial spray every four or five weeks.
“We’re getting really good at it,” he says. “We’ve gone from disaster looking and turned it around. The [trees] have fruit on them and look pretty healthy.”
How long the practice will continue to work is unknown, but Boyd says he’s more optimistic now than he was in the past.
“Right now I’m thinking we can keep going for a long, long time,” he says.
Sorting things out
Bob Rouse, a citrus horticulturist at the IFAS Southwest Florida Research and Education Center, is trying to sort out the most effective components of the chemical cocktails Boyd is working with.
In 2009, he analyzed groups, such as phosphites, micronutrients and macronutrients, as well as systemic acquired resistance, or SAR, compounds. This year, he will study the effects of individual components of the cocktail.
So far, he’s learned that it’s imperative to include the nutrients, such as magnesium, manganese, zinc and molybdenum. Adding a phosphorus acid or an SAR compound containing salycilic acid—such as SAver—appears to provide an enhanced benefit, Rouse says.
Conducting a nutritionals spraying program on top of other methods of controlling greening can add $400 per acre, he says. But by not incurring the expense of pulling out trees, the process may be reduced to about $200 per acre.
“Nothing out there other than this approach is showing any kind of hope at this point other than old standard of removing infected trees,” Rouse says.
Researchers also are looking into other methods of controlling the disease and/or its vector.
Stelinski is working with repellants contained in plants’ defensive chemicals that are released when insects feed on them.
“We’re trying to exploit those defensive chemicals and formulate them into release devices that will repel the psyllid and, hopefully, act as pesticide alternatives,” he says.
They need to be applied mechanically, slowly over time because of the large acreage that would require coverage.
“You would want to saturate the air in a citrus grove with this chemical to affect the psyllid population by repelling them and/or preventing them from immigrating into the orchard,” Stelinski says.
Research is ongoing and has not yet resulted in any chemicals that are available or even practical, he says.
Stelinski also is looking at antifeedants—chemicals that can inhibit a psyllid’s feeding either by affecting the feeding muscles or the way it finds a location to insert its stylet into the plant.
These chemicals will not be direct toxins but would interrupt the feeding process and from that, acquisition of bacteria and transmission of bacteria.
“We’re hoping they will be critical barriers to the process of acquiring bacteria and transmitting bacteria,” he says.
Karen Stauderman, IFAS residential extension agent for Volusia County, is working with a pathogenic fungus called Isaria—previously known as Paecilomyces fumosoroseus or PFR—as a biological control.
She is testing four strains of the fungus with the citrus psyllid, and the results look promising.
“We have gotten control in the laboratory,” she says. “Our next step is to go into the greenhouse.”
The final step will be field testing.
Researchers have inoculated flytrap-like strips with the fungus. If the insects land on them or get any of the material on them, it spreads quickly throughout the colony, destroying the pests.
The fungus is found naturally on trees, so she does not expect its cost to be excessive.
A strain should be available for release to growers later this year.
Putting wasps to work
Phil Stansly, professor of entomology at the Southwest Florida Research and Education Center, wants to use wasps to take the sting out of the psyllid crisis.
He’s working with a gnat-sized parasitic wasp called Tamarixia radiate, which was first released in Florida in 2000.
The wasp is effective in fighting the psyllid, but it’s a challenge to maintain its population throughout the year—especially in the early spring.
“In many cases, we think it’s not surviving the winter because they’re doing so much spraying,” Stansly says. The plan is to conduct a mass release in the spring, right after spraying is conducted in late winter when the psyllid population is at its lowest.
There’s no question that the wasp can be an effective control for the psyllid.
“Every wasp capable of taking out a couple hundred psyllids,” he says. “They have the biological potential to do the job.”