LAKE ALFRED -- At a time when there's much talk about building walls, Nabil Killiny hopes to build a barrier that should be much more popular, unless you're a psyllid.
Killiny, an assistant professor of entomology, is researching ways to genetically disable the Asian citrus psyllid, the insect that hosts the deadly citrus greening bacteria, that would render it incapable of spreading the disease, or at least greatly inhibit its ability to do so. He works at the Citrus Research and Education Center in Lake Alfred.
Citrus greening poses the greatest threat to the Florida citrus industry in its history dating back to the late 1800s. The disease is responsible for a 70 percent reduction in the state's biggest citrus crop, oranges, since its discovery in 2005.
Since that time, greening has spread to virtually all of the state's 501,396 commercial citrus acres. As a result, Florida orange production has dropped to 76 million boxes this season compared to 242 million boxes in 2003-04 while the grapefruit harvest has fallen to 10.7 million boxes from 40.9 million over that time.
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Psyllids spread greening
by acquiring the bacteria when feeding on an infected plant, retaining the deadly microorganisms in its intestines, then transmitting bacteria through saliva when it feeds on a healthy tree, Killiny said.
"This is the worst kind of transmission," he said, because a psyllid provides the perfect vehicle for growing the bacteria in its body and can transmit the infection through it entire life, about two months.
"A single psyllid visits several trees in a day," he said.
Finding the right RNAi tool
Killiny works with a technique called "RNA interference," or RNAi. RNA stands for "ribonucleic acid," a chemical present in all living cells that carries genetic information.
Scientists can use RNAi to inhibit the expression of individual genes in a living organism that affect, for example, wing formation or the psyllid's ability to retain the bacteria in its saliva.
Killiny has spent the past two years identifying specific genes involved in the interaction between the psyllid and greening bacteria, he said, and he has assembled at least 25 RNAi candidates that could alter those genes.
He has already demonstrated RNAi can alter a psyllid's wing formation, rendering it unable to fly and spread bacteria, Killiny said. He has also identified RNAi could alter the psyllid's ability to host greening bacteria, to mate and to suppress the insect's ability to develop a resistance to pesticides.
Killiny has also developed "target" citrus trees that carry these RNAi strands, he said, and the psyllids would acquire them when they feed. Another RNAi candidate makes those trees more attractive to psyllids.
Although introduction of the RNAi does not affect the citrus tree's production nor alter its fruit, Killiny's strategy calls for planting the target trees only around a grove's border, he said. If the RNAi works, that would eliminate the psyllid's ability to spread the disease to interior trees.
Fruit from the target trees would not be harvested for commercial production to avoid a possible consumer backlash, Killiny said.
Other insects feeding on those trees would not be affected because the RNAi candidates affect genes found only in the psyllid, he said. That's one reason the research has taken so long.
"It's very hard to find a gene in a psyllid that does not work in another insect," he said.
Killiny acknowledged a successful RNAi would not be 100 percent effective, just as no pesticide will destroy 100 percent of insects.
For that reason, RNA would become another important tool in an "integrated pest management" strategy, Killiny said. Integrated pest management uses a variety of tools, including pesticides and beneficial insects that prey on harmful insects, to control pathogen levels.
Many research projects
Killiny said he hopes to have a successful RNAi tool for Florida growers within five years. When that would become widely available would depend upon state and federal regulatory approval.
Killiny's RNAi research is one of a number of similar projects aimed at finding new solutions against greening, said Harold Browning, the chief operating officer at the Citrus Research and Development Foundation Inc. in Lake Alfred, which is funding much of that research.
"It's one of the newer areas of molecular biology," said Browning, also an entomologist who has worked on integrated pest management. "We're relying pretty heavily on pesticides for psyllid suppression. This would be another important tool to add to integrated pest management."
Browning agreed with Killiny that RNAi has a greater chance of avoiding a consumer backlash against genetically modified organisms (GMOs).
RNAi does not change an organism's genetic makeup, only RNA strands that are "ubiquitous in nature," Browning said.
"It looks like it could have some advantages over standard genetic engineering," he said. "It's a step below introducing a whole new gene."