For years, citrus growers have feared that abandoned groves provided refuge for the Asian citrus psyllid, an invasive insect that transmits citrus greening — now, University of Florida researchers say they were right.

A study published in the current issue of the Journal of Economic Entomology shows that the psyllid not only survives in abandoned groves, it often travels to commercially active groves nearby, bringing along the bacterium responsible for the disease.

First detected in Florida in 2005, greening is incurable and fatal to citrus trees. It is considered the biggest threat to the state’s $9 billion citrus industry. Asian citrus psyllids pick up the greening bacterium by feeding on sap from infected trees and later transmit the pathogen while feeding on healthy trees.

The results underscore the need for landowners to remove or destroy unmanaged trees, something the state is urging, said entomologist Lukasz Stelinski, an assistant professor with UF’s Institute of Food and Agricultural Sciences and one of the study’s authors.

“There was very much anecdotal evidence that these abandoned areas are harboring citrus psyllids,” Stelinski said. “It’s just one of those things that had to be confirmed.”

An estimated 140,000 acres of citrus groves go untended in Florida, according to the U.S. Department of Agriculture. The state has an estimated 550,000 acres of active groves.

Much of the abandoned grove acreage is believed to be owned by developers or investors who expected to clear the land rather than produce citrus, Stelinski said. Consequently, the owners never provided basic management such as pest control.

In the study, Stelinski and colleagues from UF’s Citrus Research and Education Center in Lake Alfred sprayed non-toxic “marker” chemicals on trees in seven abandoned groves, where psyllids might be present. They also placed insect traps in nearby commercially active groves.

Finally found proof

When the traps were checked, researchers found psyllids bearing the marker chemicals, indicating the pests had traveled from abandoned groves to active ones. Laboratory analysis revealed that some of these psyllids carried the bacterium that causes greening disease.

Researchers also took leaf samples from citrus trees and found the presence of greening was about the same in abandoned and managed groves. Other members of the research team were Siddharth Tiwari, Hannah Lewis-Rosenblum and Kirsten Pelz-Stelinski, all with UF’s entomology and nematology department.

Stelinski added that as-yet unpublished findings showed the insects could fly up to 1.25 miles in 10 days, and could probably travel farther over time.

“So you don’t necessarily need to be right next to an abandoned grove to be at risk,” he said.

Currently, the state is asking local property appraisers to urge landowners to remove or destroy untended citrus trees by offering tax incentives to do so, said Mike Sparks, executive vice-president and chief executive officer of Florida Citrus Mutual, Florida’s largest citrus grower trade organization.

“Even though we’ve had some success, it’s not nearly enough,” Sparks said. “This study could help us mold public policy.”

Sparks said he hopes that the UF research will persuade state and local officials to take further action to reduce the amount of abandoned citrus acreage.

“We have a $9 billion industry and 76,000 jobs at stake,” Sparks said. “Abandoned groves are putting all of that at risk and policymakers need to know that.”