Plant pest prompts citrus quarantine

12/06/2018

The discovery last month of an aphid-sized insect could spell peril for citrus plants in Marin County. The Asian citrus psyllid, which was found in a trap in northeastern Novato on Nov. 22, is one of the world’s most dangerous threats to citrus flora and related plants such as curry leaf trees. 

On its own, the insect can wreak havoc to a plant’s new growth by sucking the liquid straight out of it. But the pest’s true menace comes from a disease it can spread called greening disease, or Huanglongbing, for which there is no known cure. 

The psyllid found in Novato died before it could be sent to a lab for testing that would have determined whether it carried the bacteria. But county, state and federal agencies are taking no chances. Although Marin has no commercial citrus operations, an estimated 60 percent of homeowners have some kind of citrus plant. 

Agricultural officials have placed insect detection traps and will treat all citrus plants within 50 meters of where the insect was found, hoping to learn whether it was alone or part of a larger infestation. The California Department of Food and Agriculture established a quarantine on the movement of all citrus plants and their detritus since the discovery, and homeowners are not to move citrus trees, plant parts or foliage out of the Novato area. (The department notes that an exception may be made for nursery stock grown in U.S.D.A.-approved structures specially designed to keep the insect out). 

Quarantines are also in place in 26 other counties in California, from Solano to San Diego. Commercial citrus fruits can still be transported, provided they are cleaned of all leaves and stems prior to the move. 

Asian citrus psyllids are three to four millimeters long, with light brown heads and mottled brown bodies. Adults can live for several months. Their wings are covered in a whitish, waxy secretion, and the young leave that secretion behind on leaves—one indicator of the insect’s presence. After the disease takes hold, a plant’s leaves become yellow and mottled and its fruit becomes small, misshapen and bitter (though safe for human consumption). Once a tree is infected with the bacteria, it cannot spread the disease to other trees unless it is grafted; it is the psyllid itself that takes its bacteria to other plants. 

“It’s a huge concern,” said Dr. Beth Grafton-Cardwell, a research etymologist at the University of California, Riverside, and director of the Lindcove Research & Extension Center. “It can live anywhere citrus can live. That means it’s throughout Florida, Mexico. It’s made its way to Texas, arrived in Arizona and California. In three years, we’ve gone from 20 infected trees to over 900 in SoCal.” 

The insect was first discovered in California in 2008 and quickly spread throughout the south. Efforts to eradicate it have included releasing a wasp—Tamarixia, a natural predator—in residential areas, and spraying pesticides in commercial orchards. Although neither of those methods have proven to be completely effective, “that’s the best we can do for the moment,” Dr. Grafton-Cardwell said. 

Further north, the bug—but not greening disease—has been found in both San Francisco and Solano Counties. 

In Novato, the county will be sending out experts to use both a foliar treatment—applied directly to a plant’s leaves—and a soil treatment, which will be absorbed through plant tissue, to kill any insects. 

Dr. Grafton-Cardwell suggested that residents “wash plants, stems and leaves—anywhere the psyllids might be growing.” She also reiterated the importance of the quarantine, recalling a woman who purchased a nursery plant in Southern California and only discovered it carried the insects after she tried to plant it in her home up north. 

“One of the key messages is being able to control the A.C.P.—that’s really critical to controlling the disease,” said Stefan Parnay, the deputy agricultural commissioner for Marin County. “We’re working very closely with the city and the state and the U.S. Department of Agriculture as well; it’s a cooperative effort because it’s such a serious pest.” 

Mr. Parnay recommended that residents check their trees for signs of the psyllid and Huanglongbing, and cooperate with agricultural crews deploying treatment in the area.