Asian citrus psyllid and the devastating citrus greening disease it vectors pose a new and serious, exotic-pest threat for California's citrus and nursery plant industries.
Although the aphid-sized, mottled-brown insect has not been detected in the state, University of California and California Department of Food and Agriculture scientists say it could invade the state at any time, most likely from Florida, Mexico, or Asia.
At a recent gathering of growers, PCAs, and scientists at Lindcove, information was shared by a pair of University of Florida research entomologists, who have several years' experience in dealing with the pest. Their visit was funded thorough a grant from the California Citrus Research Board.
The psyllid, or ACP, was first found in Florida in 1998 in landscape plantings of orange jasmine, and the discovery of citrus greening disease, or CGD, in Florida in August of this year heightened alarm there.
CGD, ranked as one of the most serious pests of citrus in the world, has long been known in the Saudi Arabian Peninsula, India, and Asia. In parts of Asia, in particular, it has sharply reduced the acreage of citrus. In areas where the disease is endemic, citrus trees may live for five to eight years without producing marketable fruit. It damaged citrus in Sao Paulo State in Brazil in 2004.
Meanwhile, California's disease-free citrus budwood program and measures for detection and eradication of ACP are in place. Entomologists and plant pathologists of various industry and regulatory agencies are on alert.
The scientists say orange jasmine plants common in coastal California are apt to be hosts for the insect and the San Joaquin Valley may have other, equally suitable hosts.
The psyllid alone causes direct damage to citrus and related species by feeding on sap and depositing massive amounts of honeydew that promote sooty mold.
Its threat becomes magnified when it is infected with the bacterium that produces CGD. The disease is transmitted not only by the insect but also by grafting and possibly citrus seed.
Infected nymphal stages feed on new buds and cause yellowing of new shoots, hence the other common name, “Huanglongbing,” or yellow dragon disease, in Chinese.
Twigs, fruit affected
The disease, which invades the phloem of hosts, also brings on twig dieback and reduced fruit size and quality. Fruit fail to develop color uniformly and remain partially green. Other symptoms are dark, aborted seeds and bitter tasting fruit.
Although the bacterium was once difficult to identify, DNA and other advanced technologies have since improved detection.
Beth Grafton-Cardwell, UC Extension IPM specialist at the Kearney Agricultural Center, Parlier, is senior author of a new pamphlet, “Asian Citrus Psyllid,” that describes ACP and CGD and is available to the industry.
The publication calls for monitoring by visual inspection of new flushes of growth of citrus and related host plants for all stages of the insect. Adults are collected by aspirator and detected by sticky cards. Eggs and nymphs are difficult to see with out a hand lens.
If an infestation is discovered, protocols exist for steps such as plant destruction, insecticidal treatments of plants, quarantine of the area, and follow-up surveys of the area during period of new growth flush.
Entomologist Carl C. Childers of the University of Florida's Citrus Research and Education Center at Lake Alfred told the Lindcove group ACP is distributed throughout Florida and the Rio Grande Valley of Texas. Other psyllids also transmit CGD, but the ACP is the most common vector species.
The adult stage of the insect, which has a characteristic, tail-high posture at 30 to 45 degrees from the plant surface, congregates on new growing tips for feeding and mating. Childers said at first they may appear to be dead spurs on shoots.
“It is important to know that adults only lay eggs on immature leaves because nymphs cannot develop on mature leaves,” he said. Eggs, deposited in the folds of new tissue, are pale yellow at first and later become orange in color.
His laboratory studies indicate that egg production is substantial at temperatures of 82 to 86 degrees, although production decreases at 91 degrees.
He said a positive element for California, in relation to Florida conditions, may be that its colder winters and hot summers and absence of a summer flush tend to discourage the insect's development. Florida, on the other hand, has milder temperatures that foster longer flushes and has sufficient alternate hosts of ACP.
Childers said that if Florida growers had only to contend with damage from the insect itself, it would be manageable. The additional impact of the sooty mold that follows honeydew and the greening disease bacterium, however, make the problem severe.
Phil Stansly, entomologist at the University of Florida's Southwest Florida Research and Education Center at Immokalee, said one billion plants pass through Miami each year, overwhelming efforts to exclude exotic pests. Most of the plants entering are species that attract tiny, sucking insects like ACP, and the Florida climate is ideal for them.
“Biological control is the keystone of our psyllid management program and we don't want to mess it up with sprays,” he said.
Since the Florida industry is mainly for juice production, it is less concerned about the cosmetics of citrus fruit than the California industry and therefore has limited uses of chemical controls.
Several generalist arthropod predators in Florida are natural enemies of ACP and the list includes spiders, lacewings, hover flies or syrphids, and minute pirate bugs, plus several parasites.
Two Coccinellid predatory beetles, lady beetles, have been important predators of ACP nymphs, and on some citrus terminals all ACP nymphs are devoured by immature and adult stages of the two beetles.
Stansly said a wasp parasite from Taiwan and Vietnam, Tamarixia radiata, showed promise as a biological control in Puerto Rico and Texas. “We have it ready in the wings.”
The Tamarixia and another tiny, parasitic wasp were released in Florida, but only Tamarixia became established there. It lays eggs underneath the ACP nymph, and the larvae consume the nymph. After it develops, the parasite emerges as an adult from its victim.
Grafton-Cardwell and her colleagues say that although these biological controls have been important for the Florida citrus industry, chemical controls would likely be the primary practice for eradication if the pest arrives in California.
In Florida flushes of growth in young trees have been protected by Admire, a systemic formulation of imidacloprid. Treatments are usually confined to new trees because mature trees can withstand ACP damage.
Since the uptake of Admire is relatively slow, a follow-up treatment with Provado, a foliar-applied imidacloprid product, or Danitol to speed control. Mineral oil has also shown some limited effect.