No one knows just when the Asian citrus psyllid will arrive in California citrus, but scientists and governmental agencies agree that it — and the world-class disease it vectors - demand vigilance and preparation.
The tiny, aphid-sized ACP does direct damage to citrus as it feeds on foliage and deposits messy honeydew that brings on sooty mold. It also damages new shoot growth.
Well-established in Asia, Afghanistan, Saudi Arabia, and parts of Central and South America, ACP was first discovered in Palm Beach, Fla., in 1998 on landscape plantings of orange jasmine, a preferred host between new flushes of citrus growth.
But more damaging than the psyllid itself is the citrus greening disease it carries. The disease, or CGD, is one of the most devastating citrus pathogens in the world and is common in Asia, the Indian subcontinent, and parts of Brazil.
CGD, also known in Asia as huanglongbing, prevents fruit from sizing and coloring and causes the fruit to have a bitter taste. Another symptom is a characteristic yellowing of foliage.
In Brazil, where it may have lurked for a decade before being detected, it destroyed thousands of acres of citrus. It has spread through Central America and to the Yucatan Peninsula of Mexico. In 2005, it showed up in Florida, where entomologists say coping with the insect alone would be manageable, but the sooty mold and CGD pose a much more serious economic threat.
In areas where CGD is established, infected trees may survive for five to eight years without producing marketable fruit. It is also transmitted by infected budwood and possibly by citrus seed. It is not carried by the fruit.
Entomologists and plant pathologists of the California Department of Food and Agriculture and University of California are taking cues from counterparts in Florida, where the disease has reached epidemic proportions in citrus in the southern part of the state.
According to Beth Grafton-Cardwell, UC Extension IPM specialist at Parlier, the primary defenses for the California citrus industry at the moment are the disease-free budwood program and detection and eradication of the ACP.
She said the ACP's arrival in California is inevitable, although when remains a question. “It will likely come in from Mexico by flying from tree to tree or by riding on infected plant material.”
Using a grant from the California Citrus Research Board, Grafton-Cardwell invited University of Florida researcher Michael Rogers to speak at a recent seminar in Tulare for growers, PCAs, and others.
Rogers, assigned to the University of Florida's Research and Education Center at Lake Alfred, Fla., has been working with colleagues on management of the ACP and sharing the findings with California researchers.
His carry-home message to the California citrus industry, once the pest is detected, is “catch it early and get on an eradication program.”
A primary indication of the insect is presence of adults, often difficult to see at first but usually found feeding on the undersides of mature leaves. Yellow sticky card traps can also be used for detecting adults.
Adults have a characteristic stance with the head down and body canted at 45 degrees to the surface of a leaf. Their bright yellow-orange eggs can be found in crevices of unfolded leaves. Nymphs produce tiny, whitish tubules of honeydew.
Populations of ACP fluctuate with growth flushes and when temperatures are below about 90 degrees F. Its egg production drops off at above 91 degrees F. and remains low until cooler temperatures in the fall. Although it thrives in humidity of above 53 percent, it can survive in as little as 7 percent.
In addition to all citrus, ACP goes to the common urban ornamental orange jasmine, and plants of the genus citropsis.
Florida growers have been spraying Admire, Danitol, Lorsban and Temik (all but Temik are registered for this use in California) where control is worthwhile. Most have been spending about $20 per acre, although one grower laid out a staggering $350 per acre in his effort to manage the pest. The optimum times for treatment are when citrus is in new growth flushes.
In general, only nurseries and new plantings are treated chemically in Florida, since mature trees can withstand the physical injury by the ACP.
Rogers said there is little prospect that the disease can be eradicated in Florida at this point. With that in mind, management strategies are ensuring budwood is free of disease, solid screening of citrus nurseries, heavy insecticide use, treatment during new flush periods, and removal of infected trees when they are found. Grafton-Cardwell said anyone finding a suspected ACP should place the infested plant part in a container or place the insect in alcohol and get the sample to the local county ag commissioner's office quickly for identification.
If an infestation is determined, steps will then be taken to deal with it. Those include destruction of infested plants, insecticidal treatments of plant around the infestation, quarantine of the area, and additional surveys and treatments of the area during periods of new flush.
The yellowing of citrus foliage is a sign of the disease, and it may be mistaken for a nutritional deficiency. An infection often occurs on one side of a tree only, so the entire tree should be inspected closely.
Ted Batkin, president of the California Citrus Research Board, which is funding part of the campaign against the psyllid and citrus greening, said, “We have to go after this insect pest and we have to assume that it has the pathogen with it. Otherwise, we could find ourselves in the same situation as Florida citrus growers.”
Commenting on security in ports where plant material infested with CGD could potentially enter California, Batkin said U.S. Department of Homeland Security authorities are aware of the citrus greening threat but need to give closer attention to it.
“It's thugs, drugs and bugs with them, but that may not give our concerns a high enough priority, so they need significant training to be as effective as APHIS (Animal and Plant Health Inspection Service) was in the past.”
Grafton-Cardwell is senior author of University of California Publication 8205, “Asian Citrus Psyllid,” available at UC Extension offices. It details ACP's biology, along with detection and management measures for it.