For the first time since California enlisted biological control methods in its war against the Asian citrus psyllid (ACP), the state has moved reinforcements to the Central Valley.
Bakersfield becomes the first San Joaquin Valley city to enlist a tiny parasitic wasp – Tamarixia radiata – to help fight the ACP in the state’s urban battle against the invasive pest that can kill citrus trees.
According to Kern County Agricultural Commissioner Ruben Arroyo, the releases are as much an effort to control a growing and troublesome ACP population as an opportunity to study the effectiveness of biological control methods currently employed across Southern California.
On Aug. 23, California officials released nearly 10,000 Tamarixia at 23 different locations within the city of Bakersfield in an attempt to reduce ACP numbers. Kern County is home to over 66,000 acres of commercial citrus valued at nearly $900 million in 2014. Much of county’s commercial citrus production skirts the east and southern ends of Bakersfield.
To date over 70 percent of the county’s commercial citrus is under state quarantine restrictions for the ACP, meaning certain protocols are necessary before shipping fruit out of the quarantine zone. The purpose behind the quarantine regulations is to prevent the spread of ACP to non-infected areas.
In spite of this, the ACP continues to spread across California.
David Morgan, biological program manager for the California Department of Food and Agriculture, released the first 200 Tamarixia in two citrus trees at a private residence in north Bakersfield.
The goal is to introduce the beneficial insect in pre-determined grids across the city. Releases were made about every three miles across the city. Once it can be determined that the Tamarixia are reproducing officials can come back and reintroduce more in an effort to better blanket the city, Morgan says.
Tamarixia is a tiny parasitic wasp discovered in Pakistan by University of California Entomologist Mark Hoddle.
Hoddle gained U.S. Department of Agriculture approval to study the wasp at his lab in Riverside, Calif. He was later granted approval for releases as the wasp was proven safe and determined to only feed on the ACP.
Until now Tamarixia has only been used in southern California counties of Los Angeles, Riverside, San Bernardino, Orange, San Diego and Ventura. Those releases began in 2011 after Hoddle gained USDA approval to employ the tiny wasp. Since then Hoddle also received a permit to study and eventually release a second parasitic wasp, Diaphorencyrtus aligarhensis.
Studies indicate Tamarixia will fly about eight miles in search of ACP to parasitize.
“If the insect cannot find an ACP to parasitize they will die,” Morgan continued.
Arroyo said he pushed for the releases, even though scientists initially suggested they may not work because of low ACP numbers in the city.
Arroyo did not necessarily buy the “low ACP population” argument as earlier this year his staff was daily discovering ACP on traps within the city. Eventually officials found 136 nymphs on non-commercial trees earlier this year, which for Arroyo was enough to suggest the integrated pest management (IPM) approach.
Over 700 adult psyllids have either been trapped or found alive in Bakersfield since 2013. Most of those finds happened earlier this year.
As news continued to be reported about the increased ACP finds, Arroyo says he was being asked by commercial growers why biological control measures weren’t being used in Bakersfield.
“I thought that was a good question, so I began to ask around,” Arroyo said.
For Arroyo, using biological control in the city could help reduce populations of the psyllid that inspectors have not been able to find, which at the end of the day is what he wants to do to protect the county’s considerable commercial citrus industry.
The increased psyllid finds in Kern County raised concerns among agricultural officials and growers as the pest is capable of vectoring an incurable and fatal bacterial disease called Huanglongbing (HLB) – also known as citrus greening disease.
Slowing HLB spread
Since 2012 California has removed 26 residential citrus trees after they tested positive for HLB. Trees that test positive for HLB in a non-commercial setting are removed and destroyed by the state of California.
The disease is not automatic. Insects need to be infected with the bacteria Candidatus Liberibacter asiaticus to vector the disease. They can get the bacteria from feeding on infected plant material or from birth if they were hatched from a psyllid that was previous infected by the bacteria.
The bacteria lead to HLB infections in citrus trees. Once infected, citrus trees will begin to die as the bacteria spread throughout the tree. Along the way the disease causes misshapen fruit that becomes bitter and unmarketable.
Victoria Hornbaker, citrus program manager with the state’s Citrus Pest & Disease Prevention Program, said Kern County’s commercial growers will be asked to conduct coordinated insecticide applications later this year in a timed approach to address possible psyllid populations in commercial groves east and south of Bakersfield.
Tamarixia are not released in commercial citrus groves because of the insecticides used by growers to address ACP and other pests. There are no “soft” chemistries that will spare the Tamarixia. Even the insecticides labeled for organic use, though much less effective at ACP control, will kill the Tamarixia.
Still, Tamarixia are believed to be a good part of an overall IPM program, particularly in urban centers and where citrus trees are left untreated.
While the Tamarixia is no “silver-bullet” control mechanism for the ACP, Morgan says they can provide significant control in some instances. In certain studies, parasitism has been close to 100 percent, though “we’re often happy if we can get 30 percent of the insects killed by this biological control method.”
Morgan used the release as an opportunity to share about the importance of ant control in citrus where beneficial insects are being used to address invasive pests.
Because the ACP in its immature, nymph stage secretes a sugary substance, ants will become protective of it as it becomes a food source for the ant colony. Studies have shown that parasitism rates by Tamarixia can fall to zero where ants protect breeding colonies of ACP.
The much-higher parasitism rates depend on a variety of factors including climate and other environmental factors, and the absence of ants to protect breeding ACP colonies, Morgan says.
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