A new species of parasitic wasp may soon be enlisted in the urban jungle battle against the Asian citrus psyllid (ACP).
While the US Department of Agriculture must approve marching orders for the wasp, the war to prevent citrus greening from claiming California’s fresh citrus market and its iconic landscaping continues.
University of California Riverside Entomologist Mark Hoddle recently submitted an 84-page report to the USDA related to the Diaphorencyrtus aligarhensis, a tiny insect he says only feeds on the ACP.
Hoddle’s report was part of a lengthy process to gain federal approval for the biological control method in California and elsewhere.
Final federal approval is expected this month.
If approved Hoddle hopes to first introduce Diaphorencyrtus in areas where officials have not already released populations of Tamarixia radiata, another parasitic wasp with a similar dislike for the Asian citrus psyllid.
Early Diaphorencyrtus releases could take place in the Coachella Valley.
Hoddle wants to allow Diaphorencyrtus the opportunity to reproduce and build populations without having to compete for its food source – the Asian citrus psyllid – which is why he is attempting to keep the two wasps separate in the field. The second reason is to better understand how the insect adapts to various climate conditions in southern California.
“We want these insects to be successful and because they only feed on the Asian citrus psyllid they will be motivated to seek them out and parasitize them rather than seek other food sources,” Hoddle said.
Hoddle is already seeing the Tamarixia colonizing in parts of southern California and he hopes to give the Diaphorencyrtus the same opportunity.
Another reason to initially keep the Tamarixia and Diaphorencyrtus apart in the field is to determine how effective each are in controlling ACP populations.
Parasitism occurs when wasps bore into developing ACP nymphs and lays eggs inside the nymph. Inspecting parasitized ACP nymph mummies will determine which wasp was responsible for destroying the ACP.
Hoddle says it is relatively easy to determine which insect has parasitized an ACP because Tamarixia leave an exit hole in the head of the nymph while the Diaphorencyrtus aligarhensis leaves an exit hole in the tail.
The psyllid is of major concern to California’s $2 billion citrus industry because it can spread a deadly bacterial disease called Huanglongbing, or citrus greening. The disease is responsible for destroying groves in Florida and has infected trees throughout the southern United States.
Citrus greening is thought to be caused by the bacterium, Candidatus Liberibacter asiaticus. It leaves citrus fruit misshapen and extremely bitter as it kills the tree. There is no known cure for the disease, which experts say is not a human health risk.
One case of the disease was reported in grafted citrus tree in a Los Angeles suburb in 2012; it has not been discovered since in California.
Researchers hope biological control will help them gain an upper hand in slowing the psyllid’s population growth in California’s urban centers. Chemically treating citrus trees – currently the most effective method in controlling the psyllid – is simply not feasible in southern California’s urban neighborhoods.
The wasps are a natural enemy of the ACP and feed only on developing ACP nymphs, according to Hoddle.
The two-wasp approach is believed to be beneficial because they tend to parasitize different life stage developments in the ACP. Diaphorencyrtus tends to parasitize the second and third instars of nymph development whereas Tamarixia tends to parasitize third and fourth instar development, Hoddle said.
Tucked against the side of Mount Rubidoux in a quiet Riverside, Calif. neighborhood sits a state facility that is rearing millions of Tamarixia. Hoddle suspects that as breeding efforts of the Diaphorencyrtus continue at UC Riverside, CDFA officials will assume mass-rearing efforts of the insect there as well.
About 100,000 Tamarixia each month are reared in small cages at a California Department of Food and Agriculture quarantine facility in Riverside. The facility also raises ACP determined to be free of the bacterium which causes citrus greening.
Running the facility is no small feat, according to David Morgan, the CDFA’s biological control program manager in charge of the site.
“This is basically an insect factory,” Morgan said. “We need to fertilize the plants properly; they need to be planted in the right soil and given the right conditions in which to grow; you have to make sure they are not contracting any other diseases or attracting unwanted pests.”
With all of that Morgan says there is no room for failure as “each step has to be perfect.”
Morgan continued: “If it’s not, it’s going to reduce the quality and quantity of your end product.”
Greenhouses at the CDFA facility contain individual enclosures that house curry plants as hosts for the ACP. The plants allow ACP to feed, breed and flourish ahead of the introduction of Tamarixia.
Each cage goes through a seven-week cycle to produce Tamarixia.
At the beginning of the process about 300 ACP are introduced into each cage. It is there they begin a two-week period to reproduce and lay eggs on the curry plants. No other insects are in the cage at that time, which gives the resident ACP population the best opportunity to flourish.
By week three the ACP eggs hatch into nymphs. This is when about 200 Tamarixia are introduced into the cages to begin their reproductive cycle, which includes parasitizing the developing ACP nymphs.
By week five the Tamarixia offspring emerge from the parasitized ACP nymphs as adults.
In weeks six and seven workers collect the Tamarixia and prepare them for release by CDFA officials throughout southern California.
According to Morgan, the entire process is quite intensive and takes a toll on the host curry plants. Some of those plants can be rehabilitated at the Riverside facility and used again after they have been trimmed, treated for pests and disease, and allowed to grow back.
Much of the plant material needed for the Riverside facility is produced at a separate CDFA facility in Arvin. That is a slow and laborious process as it takes about six months to produce enough curry plants for the Riverside facility, which needs 270 plants each week to fill its cages.
“We go through plants rather quickly around here,” Morgan said.
In the beginning
In search of biological control methods to manage a growing Asian citrus psyllid population in southern California Hoddle traveled to Pakistan in 2010 after research led him to the two parasitic wasps. He returned to the United States with the insects under USDA permit and immediately began research on them in a UC Riverside quarantine lab.
“We had to prioritize which parasitoid to study and release first,” Hoddle said. “We chose to go with the Tamarixia.”
After gaining state and federal approval to release the Tamarixia in California he began measured releases of them in southern California. Over the following three years Hoddle would release more than 160,000 Tamarixia in over 400 locations in six southern California counties.
Since then those releases have expanded to include urban areas from Ventura to the Mexican border. To date over a million Tamarixia have been released in California.
With limited facilities to mass produce the Tamarixia at UC Riverside, the CDFA assumed mass-rearing efforts of the Tamarixia in 2013.
Morgan expects his facility will have produced another one million Tamarixia this year alone to add to the fight against the psyllid. It is also expected that the CDFA will assume mass rearing of the Diaphorencyrtus after UC Riverside researchers develop sustainable protocols to breed the wasp.
California officials release Tamarixia in a grid system based on the risk of citrus greening in southern California. Those grids cover over 7,000 square miles of urbanized neighborhoods in the region.
Within each one-mile grid a computer algorithm developed by the USDA determines low, medium and high-risk locations for contracting citrus greening. Under the protocols 400 insects are released every three miles in low-risk locations, 600 in medium-risk locations and 800 in high-risk locations.
In addition to urban releases Morgan says they constantly seek out abandoned citrus groves, which can provide a safe haven for ACP to reproduce quickly. Lessons learned after the ACP colonized Florida suggest abandoned groves there became a safe-haven for the psyllid, which could be easily spread by high winds common during severe storms and hurricanes.
While California’s Tamarixia releases target urban, non-commercial citrus, Morgan says they have also released Tamarixia in organic commercial groves “partially because a lot of the organic treatments are not all that effective.”
Officials do not plan to release the parasitic wasps into commercial San Joaquin Valley groves because chemical treatments still appear successful in maintaining extremely low populations of the invasive pest there.
That could change if urban centers elsewhere in California develop larger populations of the psyllid, which of late has been discovered in the northern San Joaquin Valley and the Bay Area.