Don Curlee: Introduced wasp may save citrus growers
For three years, California's nearly 4,000 citrus growers have been on pins and needles because of a tiny insect. Now an even tinier insect is putting them at ease.
The worry was caused by the Asian citrus psyllid, discovered in a backyard citrus tree in Hacienda Heights in Los Angeles County four years ago. The psyllid is known to carry and thereby transmit to trees a malady called Huanglongbing disease. Growers translate that to citrus greening disease, a real tree killer.
Growers and the rest of the industry have had to look no further than Florida to see the tree-crunching devastation of the disease that got a foothold there a dozen years ago.
To prevent the spread of the carrier psyllid, a quarantine on citrus shipments has been in effect in part of San Diego County since 2008, and imposed in other Southern California counties or parts of them at various times since. Currently, the quarantine includes all of Los Angeles, Orange and Imperial counties and parts of Riverside, San Bernardino and San Diego counties.
It took a longer look, all the way to Pakistan, to find the parasitic wasp Tamarixia radiata which kills the ravaging psyllid by eating it from the inside. Biological control specialist Mark Hoddle at the University of California, Riverside, made several trips there with his wife, Christina, to capture the tiny wasp.
He calls the wasp find encouraging although it hasn't been determined yet just how big an impact it is having since quantities of it were first released late in 2011. Hollowed-out shells of the menacing citrus psyllid, convincing evidence the wasp did its predatory work, have been found in Fontana, San Bernardino, Pomona, Azusa and Pico Rivera.
Researchers are reassured that the tiny wasp from Pakistan doesn't attack other insects, especially those that are beneficial. They don't know yet how tolerant the wasp is to chemical sprays that growers might apply to control other insects, disease or weeds.
They have found that it has enemies that might threaten its long-term existence, but have not estimated yet the number of wasps consumed by them. The primary danger occurs from Argentine ants, themselves an invasive species. One was observed snatching a T. radiata wasp out of the air as it began to take flight.
The ants like to farm the sugary waste produced by the larval stages of the citrus psyllid, so they often hang around psyllid populations. That might be a plus, or not. The term the researchers are using to describe the phenomenon of one invasive species supporting another is invasive meltdown.
Part of Hoddle's team is investigating ways to control the ants, an indication of how complicated this or any biological control project can be. Such efforts are also time-consuming, never the immediate cure that growers hope for.
Another concern about the wasp's welfare that needs to be explored is its ability to endure typical California winters, and continue its reproductive and predatory lifestyle in the spring.
The industry's confidence in the tiny T. radiata wasp is strong enough that efforts are under way to finance a facility for rearing it in volume. Releasing larger concentrations is especially appealing to urban residents with trees in their backyards, who can dismiss thoughts of sprays or sticky traps for psyllid control on their properties. With as many trees in the state's backyards as there are in commercial orchards, that is a major relief.
Releasing beneficial predators like the tiny Tamarixia radiata wasp from Pakistan can be equally effective in either setting.
CONTACT Don Curlee at email@example.com.