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study sites, psyllid eggs and nymphs were three times greater,” Hoddle said. “The ants were protecting the psyllids from natural predators because the ants wanted the honeydew the psyllids produce.” Because the ants eat the honeydew secreted by the psyllid, they protect their food source by killing and eating natural enemies of the psyllid. One way to reduce the ants, Hoddle said, is to leave small beads of water laced with sugar and very small doses of insecticide at locations where the ants travel. Another way to manage the ants is to slow their travel along the irrigation lines that run throughout the orchard. “The ants use the smooth plastic irriga- tion lines as a superhighway,” Hoddle said. Growers can slow the ants by placing col- lars that serve as roadblocks at selective loca- tions on irrigation lines. It is also possible to cover the irrigation lines with compostable mulch, rendering the superhighway invisi- ble and useless to the ants, Hoddle added. Allowing natural enemies to attack the psyllid and taking action to reduce ant populations in the citrus groves have made a difference in controlling the prob- lem. Many California study sites have been free of Asian citrus psyllids for two years or more, Hoddle said. (Bob Johnson is a reporter in Monterey County. He may be contacted at bjohn11135@gmail.com.)
Early attempts to eradicate the psyllid with insecticides failed as the vector popula- tion continued to grow. In 2010, Hoddle and his team of researchers visited the University of Agriculture Faisalabad in Pakistan to bet- ter understand the impacts natural enemies have on controlling the psyllid. A wasp that is the Asian citrus psyllid parasitoid, Tamarixia radiata, was initially discovered in the 1930s in the region that is now Pakistan and may be where the psyllid and HLB evolved. “Within that huge area, we looked for localities with climate like Bakersfield and Central Valley citrus areas because we wanted parasitoids to be pre-adapted to the climatic conditions in California,” Hoddle said. “The Punjab region of Pakistan was a match.” When researchers arrived at ground zero for the Asian citrus psyllid and huan- glongbing in the country, they did not find devastation in the orchards. “The citrus trees were green, healthy and producing tons and tons of fruit,” Hoddle said. This was due to the parasitoid wasps protecting the trees by keeping psyllid pop- ulations under control, he added. The Tamarixia radiata is a voracious de- stroyer of the psyllid, which is underscored in research trials after its introduction in California as a beneficial insect for protect- ing citrus groves.
Introduction of the parasitic Tamarixia radiata wasp, left, into citrus orchards has helped control the Asian citrus psyllid, right, which can spread huanglongbing, a fatal disease in citrus.
In addition to killing hosts by parasitism, Tamarixia radiata females kill first through third instars by feeding on psyllids. The fe- male wasp punctures the psyllid nymph with her ovipositor and consumes the ex- uding body contents. “She lays eggs underneath psyllid nymphs,” Hoddle said. “The larvae feed on the underbelly of the nymph and leave the shell of the host.” After returning to California in 2011, Hoddle released Tamarixia radiata in Southern California. Researchers found the parasitoid could travel a distance of 5 to 8 miles from the release point. A three-year study at 28 sites, including all California
citrus regions, showed that Tamarixia radi- ata reduced the populations of Asian citrus psyllid eggs by 92%, nymphs by 81% to 94% and adults by 75%. Cameras set up to document the control showed that the wasp was aided by other predators. The cameras documented 647 kills, but only 29% were by Tamarixia radiata. Syrphid flies, also known as hover flies, were responsible for 59% of the psyllid kills, while lacewings took care of the remaining 12%. A challenge to the biological control was due to Argentine ants, which are common in California orchards. “Where Argentine ants were present at
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