Without any natural enemies to keep them in check, nonnative pest insects often have a field day feeding on California crops and plants. In California, populations of two such pests - the glassy-winged sharpshooter, which threatens the state's valuable grape industry, and the red gum lerp psyllid, which attacks eucalyptus trees - are in retreat after UC researchers identified, tested, bred and released natural parasites of the pest insects.
In two peer-reviewed studies published in the October-December 2005 issue of the University of California's California Agriculture research journal, UC scientists describe their efforts to establish natural enemies of these major pests by attacking them without the use of insecticides. Rather, they employed "biological control," which involves introducing natural pests or parasites of the pest insect. The articles are posted in full online at http://californiaagriculture.ucop.edu.
The glassy-winged sharpshooter invaded California around 1990. The one-half-inch insect is a vector of Xylella fastidiosa, a bacterium that causes Pierce's disease in grapevines as well as other plant maladies such as leaf scorch in oleander and liquidambar in olive trees. Researchers from UC Riverside, the U.S. Department of Agriculture's Agricultural Research Service, and the California Department of Food and Agriculture imported four parasitic wasps from the southeastern United States - the glassy-winged sharpshooter's original home.
The tiny wasps parasitize the sharpshooters' egg masses, preventing it from reproducing. More than 1.2 million parasitoids have been released at 373 sites in 13 California counties where the sharpshooter has been found.
The parasitic wasps are slowly establishing stable populations, says Leigh Pilkington of UC Riverside, lead author of the study. "Biological control with host-specific parasitoids may be the only feasible control strategy for providing long-term, area-wide suppression of the glassy-winged sharpshooter in areas where the climate is favorable to establishment."
The red gum lerp psyllid preys on eucalyptus trees, both of which are native to Australia. The red gum lerp psyllid became established in California in the late 1990s and has caused considerable damage to eucalyptus trees around the state. To control the psyllid, UC researchers traveled to Australia and found a natural pest of the psyllid - a parasitoid called Psyllaphaegus bliteus that lays its eggs inside the psyllid's nymphs and prevents them from reaching maturity.
Nearly 50,000 P. bliteus parasitoids were released between 2000 and 2003 at 78 release sites in 42 California counties. Subsequent monitoring has found that the parasitoid is now well established in coastal areas, with concurrent declines in psyllid populations of between 78.6% and 44.8% depending on release location. However, psyllid densities have not declined significantly in some parts of the Central Valley, most likely due to climatic differences that affect the establishment of P. bliteus.
"The red gum lerp psyllid now appears to be in check in most coastal regions of California," wrote UC Berkeley's Kent Daane, lead author of the study. "As psyllid numbers have dropped, the defoliation and death of eucalyptus trees due to the psyllid have been reduced."
The red gum lerp psyllid project is a tribute to the legacy of Donald Dahlsten, UC Berkeley professor of insect biology, who was a pioneer in the biological control of pest insects (http://www.berkeley.edu/news/media/releases/2003/09/10_Dahlsten.shtml). Dahlsten initiated and oversaw the psyllid project before his death in 2003. He is first author of the study published in California Agriculture, which was written by Daane.
California Agriculture is the University of California's peer-reviewed journal of research in agricultural, human and natural resources. For a free subscription, go to http://CaliforniaAgriculture.ucop.edu, call (510) 987-0044 or write to calag@ucop.edu. For a printed copy of California Agriculture, media should e-mail janet.byron@ucop.edu or call (510) 987-0668.
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