In August 2008 the Asian citrus psyllid (Diaphorina citri), an invasive insect known to spread citrus greening disease (huanglongbing), which can be lethal to citrus trees, was detected in southern California. After initial treatments with insecticides were determined to be costly and unsustainable, a decision was made to instead try biological control by using insect parasitoids that are known to attack the psyllids. However, since the only known parasitoids are not native to North America, an environmental impact study had to be conducted before the natural enemies could be released from quarantine.
The methods and results of the study appear in the February 2014 issue of the Journal of Economic Entomology (JEE) in an article called "Host Range Testing of Tamarixia radiata (Hymenoptera: Eulophidae) Sourced From the Punjab of Pakistan for Classical Biological Control of Diaphorina citri (Hemiptera: Liviidae: Euphyllurinae: Diaphorinini) in California."
In it, authors Mark Hoddle and Raju Pandey of the University of California, Riverside, describe how they tested Tamarixia radiata (Waterson), a parasitoid wasp native to Pakistan which is a natural enemy of the Asian citrus psyllid -- the female wasps lay eggs under the psyllid nymphs, which then hatch and feed on them.
The researchers quarantined the imported wasps and then observed how they reacted to seven non-target psyllid species, including ones that are taxonomically related to D. citri, as well as others that are native to California that are also found on other plant species near citrus trees. Of the seven non-target psyllids, only one -- a pest species, the potato psyllid, Bactericera cockerelli -- was parasitized by the wasps, and even that occurred at very low levels of less than 5 percent.
The results suggested that the likelihood of significant impacts on non-target insects was low, and that the establishment of T. radiata in southern California for the classical biological control of D. citri posed negligible environmental risk.
In November 2011, an Environmental Assessment Report was submitted to the U.S. Department of Agriculture (USDA), which authorized the release of the parasitoid wasps later that year. By January 2014 more than 200,000 wasps had been released at more than 400 sites in southern California. Tamarixia has in some instances been recovered up to 5-7 miles from the nearest release site, and DNA testing confirmed the Pakistani origin of recovered parasitoids.
This JEE article provides even greater analytical detail than the Environmental Assessment Report that was submitted to the USDA in 2011, and could serve as a model for future biological control assessments evaluating the safety of arthropod natural enemies.
"Growing disquiet over arthropod biocontrol safety has driven rapid development of testing protocols that assess host range and host specificity to determine if more than one host/prey species is attacked and what the potential intensity of attack may be like outside of the laboratory," said Dr. Hoddle. "This enhanced pre-release testing approach will likely increase safety and efficacy of classical biological control agents used for the management of invasive species."
The full article is available at DOI: http://dx.doi.org/10.1603/EC13318.
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