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Homalodisca coagulata (Say) (Homoptera : Cicadellidae)

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          The Glassy Winged Sharpshooter invaded California about 1990 (Sorensen and Gill 1996)] and in French Polynesia [established 1999 (Cheou 2002)], Hawaii [established 2004 (Hoover 2004)], Easter Island [established 2005 (Sandra Ide pers. comm.)], and the Cook Islands [established 2007 (Disna Gunawardana pers. comm.)].Modeling that combines regional climate data and relevant biological information has indicated California’s premier wine growing areas of Napa, Sonoma, and Mendocino counties may be vulnerable to invasion by sharpshooter while states north of California, also with substantial grape industries, may be too cold to harbor permanent populations of sharpshooter (Hoddle 2004a).


          The major wine-growing regions of New Zealand, Australia, the Bordeaux region of France, most areas of Spain, as well as central and southern Italy have climates that could be conducive to sharpshooter establishment and proliferation should it be accidentally introduced (Hoddle 2004a). It was likely introduced accidentally on ornamental plants imported from California. Tahiti is the largest and most populous island in the group of islands collectively known as the Windward Islands in the Society Islands Archipelago in French Polynesia in the South Pacific. French Polynesia is very remote; it is 6,000 km west of Chile, and 5,200 km east of Australia. The Cook Islands lie 900 km to the west of French Polynesia and Easter Island is 5,178 km to the east. sharpshooter flourished in Tahiti because of an extremely mild year round climate and an abundant supply of exotic and ornamental plants maintained in gardens. In contrast to California, where there are just two generations [spring and summer] each year, natural enemies are lacking, and no obvious competitors exist in urban or natural settings, sharpshooter populations underwent exponential growth


          Naturally occurring parasitism of sharpshooter eggs was very low on the island of Mo’orea, the immediate neighboring island to Tahiti. Surveys indicated that less than 2% of individual eggs were attacked by generalist egg parasitoids in just 4% of egg masses collected. Of those egg masses attacked, 44% of the eggs were parasitized. Because only a few eggs in an egg mass were attacked (indicating inefficient and opportunistic exploitation) and only males were reared from sharpshooter eggs (which suggest poor host quality as females did not oviposit fertilized female eggs), and the wasp responsible for attacking sharpshooter egg masses was a species of platygasterid, a family that does not specialize on sharpshooter eggs, but will parasitize various species of leafhoppers, indicated there were no native specialized parasitoids attacking sharpshooter. Consequently, sharpshooter populations in French Polynesia were free of the pressures associated with natural enemies and this pest reached extraordinarily high densities in Tahiti. Watery excreta known as sharpshooter rain, literally rained from infested trees because there were so many sharpshooter feeding on trees. Such high populations retarded plant growth and reduced local fruit production. Further, the use of shade trees was reduced because so much sharpshooter rain made them unsuitable for escaping the intense tropical sun. Finally, at night, incredible numbers of sharpshooter would be attracted to lights and large numbers would fly into homes, shops, and business at night. This was very annoying because adult sharpshooter would die and need to be swept up, the wingbeat frequency of adults flying past heads caused an unpleasant “buzz” in ears, and adults would occasionally “bite” people when they landed on exposed skin and probed with their needle-like mouthparts.


          Because of high levels of human activity and interisland movement of plants between Tahiti and other islands in French Polynesia, sharpshooter was moved rapidly to other island groups in French Polynesia. In 2001, it was found in Raiatea (Leeward Islands) and in 2002 in Moorea. Populations were then discovered in the Leeward Islands of Huahine and Bora Bora in 2003, and in Tahaa and Maupiti in 2005. At the end of 2004 and the beginning of 2005, sharpshooter populations were discovered outside of the Society Islands in two other archipelagos of French Polynesia substantially distant from Tahiti: the Australs, where two islands were infested (Rurutu and Tubuai) (January 2005) and the Marquesas, where one island, Nuku Hiva, was found infested in November 2004.


          To combat the sharpshooter infestation in French Polynesia the best control to use would be classical biological control. To this end, the mymarid egg parasitoid Gonatocerus ashmeadi was imported into quarantine in Tahiti in September 2004 and studied to make sure it did not pose any undue risk to other insects already present in French Polynesia. Following these studies, the accumulated evidence strongly indicated that the parasitoid was likely safe for release and posed no undue risk to non-target species in French Polynesia. The parasitoid was released on May 2, 2005 on Tahiti.  Between May and October 2005, 13,786 parasitoids were released at 27 sites in Tahiti. The parasitoid established readily, and within seven months of release sharpshooter had been completely controlled in Tahiti and sharpshooter populations were reduced by over 95%. The parasitoid also spread unassisted to every other island infested by sharpshooter. This rapid movement between islands strongly suggested that quarantines that were established to reduce sharpshooter were not working and people were still moving plants infested with sharpshooter eggs. However, this time eggs were parasitized by G. ashmeadi, and this is how it was moved unintentionally between islands and archipelagos.


          Reduction of sharpshooter populations on Tahiti following the release of G. ashmeadi. (A) sharpshooter densities on Tahtiti from one minute sweep net sampling of hisbiscus bushes before the release of G. ashmeadi on May 2 2005; (B) sharpshooter densities five months (October 1 2005) after parasitoid release; (C) sharpshooter densities seven months after parasitoid release (December 12 2005); (D) sharpshooter densities two years after the release of G. ashmeadi (Figure courtesy of J. Grandgirard and J. Petit.).




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