Results |
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By October 2005, the parasitoid had completely colonized Tahiti (including non-release control sites), and was even found in the mountains at an elevation of 1400 m. Survey results indicate that G. ashmeadi has had a catastrophic impact on GWSS populations. Prior to parasitoid release, GWSS densities on Tahiti were averaging around 100 to 240 nymphs being collected in a sweep net per minute of sampling effort. After the parasitoid release, an abrupt decline in GWSS abundance was observed at all release sites. |
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Number of GWSS nymphs collected in a sweep net per minute of sampling effort in Tahiti in April 2005 and December 2005 |
| Parasitism of GWSS eggs has averaged around 88-100%, and since December 2005, the number of GWSS nymphs has been maintained at a very low level with less than 3 nymphs per site on average. This represents a decrease of ~ 97-99% in GWSS nymph densities. |
The results obtained from this classical biological control project have clearly demonstrated that the parasitoid G. ashmeadi readily established in Tahiti and rapidly colonized the whole island including high elevation mountainous interior sites. The results of egg parasitism and the comparison of GWSS abundance before and after releases at the release sites and parasitoid invasion into control sites indicated clearly an extremely large decrease in H. coagulata abundance in Tahiti (> 95%). This observed population collapse is due directly to the action of the introduced egg parasitoid. As G. ashmeadi attacks the eggs of its host, the initial impact on pest population structure was first seen on the abundance of nymphs (maximum impact observed 2 to 3 months after the parasitoid arrival), then on adults (maximum impact observed 5 to 6 months after the parasitoid arrival) and finally on the number of egg masses (maximum impact observed 6 to 7 months after parasitoid release) being counted in surveys. Since very good results were obtained by G. ashmeadi in Tahiti, the biological control program will now be extended to all islands in French Polynesia infested with GWSS. <more data available soon> |