Engineering an invasion: classical biological control of the glassy-winged sharpshooter, Homalodisca vitripennis, by the egg parasitoid Gonatocerus ashmeadi in Tahiti and Moorea, French Polynesia

Published source details Grandgirard J., Hoddle M.S., Petit J.N., Roderick G.K. & Davies N. (2008) Engineering an invasion: classical biological control of the glassy-winged sharpshooter, Homalodisca vitripennis, by the egg parasitoid Gonatocerus ashmeadi in Tahiti and Moorea, French Polynesia. Biological Invasions, 10, 135-148.
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Summary

The glassy-winged sharpshooter Homalodisca vitripennis is a cicadellid Hemipteran native to southern USA and northern Mexico. Females lay eggs within tissue of a great variety of plants, mostly shrubs and trees. It is a major pest of agricultural, ornamental and native plants as it can spread the lethal plant bacterium, Xylella fastidiosa. It invaded Tahiti in 1999 and spread quickly to the main French Polynesian island groups. It became a major pest threatening agriculture and native biodiversity, as well as becoming a social nuisance; and represented a threat to other South Pacific islands. In 2004, a biological control program was initiated on the islands of Tahiti and Moorea (French Polynesia).

It was decided to trial using the highly host-specific egg parasitoid Gonatocerus ashmeadi (Hymenoptera: Mymaridae) shown to be successful in reducingH.vitripennis populations elsewhere. Risk assessment studies for non-target cicadellid species in French Polynesia commenced in May 2004; these studies indicated an acceptably low level of risk to non-target species.

Releases and monitoring on Tahiti: G.ashmeadi were reared in a controlled quarantine facility. Between May and October 2005, 13,786 parasitoids were released at 27 sites spread throughout Tahiti in a variety of habitats. Four paired-sites (release site and non-release control site) were regularly monitored between August 2004 and May 2006.

Monitoring on Moorea: On Moorea (a small island lying 17 km west of Tahiti), monitoring of two tiare Gardenia tahitiensis plantations was undertaken to provide data on H.vitripennis abundance prior to planned parasitoid introduction and to detect if G.ashmeadi arrived from Tahiti prior to a planned release.

The impacts of G.ashmeadi on H.vitripennis during the first year (until mid-May 2006) are presented.

The impact of G.ashmeadi on H.vitripennis was rapid and pronounced. Parasitism of H.vitripennis egg masses averaged 80–100% in Tahiti subsequent to parasitoid releases; H.vitripennis nymph and adult abundance decreased by more than 90% since December 2005 and were maintained at this level (until the end of the monitoring period) for over a year. On Moorea, where the parasitoid appeared prior to a planned release (probably by transport of plants infested with parasitized H.vitripennis eggs) similar reductions of H.vitripennis were apparent. Population monitoring is ongoing.

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