Wednesday, 2 May 2012

Rapid dispersal of Tahitian biological control agents

Glassy-winged sharpshooter Homalodisca vitripennis (Hemiptera: Cicadellidae)
Gonatocerus ashmeadi (Hymenoptera: Mymaridae)
The glassy-winged sharpshooter (above) and its parasitoid (below). Pictures courtesy of the Center for Invasive Species Research. License: CC: BY-NC-ND

In 1999, Tahiti was invaded by the glassy-winged sharpshooter (Homalodisca vitripennis, a sap-sucking bug that feeds on a huge variety of plants. In the absence of its natural predators, the bug became extremely numerous. This was of concern for two reasons. The first was that the sharpshooter had the potential to spread bacteria that kill plants. The second was more aesthetic. The sharpshooter feeds on the xlyem fluid of plants, which is high in water and low in nutrients. The result is that the insect needs to drink a lot to get the nutrition it needs. In doing so, it excretes the excess water. When sharpshooter numbers get high, the result can be fairly unpleasant as this video shows. The "rain" in the video is actually water secreted by hundreds of glassy-winged sharpshooters.

Something had to be done, and it was. After pre-release testing, a minute parasitic wasp Gonatocerus ashmeadi (pictured above) was released on the island in 2005. This wasp parasitises the eggs of the sharpshooter. It rapidly became established, and glassy-winged sharpshooter numbers plunged dramatically. When I visited Tahiti in 2008, I found very few sharpshooters despite extensive collecting on the island.

A paper by Petit and co-authors investigated the dispersal of the wasp from two release sites on the island. Sites up to 5 km away were regularly monitored after the wasps were released, to determine how quickly they were moving around. They found that the wasps only took 50 days to be collected 1 km away, and were first collected from the 5 km sites 106 days after the initial release. The team calculated that this insects that is less than 2 mm in length was travelling at around 40 m/day.

This biological control scheme has been especially effective, and will (or should) become a textbook example of successful biological control introductions. Results have been clearly recognised within a short time period, thanks to this parasitoid which has managed to disperse rapidly despite its size.

Petit JN, Hoddle MS, Grandgirard J, Roderick GK, Davies N. 2008. Short-distance dispersal behaviour and establishment of the parasitoid Gonatocerus ashmeadi (Hymenoptera: Mymaridae) in Tahiti: Implications for its use as a biological control agent against Homalodisca vitripennis (Hemiptera: Cicadellidae). Biological Control 45: 344-352

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