Thursday, 18 October 2007

French Polynesia and the invasion of the Cicadellids...

1999 was a bad year for French Polynesian plants. In that year two species of highly generalist leafhoppers (Homoptera: Cidadellidae) were found on Tahiti. These were the catchy-named but highly harmful glassy-winged sharpshooter (Homalodisca vitripennis) and the two-spotted leafhopper (Sophonia orientalis). Since then the two have spread around French Polynesia, and are now in the Austral Islands and the Marquesas. In doing so, they have sparked a frenzy of research into their biology which has revealed a number of taxonomic difficulties. The story provides insight into the importance of good taxonomy, and its usefulness in real life.

The glassy-winged sharpshooter aptly got a lot of attention from its bold invasion. The insect is a hugely important vector for a number of plant diseases, is large, and is easily transported. It also excretes a lot of watery fluid, weakening plants and causing a nuisance for humans. As an added twist, it poisons predators that try to eat it. Since then it has been the subject of a biocontrol programme with the wasp Gonatocerus ashmeadi (Hymenoptera: Mymaridae).

While this was happening, the two-spotted leafhopper was slipping under the radar. My attention was brought to it when reading a paper by Aguin-Pombo, Aguiar and Kuznetsova in the Annals of the Entomological Society of America. The insect has made a hop over to the eastern Atlantic island of Madeira, where it was discovered in 2000. In their paper, Aguin-Pombo et al summarise what is known about the leafhopper, which has made itself a nuisance in Hawaii, California and now Madeira. They also provide a redescription of the insect, and comment on its taxonomy.

It turns out that there is a lot of confusion in the literature regarding the correct name and identity of the leafhopper. In this paper, the authors follow the rather convoluted path to the leafhopper's currently accepted name of Sophonia orientalis. Unfortunately though, there remains some confusion, in that the name given in most of the biological literature is S. rufofascia, currently a synonym of S. orientalis. The confusion seems to have arisen through a combined effect of scattered taxonomic literature, poor and incomplete descriptions, differing taxonomic opinion, and the inability to access type specimens. This is unfortunate, particularly in the situation of biological invasions, where it is necessary to quickly and confidently identify the species in question in order to evaluate the possible impact and to put managment strategies in place.

The taxonomic confusion hasn't been all one way though. I see that the glassy-winged sharpshooter has also had a recent name change from Homalodisca coagulata to H. vitripennis. The paper by Takiya, McKamey and Cavichiolil informing of the change is a paper of fine taxonomic detective work, and a good explanation of the rationale behind the change.

The Aguin-Pombo et al paper is also a good example of how taxonomic literature can be incorporated with other information to produce a much more useful document than a description of a single species. This has been the subject of discussion amongst some of my colleagues recently, and it is good to have a recent example. As well as the description, the paper by Aguin-Pombo et al incorporates karyological results (chromosome stuff) and information on the host plants, abundance, and establishment of the two-spotted leafhopper on Madeira.

It is unfortunate that these species invasions occurred at the same time, as resources which could have been used to control the two-spotted leafhopper were used to combat the sharpshooter. While the sharpshooter is an important pest, the leafhopper is not negligible either. In Hawaii and Madeira, it has been found to be malevolent towards banana, taro, sweet potato, papaya and mango - all of which are important food plants in the South Pacific.

The whole scenario is a good case study of the practical importance of taxonomy, and the necessity of good taxonomic work in the first place. It's a timely lesson for those of us entering the field, which we'd do well to heed.

Aguin-Pombo A, Aguiar AMF, Kuznetsova VG. 2007
Bionomics and taxonomy of leafhopper
Sophonia orientalis (Homoptera:Cicadellidae), a Pacific pest species in the Macaronesian archipelagos.
Annals of the Entomological Society of America 100(1): 19-26

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