The genus Encarsia is a diverse and cosmopolitan group of small parasitic wasps. These species are usually parasitic on whiteflies, hard scales or sometimes even themselves as autoparasitoids. As such, Encarsia have demonstrated economic importance in the biological control of whitefly and scale insects and have been effectively used for the control of San Jose Scale (E. perniciosci), Greenhouse whitefly (E. formosa), Ash whitefly (E. inaron), and Citrus whitefly (E. lahorensis). New programs are focusing on the control of Citrus whitefly in California using E. variegata, and against the Silverleaf whitefly with E. strenua and E. transvena. At present there are more than 230 described species of Encarsia and new species are being identified and imported annually as part of efforts searching for new biological control agents.
In choosing a species of Encarsia to test for effectiveness in biological control, researchers commonly choose a species closely related to a known control agent in the hopes that they share similar biology such as host choice or effectiveness in control of whiteflies. Unfortunately, Encarsia species are currently grouped somewhat arbitrarily on the basis of overall morphological similarity and there are many differing opinions as to the relationships, composition and placement of species into groups of Encarsia. This can lead to misconceptions about behavior and host associations which are crucial to biological control programs.
We are using a molecular approach to compare species of Encarsia based on their genetic similarities (molecular systematics). This project will enable us to address the following:
1. Test present concepts of relationships among Encarsia species.
2. Evaluate the accuracy of morphological characters currently used in placing species into existing species-groups of Encarsia.
One genetic region we are using in our study is the variable D2 region of the 28S rRNA gene. Currently we have included sequences of 28S-D2 from 19 species of Encarsia, the sister taxa Encarsiella, and an outgroup Coccophagoides fuscipennis. The phylogenetic tree shown is the single most parsimonious tree generated using Paup 3.1.1.
[download cladogram - enctree.tif.pdf]
Note on the figurethat the strenua and luteola species-groups are each monophyletic and readily distinguishable. Based upon our current data, the sister taxa, Encarsiella, is not an outgroup to all of the Encarsia species. The aurantii group (aurantii, perniciosi) is polyphyletic. The citrina (citrina) and parvella (pergandiella) groups, which have the distinct bare spot on the forewing, are paraphyletic and embedded within Encarsia (although under some iterations they are sister to the remaining Encarsia. Scale parasitism appears to have arisen three times in aurantii, perniciosi and citrina. These results are extremely tentative at the moment and other than group support for the luteola and strenua groups, little can be said of the other relationships.
The phylogenetic approach used in this study will help us group related species based on evolutionary history and be a reliable means for creating a more rigid set of criteria for assigning species to particular groups. This information will be useful to the scientific community for the evaluation of species with potential for the biological control of whiteflies.
We are also working to define molecular markers for each species of Encarsia - especially those commonly reared for biological control. This should be particularly useful in distinguishing morphologically similar species and will offer a genetic alternative to morphological keys and classification.
This project is currently funded by the USDA/NBCI posdoctoral and the Citrus Research Board.
Many thanks to Bruce Campbell, Paul DeBarro and Felice Driver for contributing some of the 28S sequences. Molly Hunter, Bill Rolsch, Ru Nguyen, Don Vacek, Dan Gerling, Tom Bellows, Marta Guillen and Michael Gates have all helped by ontributing specimens of Encarsia.
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Department of Entomology
University of California, Riverside
Riverside, CA 92507