February 7, 2013

The taxonomic conundrum of Cotesia xylina and Cotesia yakutatensis

One of the main purposes of this blog is to provide comments on species of Microgastrinae that are of interest to a broader audience. I am still thinking on how to organize those posts to make them more useful to potential users, and hopefully soon will come with a system to do that. In the meantime, I am tagging them with the label "Species Profile", to make it easier to search and retrieve that kind of data.

Today I will briefly comment on two Nearctic species of Cotesia that present an interesting challenge to taxonomists and biocontrol workers alike: Cotesia xylina (Say, 1836) and C. yakutatensis (Ashmead, 1902).  Both species are only found in the Nearctic region, although yakutatensis has a wider and more northern range, extending up to Alaska in the West and up to Greenland in the East.
Distribution of Cotesia xylina and Cotesia yakutatensis. Data taken from Taxapad (2012). The coloured areas represent the Provinces/States where the species have been recorded, but no information on exact localities is shown here.

Both species have been reared from several species of Lepidoptera, mostly from the family Noctuidae, with some of the hosts representing agricultural pests. The validity of some records is questionable, especially the ones from Pyralidae and Sphingidae -those were reported in papers before 1920 (mostly late XIX century), and the wasps species may just have been missidentified.

C. xylina has been recorded parasitizing eight species of caterpillars: Epiglaea apiata, Mamestra configurata, Peridroma saucia, Xestia c-nigrum, Xylena nupera (all noctuids), Evergestis rimosalis (Pyralidae), Lampara bombycoides, and Smerinthus jamaicensis (Sphingidae). As written above, the last two records are suspicious.

As far as I know, there is no published paper providing illustrations or drawings of C. xylina, so here I present one colour photo taken a couple of years ago in the CNC.  

Female specimen of Cotesia xylina. Specimen deposited in the Canadian National Collection (CNC). Photo taken by Caroline Boudreault (CNC).

On the other hand, C. yakutatensis has been recorded only from four species of pyralids: Autographa californica, Autoplusia egena, Trichplusia ni, and Xestia c-nigrum. Those records are in line with what could be expected, and all come from more recent studies -but still, that does not guarantee they are right.

There are some line drawings for this species in a recent paper about braconid wasps of Greenland (Achterberg, 2006), and a general habitus from an historical reference (Hyslop, 1912). But, for the sake of comparison, I am also providing a colour picture of one specimen here. Please, note that the size of the two Cotesia species is similar, the differences in the photos are due to different magnification when taking the pictures.
Specimen of Cotesia yakutatensis deposited in the Canadian National Collection (CNC). Photo taken by Caroline Boudreault (CNC).

The main problem with these two species is that we are not sure what their identities are. The validity of Cotesia xylina as a species has been questioned by Muesebeck (1921) and Papp (1986) who considered it a synonym of C. yakutatensis or Cotesia tibialis (Curtis) respectively. Some of the Canadian specimens indeed seem closely related to those of yakutatensis. However, when barcoded specimens from Canadian specimens are analyzed, many distinct clusters are obtained, suggesting that there are even more potential species.

Neighbour joining tree (K2P) of 44 sequences in BOLD (http://www.boldsystems.org/) identified as either Cotesia xylina or C. yakutatensis. All sequences are over 600 base pairs (bp) in length. Bar on top shows 0.5 % of bp differences.

Based on the taxonomic keys of Muesebeck (1921), Papp (1986, 1987, 1989) and van Achterberg (2006), some of those specimens might be Cotesia halli, C. melanoscela, C. eliniae or C. tetricus (the last two not even recorded from the Nearctic as of today). Or they might just be new species on their own. The only way to solve this species-complex would be to embark on a serious study of the group at Holarctic level; including careful examination of the types, analysis of molecular data, precise geographical distribution and biology (host info), as well as detailed morphological measurements... all of that would require a significant amount of time, effort, and probably collaboration between different researchers.

In the meantime there are two things we can do: 1) We need more data of carefully reared specimens, to double-check and improve the biology (host) information of the species. 2) We need more molecular data, meaning at least more DNA barcodes from specimens, ideally covering the whole distributional range of the species.

I am calling for help among my colleagues working in biocontrol, and I am asking them to check their "stocks" of reared Cotesia. Perhaps together we could hopefully solve this taxonomic conundrum in the near future...

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