Rarely reported, widely distributed, and unexpectedly diverse: Molecular characterization of mermithid nematodes (Nematoda: Mermithidae) infecting bumble bees (Hymenoptera: Apidae: Bombus) in the USA

Authors: Tripodi, Amber; Strange, James

Submitted to: Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/13/2018
Publication Date: 3/19/2018
Citation: Tripodi, A.D., Strange, J.P. 2018. Rarely reported, widely distributed, and unexpectedly diverse: Molecular characterization of mermithid nematodes (Nematoda: Mermithidae) infecting bumble bees (Hymenoptera: Apidae: Bombus) in the USA. Parasitology. 145(12):1558-1563. https://doi.org/10.1017/S0031182018000410.
DOI: https://doi.org/10.1017/S0031182018000410

Interpretive Summary: Mermithid nematodes are internal parasites of a wide range of aquatic and terrestrial insects and other invertebrates. Although commonly encountered as parasites of grasshoppers, earwigs, and immature mosquitoes, there have only been six records of mermithids parasitizing bumble bees throughout history. Because bumble bees are important pollinators and many are in decline, we set out to characterize the parasites of bumble bees in the United States. Out of the 3,646 bumble bees we dissected, six bees were parasitized by large (2.6–5.1 inches long) mermithid nematodes. Three of these nematodes were parasites of the twospotted bumble bee, the common eastern bumble bee, and the half-black bumble bee from the Northeast. Using genetic tools, we identified these nematodes as Mermis nigrescens, a species that is mainly known to be a parasite of grasshoppers and earwigs. Three bumble bee samples from a single Arkansas farm site, two of the common eastern bumble bee and one of the brownbelted bumble bee, had mermithids as well. Because there is so little genetic information available on mermithid parasites, these Arkansas specimens were not identifiable. They were genetically similar to the only mermithid found in a bumble bee host with associated genetic data, and that record was from Japan. Using genetic data to identify mermithid parasites is important, since the parasitic-stage of these animals is lacking in physical characters that allow their identification. This work doubles the historical records of mermithid parasites of bumble bees, identifies one parasite, and adds genetic data that can help solve the mystery of who these parasites really are to the record.

Technical Abstract: Mermithid nematodes (Nematoda: Mermithida: Mermithidae) parasitize a wide range of both terrestrial and aquatic invertebrate hosts, yet are recorded in bumble bees (Insecta: Hymenoptera: Apidae: Bombus) only six times historically. Little is known about the specific identity of these parasites. In a single-season nationwide survey of internal parasites of 3,646 bumble bees, we encountered six additional instances of mermithid parasitism in four bumble bee species and genetically characterized them using two regions of 18S to identify the specific host-parasite relationships. Three samples from the northeastern U.S.A. are morphologically and genetically identified as Mermis nigrescens, whereas three specimens collected from a single agricultural locality in the southeast U.S.A. fell into a clade with a currently undescribed species. Nucleotide sequences of the V2–V6 region of 18S from the southeastern specimens were 2.6–3.0% divergent from one another, and 2.2–4% dissimilar to the nearest matches to available data. The dearth of available data prohibits positive identification of this parasite and its affinity for specific bumble bee hosts. By doubling the records of mermithid parasitism of bumble bee hosts, and providing genetic data, this work will inform future investigations of this rare phenomenon.