The latest Paper of the Month from Parasitology is ‘Rarely reported, widely distributed, and unexpectedly diverse: Molecular characterization of mermithid nematodes infecting bumble bees in the United States’ by Amber D. Tripodi and James P. Strange.

Like most animals, bumble bees are plagued by a suite of parasites. Increasing interest in declining pollinators has inspired a renewed interest in discovering what organisms parasitize bumble bees. We collected and dissected 3646 bees from 121 sites across the USA, and six bees held a rare and unexpected parasitic surprise:  mermithid nematodes. Although bumble bees and their parasites are relatively well-studied, our large scale survey showed that there are plenty of mysteries still waiting to be solved.

Mermithids parasitize a wide range of insects and other invertebrates. Although commonly encountered in grasshoppers, earwigs, and immature mosquitoes, there have only been six records of mermithids parasitizing bumble bees throughout history. These six records span four continents, however, suggesting that, although this phenomenon is rare, it is also widespread. In a single study, our six records double the known instances of mermithids parasites in bumble bee hosts.

Knowing the identity of these parasites is key to understanding their life histories, host-parasite relationships, and their potential to affect bumble bee populations. Unfortunately, parasitic stage mermithids are juveniles that have none of the adult morphological features that allow species determination. Because of this, no one has been able to name the mermithid species found in bumble bee hosts, up until now.

Using genetic tools, we identified three of the nematodes we found as Mermis nigrescens, a common and widespread species that is known to parasitize grasshoppers and earwigs, but that has not been reported from bumble bee hosts before. Mermis nigrescens uses an unusual method of dispersal, scattering its eggs on vegetation to be inadvertently consumed by herbivorous hosts as they feed. These three bumble bees thus likely became infected while feeding on nectar and pollen in flowers. Interestingly, these three M. nigrescens infected bumble bees occurred at three different sites throughout the Northeastern region of the USA, highlighting again that mermithid parasitism is both rare and widely distributed.

On the other hand, 3% of the 94 bumble bees collected at a single agricultural site in Arkansas were parasitized by mermithids, a high proportion for such a rare parasite. Genetic analysis showed that these mermithids were not M. nigrescens, but with so little comparative information available, they were not identifiable to species. Phylogenetic analysis indicated that they were genetically similar to the only mermithid found in a bumble bee host with associated genetic data, and that record was from Japan. Could these sparse records represent a widespread group of mermithids that target bumble bee hosts?

Though rare, mermithid parasites of bumble bees are widespread and unexpectedly diverse. 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. Only by accumulating more data can we solve some of these remaining mysteries, and we encourage others to genetically characterize new records of these fascinating parasites.

 

Read the full article ‘Rarely reported, widely distributed, and unexpectedly diverse: molecular characterization of mermithid nematodes (Nematoda: Mermithidae) infecting bumble bees (Hymenoptera: Apidae: Bombus) in the USA’ in full for free until 6th July, 2018.

Photographs:

A Mermis nigrescens in the abdominal cavity of a Bombus bimaculatus worker collected in Pennsylvania, USA.

 

 

Jessica Mullins reacts to a mermithid parasite dissected out of a bumble bee host.