Understanding trends in prevalence and abundance of Acarapis dorsalis and Acarapis externus in Apis mellifera colonies

Authors: De Guzman, Lilia; RINDERER, THOMAS - Former ARS Employee; BURGETT, D. - Oregon State University; Frake, Amanda

Submitted to: Trends in Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/22/2019
Publication Date: 11/26/2019
Citation: De Guzman, L.I., Rinderer, T.E., Burgett, D.M., Frake, A.M. 2019. Understanding trends in prevalence and abundance of Acarapis dorsalis and Acarapis externus in Apis mellifera colonies. Trends in Entomology. 15:85-94.

Interpretive Summary: There are three Acarapis species that are parasitic to adult honey bees. Acarapis woodi lives and reproduces inside the tracheae, A. dorsalis infests the dorsal groove of the thorax, and A. externus inhabits the neck region of adult bees. While tracheal mites are well-studied, the two external Acarapis are largely ignored. Since their introduction into the US in 1984, tracheal mite infestations have significantly declined. Effective acaricide treatment and use of tracheal-mite resistant stocks are putative causes of such a decline. Here, we presented historical data and examined historical and recently collected bee samples to track and record changes in external Acarapis population through time. We found that both Acarapis species were frequently observed in the 1980s to early 2000 when infestations reached up to 100%. Results of our survey from 2007 to 2019 showed that A. externus infestations were sporadic with the highest infestation of 87% observed in 2009. On the other hand, A. dorsalis were detected every year with the highest infestations of 60% and 53% observed in 2007 and 2019, respectively. Just like any other parasitic mites, these external Acarapis obtain nourishment by feeding on their honey bee hosts. Whether or not Acarapis mites aid in the transmission or activation of various mite-borne pathogens needs to be studied.

Technical Abstract: Acarapis dorsalis and Acarapis externus are parasites of adult honey bees in the United States since the 1930s. Here, we present historical and current data on their prevalence and abundance. In the late 1980s to early 2000, these two Acarapis species were frequently detected with A. externus being found at higher levels than A. dorsalis. The abundance of A. externus over A. dorsalis may be due to the lack of host age preference by A. externus as their prevalence and intensity remained high on bees up to 35 days old. In contrast, infestation rate and mite load of A. dorsalis decreased as bees become older. By examining 16,515 worker bees from 2007 to 2019, A. dorsalis was detected yearly while A. externus infestation was sporadic. The higher frequency of detecting A. dorsalis over A. externus may be due to their differences in colonization ability. A. dorsalis was faster in establishing their population in mite-free colonies than A. externus and was also successful in invading A. externus-infested colonies. The introduction of 50 A. dorsalis in mite-free colonies was sufficient to found a population while 500 A. externus may be too small to establish a population. Variation in responses to parasitic mites by different honey bee stocks also influenced Acarapis population. A. dorsalis was most prevalent in the Hastings stock while levels of A. externus were higher on the ARS-Y- C-1, Hastings x ARS-Y-C-1 hybrid and Louisiana stocks. The Russian honey bees also had higher levels of A. dorsalis than the Italian bees. However, both stocks’ responses to A. externus were inconsistent. Nonetheless, both ARS-Y-C-1 and Russian honey bees are known to be resistant to another Acarapis species, A. woodi, which is known to be a more serious parasite of honey bees than these two external Acarapis. The potential role of external Acarapis in virus transmission especially in Varroa-infested colonies needs to be studied.