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

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 the 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 honey 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.