2.2. Host specificity
Some behavioural and morphological features of Tropilaelaps mites, such as their fast movement and ‘pincer-shaped’ chelicerae, suggests they may have only recently adopted a parasitic life-style on honey bees and that they may not be very host-specific. Indeed, when they were first discovered in the Philippines in the 1960’s they were found inside an A. mellifera colony and on field rats nesting nearby (Delfinado and Baker, 1961). It is likely that those rats picked up the mites after they entered the A. mellifera colonies, which rats often do in the tropics, because evidence gathered since that initial discovery indicates that Tropilaelaps mites are highly specialized parasites of honey bees.
Tropilaelaps mites are now recognized as common natural parasites of giant honey bees distributed throughout Asia. They have not colonized any other host organism, other than A. mellifera. Molecular studies have confirmed that T. clareae is native to A. breviligula in the Philippines (except on Palawan Island), T. mercedesae and T. koenigerum to A. dorsata and A. laboriosa in other parts of Asia (including Palawan Island) and T. thaii to A. laboriosa, in mountainous region of Mainland Asia (Laigo and Morse, 1969; Delfinado-Baker and Baker, 1982; Tangjingjai et al., 2003; Anderson and Morgan, 2007).
Tropilaelaps mercedesae and T. clareae colonized A. mellifera after humans introduced that bee into Asia. However, in contrast with Varroa mites, of which only a few genotypes have switched-host to A. mellifera, many different genotypes of T. mercedesae and T. clareae now utilize A. mellifera as a host. In this respect, Tropilaelaps mites are less host-specific than Varroa mites but they are still relatively host-specific compared to some mites, such as the water mite Protzia eximia that parasitizes a wide variety of insect hosts (Walter and Proctor, 1999). Evidence suggests that T. koenigerum and T. thaii are restricted to their Asian bee hosts and are harmless to A. mellifera (Anderson and Morgan, 2007).
Very occasionally, Tropilaelaps mites are found in A. cerana and A. florea colonies in Asia, but in none of these instances have the mites been found to be producing offspring (Otis and Kralj, 2001). The exception is a report of a single female T. mercedesae found parasitizing and producing offspring on A. cerana brood in Thailand (Anderson and Morgan, 2007). The authors, when commenting on this find, stated that their observation was the exception rather the rule, as T. mercedesae mites are rarely found in A. cerana colonies and, when they are, they are not found inside brood cells or with offspring. Obviously, other factors, other than the ability to reproduce offspring on that bee, are responsible for preventing T. mercedesae from colonizing A. cerana.