1. Introduction

Mites in the genus Tropilaelaps  are native brood parasites of the non-domesticated giant Asian honey bees, Apis dorsata, A. breviligula and A. laboriosa. They spread onto the managed European honey bee (A. mellifera) some time after humans introduced that bee into Asia. Nowadays, A. mellifera is kept for beekeeping throughout Asia and Tropilaelaps mites are one of its most damaging pests (Burgett et al., 1983; Woyke, 1985a; Anderson and Morgan, 2007; Dainat et al., 2009). At present, these mites remain confined to Asia and bordering areas but are recognized as emerging threats to world apiculture.

A question that is constantly asked of Tropilaelaps mites, the answer to which justifies or counters the need to direct scarce resources to study them, is: can they become a serious global pest of A. mellifera, like Varroa destructor?  The answer is emphatically yes and it lies in how the mites feed and reproduce on their bee hosts, how they disperse, how they spread among bee colonies and how they might survive on A. mellifera in temperate zones outside of Asia.

The breeding cycle of Tropilaelaps mites on their honey bee hosts superficially resembles that of Varroa mites, in that a mature mated female enters a bee brood cell that contains a developing bee larva that is in the process of being capped by worker bees with a wax covering. Safely concealed within the sealed cell, the mother mite produces several offspring that all feed on blood (haemolymph) of the developing bee. Eventually the mites are released from the cell when the developing bee (which by now may or may not be physically damaged) chews its way out of the cell through the wax capping (Woyke, 1987; 1994). Unlike Varroa mites, the survival of Tropilaelaps mites depends solely on them having regular access to bee brood (larvae or pupae) on which to feed, as their mouthparts and body shape do not allow them to feed on adult bees, as do Varroa mites. The mites can only survive for a few days in the absence of bee brood (Woyke, 1984; Koeniger and Musaffar, 1988; Rinderer et al., 1994). This limited food source restricts their ability to disperse, as they can only disperse on adult bees on which they cannot feed.

The ever-increasing global trade of live honey bees, which provides a potential pathway for Tropilaelaps mites to disperse out of Asia, has not yet contributed to any increase in their geographical range. This is probably because live bee trade involves movements of adult bees (in the form of ‘package bees’) and live adult queen bees, on which Tropilaelaps mites cannot feed or survive for more than about 74 hours (Wilde, 2000). Because the mites cannot feed on adult bees, very few are found on them at any one time, even in heavily infested bee colonies (Woyke, 1984; 1985b). Those mites that do venture onto adult bees can spread to neighbouring bee colonies by various means, such as on swarms, on worker bees that rob resources from other colonies, on foraging bees that become disorientated and enter the wrong colony or simply by moving between forager bees from different colonies that visit the same flowers. In the Philippines, it has been suggested that Tropilaelaps mites spread between A. breviligula (their natural host) and A. mellifera colonies by interspecific robbing (Laigo and Morse, 1969).

As Tropilaelaps mites venture onto adult bees at some stage of their life, then live bee exports from Asia could potentially carry them, albeit at low numbers, and therefore aid their dispersion. However, for mites to survive this pathway, the exported bees would need to be moved quickly to their destination country and, on arrival, come in contact with local brood-right bee colonies into which mites could disperse and feed on brood before they starve to death. Another new potential pathway for the mites’ spread out of Asia was recently uncovered in Australia, when A. dorsata worker bees were detected on air cargo that arrived at an international airport from Malaysia. These bees were probably night-foragers that had become disoriented by airport lights in Malaysia and rested and became stranded on cargo that was being loaded into an airplane bound for Australia.

Even given these and other potential pathways for Tropilaelaps mites to spread, they nevertheless still remain restricted to Asia and bordering areas. They currently occur as far west as Afghanistan-Pakistan and as far east as the large Melanesian island of New Guinea, where they were introduced in the 1980s in brood-right hived colonies of A. mellifera imported from Java (Delfinado and Aggarwal, 1987; Anderson, 1994; Baker et al., 2005). They were also reported from Kenya during the early 1990s (Kumar et al., 1993; Matheson, 1997), but this report has not been verified and it may have resulted from a false identification, as recent testing in Australia of mites that had been collected from honey bees in Kenya, and assumed to be Tropilaelaps, were found to be plant mites (Anderson, unpublished data).

So the question remains: does the current restricted distribution of Tropilaelaps mites reflect their inability to survive outside of Asia in temperate zones in the absence of their native bee hosts? The successful establishment of Tropilaelaps mites on A. mellifera in New Guinea suggests the answer is no, and that they can survive in temperate zones given an important proviso.

New Guinea is located to the north of Australia and it contains no native Apis species. Humans introduced colonies of A. mellifera to the island last century and their descendants have since thrived in the cool temperate-like highland regions, but not so well in the hotter humid tropical lowland regions (Clinch, 1979). Since their introduction to New Guinea in the 1980’s, Tropilaelaps mites have become an endemic damaging pest of A. mellifera in the western half of the island (Irian Jaya). Their success is thought to be due to the unbroken year-round production of brood by the A. mellifera colonies, which provides a continuous food source for the mites and an ideal environment for their reproduction.

Hence, the New Guinea situation confirms that Tropilaelaps mites can survive and become an endemic pest of A. mellifera in temperate zones in the absence of their native hosts provided they have access to A. mellifera brood on a year-round basis. Such conditions are found in many temperate countries, such as parts of the USA, Australia and Europe. It has also been suggested that, in coming years, temperate areas in which A. mellifera can produce brood all year round may increase, as colder regions become warmer, due to the effects of climate change (Le Conte and Navajas, 2008).

This all adds up to a situation where it appears that good fortune has played a major role in restricting the distribution of Tropilaelaps mites and that it may be only a matter of time until they spread outside of Asia to cause hardship for temperate zone beekeepers. It is therefore no surprise that the mites are currently recognized as emerging threats to world apiculture (OIE, 2004) and hence deserve immediate attention from the global research community. The methods presented here should assist those future research efforts.