No abstract

Queens play an essential role in the colonies of stingless bees. Typically, only one queen occurs in a colony at any time, and she dominates the egg laying. Their presence maintains colony cohesion and ensures the development and survivorship of these matriarchal societies. Yet there remain significant gaps in our knowledge of queen life cycles as compared to their daughters, the workers. In this review, we follow chronologically the life of queens from pre-emergence inside brood cells (caste determination), to their interaction with workers (queen selection) and males (sexual selection), and up to adulthood. Stingless bee queens can be determined either trophically or genetically. After emergence, the virgin queens undergo a selection process whereby many are executed by workers. The body size, pheromones, age and behaviour of virgin queens may play a role in queen selection. Queens then leave the nest on a nuptial flight during which they mate just once. After mating, queens are still susceptible to workers’ harassment. For example, if they produce diploid males they are killed by workers. Previous studies have successfully in vitro reared and mated virgin queens under laboratory conditions, which have revealed new insights of queen development time, the threshold of minimum and maximum provided food to larvae developing into queens, and lethal and sublethal effects of agrochemical substances. These new techniques have also provided new information about queen physiology. However, the daily routine of queens inside colonies demands further well-designed experiments to capture other patterns of behaviour which remain poorly understood, including their interactions with other queens, workers and even males.

Improving knowledge of virgin queen behaviour and mating biology in stingless bees (Meliponini) is an important step towards artificial selection programmes for managed colonies. Here we study the Australian stingless bee Tetragonulacarbonaria to (1) describe the behaviour of queens from eclosion until egg-laying, and (2) assess whether young queens can be reared and mated under constrained conditions. We extracted mature virgin queen cells of T.carbonaria from large colonies and reared them in queen maturation boxes containing a limited number of workers (“micro-colonies”). Queen behaviour followed a typical pattern with three phases: a period of high activity and wing-flapping on top of the brood (5 ± 2 days of age), attempts to leave the colony for the nuptial flight (12 ± 3 days of age), and oviposition (20 ± 6 days of age). In total, 71% (68 of 95) of queen cells hatched in our maturation boxes and 57% (39 of 68) survived to mating age, with some observed to be actively killed by workers within the first week of life (8 of 68; 11%). For a subset of our queens, we performed controlled matings (either with or without CO2 narcosis) by presenting constrained females to males in mating aggregations, allowing one male to mate (deposit mating plug) and then returning queens to their box. In the absence of CO2 narcosis, 30% of queens mated this way were subsequently found to have sperm in their spermathecae, indicative of a successful mating (5 of 17). The remaining 70% however contained no sperm, despite the presence of the male mating plug. Amongst queens that were CO2 narcotized before and during the controlled mating, none had sperm in their spermathecae (0 of 12), though most went on to activate their ovaries following the mating and lay male (haploid) eggs. Together, these trials indicate that controlled matings with T.carbonaria are possible, but suggest that queens may need to actively accept mating for successful sperm transfer.

Human-induced shifts in species’ ranges can increase contact between closely related species and lead to reproductive interference. In Australia, climate change and trade in stingless bee colonies is increasing the range overlap of two cryptic species: Tetragonula carbonaria and T. hockingsi. To investigate reproductive interactions between these species, we validated a diagnostic-PCR test based on the mitochondrial gene COI to ID field specimens to species. We then assessed the likelihood of reproductive interference in four ways. First, we imaged the male genitalia of each species and found no evidence of reproductive character displacement. Second, we assessed species composition of mating aggregations in an area of sympatry (Southeast Queensland) and confirmed that some males join the mating aggregations of interspecific colonies. Third, we translocated T. hockingsi colonies into the southern range of T. carbonaria (Sydney) and tracked their ability to requeen. These translocated colonies attracted mating aggregations comprised almost entirely of interspecific males, but never formed hybrid colonies; instead, queens either mated with their brothers, or the colony failed to requeen at all. Finally, we presented T. carbonaria males with either conspecific or interspecific virgin queens and found that males attempted to mate only with their own species’ queens. In all, we conclude that reproductive barriers between these species are complete with respect to “short-range” mating cues, but not for “long-range” mate attraction cues. Our study highlights that hive movements can increase some forms of pre-mating reproductive interference between managed bee species, even where the species do not actually mate or hybridize.

Pollinator conservation is aided by knowledge of dispersal behavior, which shapes gene flow and population structure. In many bees, dispersal is thought to be male-biased, and males’ movements may be critical to maintaining gene flow in disturbed and fragmented habitats. Yet male bee movements are challenging to track directly and male dispersal ability remains poorly understood in most species. Here, we combine field manipulations and models to assess male dispersal ability in a stingless bee (Tetragonula carbonaria). We placed colonies with virgin queens at varying distances apart (1–48 km), genotyped the males that gathered at mating aggregations outside each colony, and used pairwise sibship assignment to determine the distribution of likely brothers across aggregations. We then compared simulations of male dispersal to our observed distributions and found best-fit models when males dispersed an average of 2–3 km (>2-fold female flight ranges), and maximum of 20 km (30-fold female flight ranges). Our data supports the view that male bee dispersal can facilitate gene flow over long-distances, and thus play a key role in bee populations’ resilience to habitat loss and fragmentation. In addition, we show that the number of families contributing to male aggregations can be used to estimate local stingless bee colony densities, allowing population monitoring of these important tropical pollinators.