Social parasitism
Social parasitism is a common and intriguing phenomenon in social insects. It can occur within a single species (intraspecific) and between two species (interspecific). Social parasitic species evolve from their social ancestors by developing mechanisms to exploit the resources of their social hosts. There are several forms of social parasitism. In some cases workers raid the nests of their own or other species to take food resources (e.g. robbing behaviour of honeybees). Some species show only temporary social parasitism in the nest founding phase, when mated queens usurp the nests of host species instead of establishing nests by themselves (e.g. wood ants of the genus Formica). Other species take slaves, by stealing brood from hosts nests. The host brood is raised in the slave maker nest and performs all tasks necessary for the maintenance of the parasite colony. An advanced form of social parasitism are so-called inquiline species, where the worker caste is either reduced or has been lost altogether. Some of such species spent their entire life in the host nest.
Social parasite species are often closely related to their hosts (Emery’s rule). This might be related to the communication between host and parasite. In order to successfully pass the host defense mechanism, social parasites must have evolved communication systems, which are very similar to their host species.
We study the evolution of social parasitism by laying workers of the Cape honeybee (Apis mellifera capensis Esch.), which is native to the Eastern and Western Cape provinces of South Africa (see Special Issue of Apidologie). Honeybee workers don't usually reproduce, but they can activate their ovaries under queenless conditions to parthenogenetically produce haploid male sexuals (=arrhenotoky). As an exception to this rule, laying workers of the Cape honeybee produce diploid female offspring (=thelytoky, Neumann et al 2000a, Radloff et al 2002). Although the thelytoky is via automictic parthenogenesis following meiosis, recombination through crossing over is rare. Thus, a worker’s offspring are almost clonal and usually develop into workers and sometimes into queens. A. m. capensis workers can develop a so-called pseudoqueen phenotype with both a high ovarial development and a queenlike pheromonal bouquet. Such workers truly resemble queens and can e.g. suppress queen rearing and ovarial development in other workers and induce retinue behaviour in the host. Therefore, these pseudoqueens have a high reproductive potential and are predisposed for reproductive conflicts. This led to a facultative social parasitic pathway of laying workers as part of the life history of the Cape honeybee (Neumann and Moritz 2002). This pathway is particularly expressed, when susceptible host colonies of other honeybee subspecies are available. A. m. capensis workers show a series of behavioral adaptations for both host finding and the take over of the host nest ( Neumann et al 2001a, 2001b, Neumann and Hepburn 2002). In addition to accidental drifting into closely neighboring colonies (Neumann et al 2000b) they also have an increased potential for invading distant foreign colonies (Neumann et al 2001b). In the host nest, parasitic Cape honeybee workers produce queenlike pheromones and quickly activate the ovaries despite the presence of the host queen. Eventually they establish themselves as pseudoqueens and replace the host queen. The parasitic worker offspring is preferentially fed by the host workers and many parasitic workers can be reared. These workers can infest new host colonies thereby completing the social parasitic life cycle of laying A. m. capensis workers. Because the parasitic workers do not participate in normal hive duties, such as brood rearing, an infested colony dwindles down to a few host workers and eventually dies. After the introduction of Cape honeybee colonies from the Western Cape province of South Africa (Neumann et al 2002) into the area of the neighboring subspecies Apis mellifera scutellata an estimated number of 100,000 host colonies dies each year, leading to the so-called “capensis calamity” for South African bee keeping enterprises. Because there is reproductive isolation of the parasitic clones from the host gene pool, this sets the stage for the evolution of a queenless social parasitic honeybee Neumann and Moritz 2002) Therefore, the Cape honeybee appears to be a unique subject for studying sympatric speciation of a social parasite.