The effect of brood and queen pheromones, as well as the colony environment, in the success of Apis mellifera capensis social parasites
Honeybee queens typically inhibit the reproductive development of workers in the colony. However, African, Apis mellifera scutellata, honeybee queens seem to have little effect on neighbouring A. m. capensis honeybee workers as is evident in the huge losses of African honeybee colonies due to the invasion by ‘social parasitic’ Cape honeybees (pseudoclones). Certain factors; such as queen and brood presence, the level of colony defence and food availability may render host colonies more vulnerable to invasion by the Cape worker honeybees. In this study host African colonies were split to determine whether a “window of opportunity” existed for Cape honeybee infiltration and thus critical to the capensis problem. Nine African colonies were infected with native and pseudoclone Cape workers over different time periods; before, during and after splitting (treatments). I measured survival rates, as well as reproductive and pheromone development of introduced workers. The effect of brood pheromones on Cape worker reproduction was also examined. Approximately 70% of all workers were removed within 72 hours, a critical period to avoid detection by Cape workers. Queen absence significantly affected the success rate of intrusion and establishment by Cape honeybee workers (GLZ; Wald χ² = 4.49, df = 1, P = 0.033). 21% of 21-day old pseudoclones survived African queenless colonies and only 6% queenright colonies. Native Cape workers showed no difference in survival rates between African queenless (12%) and queenright (11%) colonies. Looking at introduction time, considerably more pseudoclone honeybee workers survived in treatment 1 than did native Cape honeybee workers while for treatment 3 the converse was true. These data show no obvious ‘window of opportunity’ surrounding the swarming process promoting Cape honeybee infiltration and establishment of African honeybee colonies, however the period immediately prior to colony fission represents the best opportunity for invasion by pseudoclones. As for ovary and mandibular gland secretion development, all surviving pseudoclones, irrespective of A. m. scutellata queen presence, fully developed their ovaries and concomitantly produced a mandibular gland secretion dominated by 9- oxo-2-decenoic acid (9ODA). Native Cape workers showed low levels of ovary development in queenright host colonies (8-17%) but this was not true for queenless colonies, with all but one worker developing their ovaries when introduced during and after splitting. Only 40% of native Cape workers introduced before splitting developed their ovaries suggesting that queen pheromones in the three days before splitting retarded ovary development in native Cape workers. These data strengthens the suggestion that the pseudoclone honeybee workers have advanced along the queen-worker developmental continuum. Preliminary studies on brood pheromones, an important factor regulating worker reproduction, indicated that Cape workers reproduce quicker and more eggs when exposed to African brood pheromones, compared to both A. m. capensis brood pheromones and no brood pheromones. Pheromones produced by African larvae therefore do not simply inhibit Cape worker reproductive development but accelerate the commencement of egg laying by these workers. On the whole, host African colonies, especially in the absence of their queen, appear vulnerable surrounding colony fission to invasion by both Cape honeybee worker populations even though there are low survival rates. I conclude that these two Cape honeybee worker populations do differ significantly regarding their reproductive capacity and ability in becoming social parasites.