The evolution of cooperation and task sharing in Xyleborina (Curculionidae: Scolytinae), fungus gardening ambrosia beetles

Ambrosia beetle adults and their offspring depend on the use of ascomycete fungi as their main nutrient source, which they cultivate on the walls of tunnels (“galleries”) excavated in the heartwood of trees. Fungus gardening has long been hypothesized in ambrosia beetles but was never actually observed. This behaviour would imply intraspecific cooperation, because it is unlikely that one single individual is able to tend a fungus garden in an entire gallery system on its own. Delayed dispersal of adult daughters from maternal galleries has been shown in the haplodiploid Xyleborina as a first step towards higher sociality. Here we present the first detailed behavioural studies on ambrosia beetles and show that daughters help with fungus gardening and brood care, accumulate reserves and occasionally reproduce in the natal gallery. Helping behaviours depend on the species’ gallery morphology. Although lifelong non reproductive castes were not yet found in Xyleborina, age-specific task sharing reveals high levels of cooperation and a form of polyethism that is comparable to the socially most advanced hymenoptera. Thus, ambrosia beetles provide one of the best model systems for studying the role of genetic and environmental factors for the evolution of intraspecific cooperation and highly advanced sociality.