The evolution and maintenance of sexual reproduction is believed to involve important tradeoffs. Queens of social insects are in a dilemma over the costs and benefits of sexual and asexual reproduction. Asexual reproduction by a queen doubles her contribution to the gene pool. However, overuse of asexual reproduction reduces the offspring's genetic diversity and thus the colony's ability to adapt to environmental stress. Recent research on breeding systems using molecular markers revealed that queens of some ants and termites can solve this tradeoff by their conditional use of sexual and asexual reproduction. On the other hand, parthenogenesis can also function as ‘social cancer’ in the parthenogenetic ant. In this symposium, we show that eusociality with its attendant caste structure and unique life histories can generate novel reproductive and genetic systems with mixed modes of reproduction that can provide important insights into the advantages and disadvantages of sexual reproduction.