Soon after swarming, the alates of the Formosan subterranean termite, Coptotermes formosanus Shiraki (Isoptera: Rhinotermitidae), drop their wings, form tandem pairs, and look for new nesting sites to form incipient colonies. In ≈10% of the several hundred incipient colonies kept in laboratory, both the male and female were found to cannibalize their progeny and die of starvation. Examination of such adults revealed that all of them had lost most of their gut fauna, essential for the digestion of wood. We used antibiotic (gentamicin) and a combination of gentamicin and antiprotozoal (metronidazole) compounds during 2002 and 2003, respectively, to induce defaunation in 30-d-old adults after they had laid 30–40 eggs. The progeny of treated pairs compared with control pairs was significantly lower 30 and 60 d after treatment. Feeding on these chemicals during 2003 caused total loss of spirochetes and one of the three flagellates, while the numbers of the other two flagellates were greatly reduced. It is suggested that, in the absence of gut fauna, the adult termites cannot effectively feed on wood, and the ensuing hunger results in cannibalism of their progeny.