Genetic technology is a transformative new tool now essential for stoat management. It can distinguish survivors from reinvaders after an apparently successful eradication. On Secretary Island in 2015, 205 stoats caught on the island and 40 from the adjacent mainland confirmed a resident population of trap-shy females on the island, supplemented by occasional swimmers across Acheron Passage. After a successful eradication operation on Rangitoto, a single stoat found on the island was identified as reinvader from the mainland, 3 km away. Stoats survive because their high productivity normally compensates for high natural or imposed mortality, but early experiments in chemo sterilisation were not encouraging, and depended on edible baits that could be refused. By contrast, gene drives work by changing the natural patterns of inheritance to produce only young of one sex, so the population should eventually die out. But the development of a heritable gene drive would be impossible for stoats with a generation time of 2 years, and modelling of possible outcomes predicted uncertain results and the rapid appearance of mutations conferring resistance. New Zealand’s history of releasing mustelids as irretrievable self-perpetuating pest control agents is a stark warning against the modern equivalent of the same logic.