The tawny crazy ant, Nylanderia fulva Mayr (formerly Paratrechina fulva) (Hymenoptera: Formicidae), is a South American species first recorded in the US in 1938 from Brownsville, Texas. Recent population explosions of N. fulva in parts of the southeastern US have received much media attention, often including predictions of tremendous long-term ecological and economic impacts. Here, we examined the status of on-going population explosions of N. fulva on the island of St. Croix, US Virgin Islands. Beginning in 2002, St. Croix local inhabitants started reporting dense populations of N. fulva. All early reports of N. fulva came from the north-central part of the island. A 2006 study found 3 geographically discrete populations of N. fulva: a main population in north-central St. Croix, and 2 smaller area populations in northwestern and south-central St. Croix. Our new survey in December 2013 indicates that N. fulva populations have expanded tremendously in northwestern St. Croix, while N. fulva populations have completely crashed throughout its 2006 range in northcentral and south-central St. Croix. This pattern of an extreme population boom followed by a bust may be a common characteristic of N. fulva and might distinguish this species from the closely related Nylanderia pubens. It seems likely that the current dense populations of N. fulva at sites in St. Croix and the southeastern US will crash within a few years, leaving little long-term impact, but that new population explosions of this species will arise elsewhere.