MaleManduca sexta(L.) irradiated as pupae with an average dose of 37.7 krad of gamma irradiation from a cobalt-60 source were released on Vieques, a 56 mile2Caribbean island, over a 17-month period to determine the effect on the native population. The highest ratio of released (sterile) to native males collected subsequently in blacklight traps was 3.7:1, but egg hatch in the native population was not reduced proportionately, and the population increased during the releases. Collections of tobacco hornworms from BL traps baited or unbaited with virgin females showed that the released males were less attracted to the bait females than the native males. Similar sterile male moths from pupae that were irradiated when they were 1–2 days older than those released on Vieques appeared more competitive when they were released on St. Croix island, based on the greater reduction in egg hatch.