Efficiency of the sterile insect release method as an eradication measure for the sweet potato weevil, Cylas formicarius (Fabricius) (Coleoptera: Brentidae) in the field

A small scale experiment was carried out to eradicate the sweet potato weevil, Cylas formicarius (Fabricius) by the sterile weevil release method from 1994 to 1996 on the islet, Kiyamajima, 35 ha, of the Amami Islands, Kagoshima Prefecture, Japan. Weevils were mass-reared with fresh sweet potato roots at 27 degreesC and roots filled with the weevils were irradiated with gamma ray, 80 Gy on the 27th to 28th days after oviposition (newly eclosed adult). Sterile weevils were stained with fluorescent dyes and released by hand on host plant foliages. Monitoring was done by both pheromone traps and root traps throughout the experimental period. From 11 January, 1994 to 19 July, 1994, ca. 32,000 sterile weevils were released every 10 d as a rule all over the island. This trial suggested the necessity of a denser release of sterile weevils for successful eradication. Thus, for intensive release ca. 16,000 sterile weevils were released every 10 d as a rule from 29 July, 1994 to 5 September, 1995 in a restricted area, 13 ha, as the release zone. The wild population in the release zone was controlled to zero or at least nearly to zero after summer in 1995. Only a few unmarked males were captured by pheromone traps for one year after the final release of sterile weevils on 5 September, 1995, which was probably immigrants from outside of the release zone. No weevils were found in the reexamination of root traps and dissection of wild host plants carried out in September in 1996.