Influence of irradiation on dispersal ability and survival rate in the sugarcane click beetle Melanotus okinawensis Ôhira (Coleoptera: Elateridae)

Abstract

In estimating the population abundance by the mark-recapture method using male-attractant pheromone traps, the release of sterile males is preferable, because they do not increase the reproductive rate of wild females by increasing the mating rate. We estimated the influence of gamma radiation on males of the sugarcane click beetle Melanotus okinawensis Ohira (Coleoptera: Elateridae) to determine an appropriate dose of gamma radiation in laboratory and field experiments. The hatchability of eggs was examined for 0, 50, 70, 90, and 150 Gy. No hatchings were observed in eggs laid by females which mated with males treated with doses of 70, 90 and 150 Gy. The longevity of adults in the laboratory was estimated with 0, 30, 50, 70, 90, 150, and 200 Gy. Analysis by the proportional hazard model indicated that irradiation significantly reduces the survival rate in the laboratory even if the dose is 30 Gy. Mean dispersal distance in the field was estimated with 0, 50, 90, and 150 Gy. Three hundred marked beetles for each dose were released at the center of Ikei Island on 1 and 3 April 2003. The estimates were 274, 219, 192, and 289 m, respectively, and we could not detect a significant influence of irradiation on the mean dispersal distance. Field survival rates were estimated using Jolly-Seber, Yamamura, and Yamamura B methods at Okinawa Prefectural Experiment Station in Naha for two doses of irradiation, 0 and 90 Gy; we could not detect significant differences between the two survival rates. The mortality added in the field was estimated to be much greater than the mortality caused by irradiation when we focus on the experiment within 12 days after release, if the dose of irradiation is lower than 90 Gy. It was therefore concluded that 90 Gy will be an appropriate dose for preparing sterile males to estimate population abundance and survival rate in the field within 12 days after release.