Effects of irradiation stages on male reproduction in the sweet potato weevil, Cylas formicarius (Coleoptera: Brentidae)

The sweet potato weevil, Cylas formicarius (Fabricius) (Coleoptera: Brentidae), is a notorious pest of sweet potato, Ipomoea batatas (L.), over tropical and subtropical countries. The eradication program for this weevil using female sex pheromone (male annihilation) and sterile insect technique (SIT) has been undertaken in Okinawa and Amami islands in Japan. The effect of the timing of irradiation on reproduction of adult male of this weevil applied with 100 Gy of radiation either 3 days before adult emergence or 1, 5, or 9 days after emergence was investigated. The survival rate in the control (no treatment) group was 100 per cent throughout the observation period (day 10 to 20 of adult life), while survival rates of irradiated groups were reduced to approximately 30 per cent by just a few days after the start of the observation period. Irradiated individuals showed reduced mating rates for several days before their death, suggesting that the mating period ended before death. The earlier the stage that was irradiated, the lower the total sperm production was recorded. However, the main factor driving decreased insemination rates in irradiated individuals reduced mating rates due to decreased sexual behavior and earlier death, rather than disruption of spermiogenesis. Since mating ability decreased several days before death, irradiation at an early stage shortened mating periods by shortening lifespan. Therefore, the release of adult males immediately after irradiation would appear to be an effective means of flooding the field with sterile males and thus suppressing populations of this pest.