Effect of irradiation on the longevity and reproduction of Pheidole megacepala (Hymenoptera: Formicidae) queens

Irradiation is a quarantine treatment option to control ants and other hitchhiker pests on fresh horticultural products exported from Hawaii. The radiotolerance of the big-headed ant, Pheidole megacephala (F.), was studied to determine a dose sufficient for its control. This ant was chosen as a representative species because it is a common hitchhiker and rearing methods in the laboratory have been developed. The desired response with irradiation treatment of ants is sterility of reproductive females. Queens from micro-colonies were irradiated at 60, 90, 120, or 150 Gy or left untreated as controls, then followed for 19 weeks to observe colony growth. In general, queen longevity, and the number of eggs, larvae, and pupae observed in the micro-colonies decreased with increasing irradiation dose. In the 60 Gy treatment, the number of eggs observed was reduced by 89.6% compared with the untreated controls. In the 120 Gy and 150 Gy treatments, the number of eggs observed was reduced by 99.5% and 98.5%, respectively, and no eggs were found after the first observation date at 7 days after treatment. No larvae or pupae were observed in the 90, 120 or 150 Gy treatments, suggesting these irradiation doses sterilized queens. This study suggests the USDA-APHIS-approved generic irradiation dose of 400 Gy is more than sufficient for the Formicidae. Information is needed on the radiotolerance of additional species of ants to confirm our findings. (author)