Gamma radiation tolerance of a tropical species of midge, Chironomus ramosus Chaudhuri (Diptera: Chironomidae)

Purpose: The Chironomid midges are known to thrive well under adverse environmental conditions and are even found inhabiting in areas contaminated by radioactive wastes. Studies were therefore undertaken to find out the radiosensitivity of different developmental stages of the Indian tropical midge, Chironomus ramosus. Materials and methods: In order to determine the threshold levels of lethality, eggs, larvae, pupae and adults of C. ramosus were exposed to varying dosages of gamma radiation (60Co radiation source) ranging from 0-3500 Gray (Gy) at dose-rate of 5.5Gy/minute. The post-irradiation studies were conducted at three different time points: (a) Immediately after the end of irradiation, (b) 24 hours (h), and (c) 48h after the end of radiation treatments. Determination of the lethal dose required to kill 50% (LD50), 90% (LD90) and 100% population was carried out using the log-probit analysis. Results: Different developmental stages showed variable threshold levels of radiosensitivity. The radiation doses required to cause 100% mortality immediately after radiation exposure of egg, larva, pupa and adult stages were 1000Gy, 3000Gy, 3200Gy and 3500Gy, respectively, indicating eggs as the most sensitive stage. Detailed analysis of the LD50 values of different post-irradiation time points indicated that pupal stages were also sensitive at 48h post-irradiation amongst all the post-embryonic stages as described in many other insects. Interestingly detailed analysis of data indicated that amongst the adult population, females were the most radioresistant, compared to the males as reported in many other insect groups in the literature. Conclusions: The Indian tropical midge C. ramosus was found to tolerate higher dose of gamma radiation as compared to other known dipteran insects. It is evident from the present findings that C. ramosus falls in the category of radiation-tolerant group of insects.