Radiocaesium and radiostrontium transfer to an insect herbivore and an insect detritivore through holometabolous development: A comparison between the cabbage butterfly (Pieris brassicae) and the black soldier fly (Hermetia illucens).

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The presence of the long-lived radionuclides 137Cs and 90Sr in ecosystems is a major environmental concern because bioavailable forms of the radionuclides are readily transferred to living organisms. The present study investigated how holometabolous insect development influences the fate of radiocaesium and radiostrontium by examining the behaviour of tracers (134Cs and 84Sr) and stable elements during the larval feeding stage (21-23days old), the pupal stage, and the adult stage. We aimed to evaluate the degree to which an herbivore or a detritivore food chain could serve as transfer pathways to higher trophic levels in terms of accumulation potential, and during which stage of development the accumulation potential is highest. We used the Cabbage butterfly (Pieris brassicae) and the Black soldier fly (Hermetia illucens) as model insects in the herbivore food chain and the detritivore food chain, respectively. Both food chains showed similar patterns of radiocaesium transfer to the larvae, with concentration ratios of 0.26 to 1.15. Radiocaesium levels then gradually decreased during the transition from larva to adult. We also found strontium in the larvae of both model insects. However, while the transfer to P. brassicae was low and the majority of retained strontium was removed prior to the pupal stage, we found that strontium biomagnified in H. illucens larvae and pupae, showing high accumulation potentials. Overall, our results suggest that radiocaesium transfer to terrestrial holometabolous insects is predominantly determined by radiocaesium levels in their diet, whereas radiostrontium transfer is influenced by the insects' dietary need for calcium and the concentration of calcium in the diet.