The forest ecosystems of Fukushima and its adjacent prefectures were severely contaminated with radionuclides after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident on 11 March 2011 [1,2]. For decades to come, the most biologically important radionuclide will be radiocesium because of its long half-life (30.1 years for 137Cs, 2.1 years for 134Cs) [2,3]. Many studies have reported that most of the Chernobyl radiocesium fallout still resides in surface layers in bioavailable form and continues to be a major potential source for transfer into living organisms even several decades after deposition [4,5]. Similarly, in Fukushima, radiocesium is expected to remain in the litter and upper soil layers of the forest floor for the long term .
Entry of radiocesium into forest ecosystems can potentially occur through two different pathways: the plant-based food chain and the detritus-based food chain. In the plant-based food chain, radiocesium in living plants moves into grazing herbivores and then into carnivores. In the detritus-based food chain, radiocesium enters the ecosystem via organisms feeding on litter and detritus (detritivores) and carnivores. Previous studies have reported highly
contaminated litter to be the primary source of radiocesium in the forest ecosystem [6–8]. A survey of the forest invertebrate communities in the United Kingdom found the highest activity concentrations of 137Cs in invertebrate detritivores, such as earthworms (Oligochaetes) and woodlice (Isopoda) . By broadly sampling organisms in forests and adjacent streams, including both vertebrates and invertebrates (fish, amphibian, reptile, arthropod, earthworms, etc.), Murakami et al.  found that detritivores are more contaminated with 137Cs than herbivores and carnivores at Fukushima.
The forest insect community constitutes a major route of radiocesium transfer to higher trophic organisms such as small mammals and birds. The highly varied feeding habits, life histories, and habitats of insects permit investigation of how radiocesium transfer from forest components into higher organisms occurs. There is particular concern for the effects of radiation on wildlife, including insects, as a result of the FDNPP accident [9–11]. However, compared with the numerous studies examining radioactive contamination of organisms used for human consumption, such as fish and game, only a few studies have been published about radionuclide accumulation in insects after the accident . Previous studies conducted in European countries have reported on radionuclide transfer and accumulation of insects and other invertebrates [8,13–17], but it remains unclear how 137Cs uptake occurs in the entire insect food web and in relation to insect feeding habits.
In this study, we focused on radiocesium transfer in insect communities by investigating 137Cs activity concentrationsg in forest insects in the Fukushima and Ibaraki Prefectures over a period of 1.5–2.5 years after the Fukushima Dai-ichi reactor accident. To assess the distribution of radiocesium across insect communities and the influence of feeding ecology on radiocesium uptake, we collected insect samples from a wide range of insect species: We sampled species from four taxonomic orders (Coleoptera, Hemiptera, Lepidoptera, Orthoptera) and four feeding functional groups within those orders (herbivore, omnivore, carnivore, detritivore). To assess 137Cs uptake across different levels of contamination, we collected samples from a high-contamination area in Fukushima and from a low-contamination area in Ibaraki (137Cs deposition was 130–270 kBq m−2 at the Fukushima area and 13 kBq m−2 at the Ibaraki area according to the 4th Airborne Radiation Monitoring by Ministry of Education, Culture, Sports, Science and Technology in 2011) . To assess the distribution of radiocesium in insect food sources, we also sampled forest floor litter, tree leaves, grasses, bark, fungi, and bryophytes in both study areas.