Scientific Reports — Wataru Taira, Mayo Iwasaki & Joji M. Otaki (BCPH Unit of Molecular Physiology, University of the Ryukyus, Japan), published Jul 22, 2015 (emphasis added):
Body size distributions of the pale grass blue butterfly in Japan: Size rules and the status of the Fukushima population
The body size of the pale grass blue butterfly, Zizeeria maha, has been used as an environmental indicator of radioactive pollution caused by the Fukushima nuclear accident… Our study… supports the idea that the size reduction observed only in Fukushima Prefecture in 2011 was caused by the environmental stress of radioactive pollution.
Following the massive environmental pollution caused by the release of radionuclides from the collapsed Fukushima Dai-ichi Nuclear Power Plant, the pale grass blue butterfly was used to monitor and examine the biological impacts of this pollution. Forewing size, a proxy for body size, of this butterfly was utilized as an indicator of the effects of environmental stressors. Adult butterflies collected from Fukushima City, Motomiya City, and Koriyama City (all within the Fukushima Prefecture) in the spring of 2011 had smaller forewing sizes than those collected from the northern and southern localities at the same time. It has been demonstrated that small forewing size can be induced by external exposure to a cesium radiation source or internal exposure by ingesting contaminated foods.