Eating food contaminated with radioactive particles may be more perilous than thought—at least for insects. Butterfly larvae fed even slightly tainted leaves collected near the Fukushima Daiichi Nuclear Power Station were more likely to suffer physical abnormalities and low survival rates than those fed uncontaminated foliage, a new study finds. The research suggests that the environment in the Fukushima region, particularly in areas off-limits to humans because of safety concerns, will remain dangerous for wildlife for some time.

The 2011 Fukushima Daiichi Nuclear Power Station disaster released massive amounts of radiation, much of which drifted out to sea. Humans were evacuated to safety and their exposure to radiation was minimal. But local wildlife were exposed both externally to radiation in the environment and internally from contaminated food sources. Joji Otaki, a biologist at University of the Ryukyus in Nishihara, Japan, and his colleagues have been conducting field studies and lab experiments on how such radiation affected the pale grass blue butterfly (Zizeeria maha), a species found throughout most of Japan.

In a previous experiment, Otaki’s group fed butterfly larvae leaves of the creeping woodsorrel (Oxalis corniculata) with radiation in the thousands of becquerels per kilogram (Bq/kg) gathered near the power plant within a few months after the accident. (For comparison, the Japanese government set a limit of 100 Bq/kg for human consumption of rice, meat, and fish, and 50 Bq/kg for milk and infant formula.) Larvae that dined on the radiation-drenched leaves had low survival rates and high incidences of physical abnormalities such as unusually small forewings. These results corroborated field surveys by others that turned up fewer butterflies in contaminated areas than would normally be expected.

The new study shows that radiation can damage larvae even at much lower concentrations. Otaki and colleagues collected leaves 16 to 20 months after the accident, after short-lived radioactive contamination had decayed, but this time from locations ranging from 59 to 1760 kilometers from the power plant; contamination levels ranged from 161 to 0.2 Bq/kg. They found that as contamination increased, mortality rates and incidences of abnormalities increased. “These results suggest that low-dose ingestion of approximately 100 Bq/kg may be seriously toxic to certain organisms,” the team writes in a paper published today in BMC Evolutionary Biology.

In another experiment, the researchers divided offspring of the butterflies into two groups, feeding larvae either the same contaminated leaves their parents had eaten or uncontaminated leaves. Larvae fed the contaminated leaves had even lower survival rates and more abnormalities than their parents, whereas those feeding on clean leaves largely reverted to near-normal in both mortality rates and frequency of abnormalities.

The findings from Otaki’s group are “groundbreaking,” says Timothy Mousseau, a biologist at the University of South Carolina, Columbia, who also studies the effects of radiation on wildlife near Fukushima and Chernobyl. He notes that there have been “almost no studies” on how ingestion of radiation-tainted foods affect wildlife. Still, Mousseau cautions that the results should not be directly extrapolated to humans. “I think butterflies as a group are likely to be much more sensitive than humans to radiocontaminants,” he says. He adds that Otaki’s findings suggest that insects that survive after eating contaminated leaves might evolve tolerance to the low levels of radiation likely to persist in the Fukushima region for decades.