Although it’s been 17 months since what has been called the worst nuclear disaster since Chernobyl occurred in Japan, officials are still testing animals and insects in the area for traces of radiation. Recently, they found a large cluster of pale grass blue butterflies which exhibited abnormalities, and residents in the affected area are concerned about what it could mean for them.

Several butterflies were collected for a study around two months after the accident at Fukushima Daiichi which showed smaller-than-normal wings–about 12 percent of the total number was affected. But in later studies, it was also found that the abnormalities doubled in their offspring. In a third generation of the species–created when healthy butterflies mated with those affected by the accident–the abnormalities rose to over 30%.

“At the time of the accident, the populations of this species were overwintering as larvae and were externally exposed to artificial radiation,” the study reads. “It is possible that they ate contaminated leaves during the spring and were thus also exposed to internal radiation.”

Because every species reacts differently to such a volatile event as a nuclear disaster, officials are asking the public not to jump to any conclusions.

“Sensitivity (to irradiation) varies between species, so research should be conducted on other animals,”Joji Otaki, associate professor at the University of the Ryukyus in Okinawa, said. “Humans are totally different from butterflies and they should be far more resistant to the health effects of radiation.”

Though high levels of radioactive cesium has been found in seafood near the Fukushima coast–and has made its way to tuna found as far away as San Diego–officials say it is well below the amount considered dangerous to humans.

Image credit: Khew SK