APPLE VALLEY, Minn. (AP) — The 20 tiny butterfly larvae being cared for by a Minnesota Zoo biologist could represent the last hope for a species that used to be ubiquitous across the state's prairies.

The Poweshiek Skipperling used to flit and float above more than 2 million acres, but for reasons that aren't entirely clear they've virtually disappeared, according to a report in the St. Paul Pioneer Press (http://bit.ly/YJiyWt ).

"They just fell off the map. It may already be extinct in the state," said Erik Runquist, the Minnesota Zoo conservation biologist who leads its butterfly conservation efforts.

The Poweshiek, a brown butterfly with white stripes, is about the size of a quarter as an adult. It hasn't been seen in the state since 2007, and its near-extinction suggests that the butterfly's habitat is ailing.

"When you consider the prairie ecosystem, and things start dropping out, something is not right," said Robert Dana, an ecologist with the state Department of Natural Resources.

The butterflies aren't entirely gone, as populations still exist in Wisconsin and Michigan. That's where zoo officials obtained the tiny green butterfly larvae, each about the size of a grain of rice.

Runquist and other ecologists are hoping the larvae can survive the winter and eventually breed, perhaps leading to clues as to what killed off the butterflies' predecessors.

"It would be nice to be able to find out what's killing them," Dana said. "Right now, we have no corpse to do a postmortem."

Possible causes include aggressive use of pesticides, the introduction of nonnative predators or the overzealous use of fire in managing the remaining prairie land.

Because of the lingering uncertainty — and because the zoo has so few larvae — officials' expectations are modest. They don't expect the Poweshiek population to ever return to its former levels, but they hope they can at least show how to help small numbers survive, said Tara Harris, the zoo's director of conservation.

"That's something no one has ever accomplished," she said.

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Information from: St. Paul Pioneer Press, http://www.twincities.com

Copyright 2012 The Associated Press.