Portland — The Oregon Zoo has released the last batch of its zoo-raised Oregon silverspot butterflies into the wild as it winds up a summer programme aimed at boosting the numbers of the once-common yellow-and-black butterfly in coastal habitats.The zoo has transported nearly 450 butterfly pupae to four sites along the Oregon Coast in the past month. There, the butterflies finish their metamorphosis in "pupae pockets" inside protective mesh, the zoo said in a statement on Monday.They flit away when they emerge."It was the perfect time of year to be out there, right in the middle of the flight season," said zoo conservation research associate Karen Lewis.The silverspot was once common in coastal grasslands from northern California to Canada. It is now listed as threatened due under the Endangered Species Act because of the loss of its host plant, the early blue violet.Only five populations of the butterfly remain.The zoo has been raising Oregon silverspots since 1998 and generally collects female butterflies each year from atop Tillamook County's Mount Hebo. The females are brought to the zoo's conservation lab, where they lay eggs. The resulting caterpillars are kept through the winter and then transported back to the wild once they have spun themselves into pupae to undergo their transformation into butterflies.This is the first summer the pupae have also been released atop Mount Hebo, which is a critical habitat for the species.Recent drought conditions have caused a dramatic decline in the silverspot population there due to a die-off of the early blue violet plants that the caterpillars feed on."Essentially, we're putting back what we took and adding quite a few more," Lewis said.