European fern is a poisonous weed that has spread and destroyed many pastures in the northern hemisphere in recent years. A cheap and self-sustaining way to deal with this weed is to introduce the plant's natural enemies. As a result, some scientists have suggested that fern-eating moths from the southern hemisphere could be re-stocked to the northern hemisphere affected by the weed as a way to control fern growth.
In addition to eating European ferns, the stocked moths also eat other weeds that grow in the northern hemisphere is necessary for the success of the scientist's proposal to control the European fern.