On a day in early June, state forester Will Phifer carried a pill bottle-orange canister into a southeastern Oklahoma forest, tied it to a shaded tree trunk and left. The area was a confirmed spot for a growing population of tree-killing beetles called emerald ash borers, which likely seeped into the state from the east. The container held what scientists hope is a solution to controlling the harmful pest: more than 100 minuscule parasitoid wasp eggs. “These emerald ash borer eggs are laid on the outer bark of the tree,” Dieter Rudolph, forest health specialist for Oklahoma Forestry Services, said. “So, this wasp will go find them and basically inject an egg into the emerald ash borer egg.” Instead of producing an emerald ash borer larva, the host egg will hatch a new wasp.