Millions of tiny wasps that are natural parasites for the emerald ash borer have been released into wooded areas in 24 states as the battle against the tree-killing borer is now biological.

The U.S. Department of Agriculture has researched and approved for release four species of parasitic wasps that target the larval and egg stages of the ash borer. Government agencies estimate the borer has killed nearly 38 million ash trees in urban and residential areas at a cost of nearly $25 billion.

The wasps, one so small it looks like a pepper flake on a white surface, lay eggs inside borer eggs or larvae preventing them from maturing.

Iowa is the 24th state to have wasps introduced in ash borer infested areas. Texas and Georgia will get them soon