Image copyright AP Image caption The emerald ash borer has cost states about $25bn

Millions of tiny, parasitic wasps have been deployed across 24 US states in an effort to stop the spread of the tree-killing emerald ash borer.

The beetle has killed about 38 million ash trees, according to the US Department of Agriculture (USDA).

The USDA approved the release of four species of wasps, which lay eggs inside the ash borer larvae and prevent them from developing into adult beetles.

The treatment and removal of the affected trees costs up to $25bn.

The wasp program is not likely to save any current trees, but is aimed at preventing the ash borer from decimating future tree populations, according to entomologist Ben Slager.

Image copyright AP Image caption The beetles destroy trees across the US

"It's really a long-term management thing," said Mr Slager, an entomologist at the Michigan laboratory producing the wasps for the federal program.

The emerald ash borer, which feeds on a tree's tissue and prevents nutrients from moving to branches, is believed to have been accidentally introduced in North America during the 1990s through wood-shipping crates from Russia, China, Japan or Korea.

The wasps have been released in 24 of the 26 states where the insect has been found. The two remaining states, Texas and Georgia, are also expected to introduce the wasp program.