In an attempt to try and halt the continual spread of the emerald ash borer beetle, which has been responsible for the deaths of scores of ash trees, the United States Department of Agriculture (USDA) has released millions of parasitic wasps to prey upon the insects. So far released in 24 states, the wasps are natural parasites in China, where the insects normally live. 
Thought to have arrived in the country sometime in the 1990s, the emerald ash borer itself isn’t actually harmful to the trees, but its larvae are. The beetle lays its eggs on the trees, which then hatch and the larvae bore under the bark and into the wood. An infestation of the larvae in a tree can eventually cause the tree to die as its ability to move water and nutrients from the soil to the branches is disrupted.
The USDA has not only used one species of wasp, but has actually released four. The wasps range in size from the smallest being as tiny as a pepper flake to the largest being comparable to a gnat. One of the wasps parasitizes the eggs of the beetles, preventing them from hatching, while the three others use the larvae as its host, stopping them from becoming adults. In trial tests, they found that the wasps caused a 90 percent decline in emerald ash borer populations.
The introduction of one species to control another invasive pest has been highly controversial ever since the cane toads, which were brought in to eat sugar cane beetles, went out of control in Australia and subsequently pushed many native species of marsupial into decline. For these reasons, any proposal to use biological control is tightly managed. This release of the wasps has therefore gone through years of testing to try and make certain that there are no unforeseen consequences of the program. 
The cost the emerald ash borer has incurred in the US alone is quite staggering. The USDA estimate that the response to the infestation, including the treatment, removal, and replacement of over 17 million ash trees, could cost up to $10.7 billion between 2009 and 2019. Yet some estimates put this figure even higher at a whopping $25 billion needed to treat 38 million trees. Either way, the massive damage the beetle is doing to ash trees is of grave concern.
It is not only in the US, however, that the insect is wreaking havoc. Europe is currently facing the threat of losing up to 95 percent of its ash trees, as they are hit not only by the borer beetle populations spreading across the continent, but by a fungal disease as well. Foresters in Europe may therefore be keeping a close eye on what exactly happens across the Atlantic with the release of the wasps.
While the wasps will not completely eradicate the beetles, it is hoped that they will slow their spread to other parts of the country currently free from the pests.
Top image in text: A tiny Oobius agrili wasp parasitizing a borer egg. USDA-APHIDS/Jian Duan/Wikimedia Commons
Bottom image in text: Another of the four species of wasp released, Tetrastichus planipennisi. Stephen Ausmus/Wikimedia Commons
