GRAFTON — Bill Davidson, a forest health specialist with the New Hampshire Division of Forests and Lands, stepped out of his truck and into the mid-morning heat and humidity at the Burnham Lot, a parcel owned by the University of New Hampshire.

Across the dirt back road in Grafton, stood an ash tree, nearly bare of leaves with bark heavily flecked from woodpeckers in search of insects — a sign that the tree had become the host of the emerald ash borer, also known as EAB.

Davidson was eager to open the cardboard shipping box he had brought with him to make sure its contents survived the trip from a Michigan rearing facility during the first heatwave of the summer. Inside, he found several lidded plastic cups containing a different Asian insect, the parasitic wasp Spathius galinae, and small bunches of wood containing the larva of another wasp, Tetrastichus planipennisi.

The wasps harmless to humans, but deadly to their hosts. These wasps, used as a biological control for EAB, lay their eggs in the insects’ larva and prevent them from maturing. A third species, Oobius agrili, will be released later in the summer to coincide with the ash borers laying their eggs, which the wasp, in turn parasitizes by laying its own eggs inside.

The EAB is an insect native to northeastern Asia and invasive in North America. The beetle lays eggs in the bark of ash trees and the larva feed on the inner cambium layer of the bark, which transports water and nutrients throughout the tree. After developing into mature insects, the beetles chew their way out through D-shaped holes and search for a new host tree. The trees are eventually girdled and die.

EAB was first found in Michigan 2002, and it can spread naturally at a rate of between 5 and 12 miles per year. The spread has been accelerated as the insects hitch rides in firewood, and perhaps on other vehicles.

After hiking into a stand of ash on the Burnham Lot, Davidson released the adult wasps from their cups and, using a rubber band, attached the small chunks of ash to a nearby branch. The wasps will emerge from the wood later after fully developing into adults.

Releases like this began in the Midwest in 2007. The New Hampshire Forest Health Program has introduced the wasps at roughly 30 sites around the state since 2014, and has three active sites this year in Grafton, Benton and Middleton. Davidson will return to each site with more wasps six to eight times through the summer.

The approach to using biological controls has greatly improved over the last 50 years, Davidson said.”I think people have learned a lot from previous attempts at some haphazard attempts at biocontrol,” he said. These wasp species are “highly host-specialized,” and have been carefully evaluated, cutting down on chances they will attack native insects, he added.

There is a four-year protocol for each site. Releases are made in each of the first two years. Nothing is done in the third year, and in the fourth the sites are evaluated to determine if the wasps are establishing and spreading into surrounding forest. Ash trees are cut in the winter and the wood is put into barrels with jars attached to their lids. The following summer, if they have been successful, the wasps will emerge, fly toward the light and collect in the jars.

The state also uses insecticide treatments as pest control at roughly 80 sites statewide. At this site, Davidson worked with UNH Land Use Coordinator Stephen Eisenhaure to treat 48 trees. Injections of the chemical are made through holes drilled at intervals around the base of the tree. Then as the tree draws water up from its roots, the chemical is distributed through all the tree’s systems and lasts for about three years. Wasps, however, are a less costly and labor-intensive approach.

Ash species are worth protecting. In addition to its use in wood products, shading streets in communites around the country, and the cultural importance of black ash for indigenous populations, “ash has a lot of unique ecological benefits,” Davidson said.

Forty or more species of insects rely solely on ash trees. Though they’re not a dominant tree species in New Hampshire forests, they are a consistent component. They leaf out late in the spring and drop their leaves early in the fall, creating pockets of increased light in the understory. The types of plants growing under ash differ from the surrounding forest, and some plants only grow in these pockets. Wildlife rely on the trees for food.

“It’s not really expected to have a controlling effect and save standing trees,” said Davidson of the control effort. “All of the mature trees are probably going to die anyway.”

After a surge in ash borer population, in what is called the aftermath forest, the only ash trees left will be less than 1 inch in diameter — too small for the EAB to use to complete its development. There will be a low density of these trees and a low density of EAB.

“The real hope,” Davidson said, is to establish a self-sustaining population of wasps that will spread alongside the ash borer, keeping them in check.

“It’s going to be a long time before we reach that stage,” he said. “This is a program that will maybe take decades to really come to fruition.”

In the Midwest, with a seven-year head start on introducing parasitic wasps, results of the approach are encouraging, Davidson said. “I’m more optimistic about the future of our forests than I was, say, five or 10 years ago.”