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To save hemlock, scientists turn to genetics, natural predators

So, starting in the 1970s, a tiny aphid-shaped insect known as woolly hemlock adelgid, originating in Japan, triggered the tree version of a pandemic in American hemlock forests. The deltagida, recognizable by the cotton-like fuzz it produces while feeding on hemlock needles, has killed millions of trees and has spilled ecosystems in the eastern United States. After transforming much of the Appalachians and New England into tree cemeteries, the insect reached the eastern shore of Lake Michigan by 2016 and threatens to continue its death march across the upper Midwest.

Many scientists and foresters have canceled hemlock as a lost cause. But some wondered if rare combinations of adelgid resistance genes could hide in the trees. These scientists searched, propagated and planted cuttings from trees that remained green when their neighbors became gray ghosts.

Researchers reported that these trees survive better and grow faster than non-hardy ones. The result could mark a milestone towards a potential hemlock return.

When scars began to die in large numbers in the late 70s and early 80s, forestry experts knew they had a problem. The tree is home to dozens of insects and birds such as the blue-headed vireo and the hermit thrush, and its shade throughout the year keeps the mountain streams cool enough for trout. Scientists have started looking for ways to keep storks.

One strategy involved searching for rare scars that seemed to tolerate the delgida. In the mid-2000s, an entomologist from the state of New Jersey examining near Delaware Water Gap found a stand of lush green storks amidst gray skeletons. University scientists cloned some cuttings from what they called “bulletproof”

; trees and, in 2015, planted them in test plots near other storks infested with adelgids. Four years later, the researchers returned to each terrain and evaluated the trees.

Ninety-six percent of the cuttings clones had survived, compared with 48 percent for the other hemlocks. Bulletproof clones were taller and had more foliage and insect-resistant chemicals called terpenes, the researchers reported in an article published in May in the journal Foreste. It’s even more impressive, says Evan Preisser, an ecologist from the University of Rhode Island who led the research, because scientists have done nothing to help trees in those four years.

Due to the small sample size of the study – only eight resistant trees and four non-resistant trees at each site – Preisser calls it a “proof of concept” that adelgid resistant trees can be found and propagated.

But some scientists are skeptical that genetics can save hemlock.

Rusty Rhea, an entomologist at the United States Forest Service in Asheville, NC, believes that bulletproof trees may have gone better due to environmental factors in which they were growing and that Preisser’s study period was too short to prove that trees they could survive in the forest.

“I am a little hesitant to give people hope that there is resistance on the basis… Propagules that are only four or five years old,” says Rhea.

Rhea’s agency, the leading source of forest research funding in the United States, has taken a different approach.

The service supported a long-term effort to identify predatory insects that could limit the number of adelgids on a permanent basis, similar, for example, to the way wolves control elk populations in Yellowstone National Park. This “biological control” strategy has already gone wrong, but, if done carefully, it can reduce populations of harmful pests, for example the introduced insects have helped the California citrus industry. But bio-control has rarely succeeded in natural forests.

Since the 1980s, scientists have searched Japan, where the aldida came from, and the west coast of North America, where the adelgidae also live, in search of insects that munch on them. The researchers spent over 20 years testing a called cockroach Laricobius nigrinus, originally from British Columbia. In April, scientists reported that on sites in the eastern United States where they released Laricobius, the beetles damaged about a third of the pouches of frozen eggs laid in the winter. As a result of feeding, less adelgids emerged in these sites in the spring.

But this is certainly not a reason for celebration, because anelidid has two life cycles per year. A follow-up article published in June by the same authors reported that the adelgid numbers in the study sites rebounded in the summer, thanks to the high reproductive rate of the insect; an adult female can produce up to 500 offspring.

Biocontrol researchers are now hoping for two tiny butterflies, both from the western United States, to control the summer adelgid generation. Flies have been shown to reduce the number of adelgids in sacks tied on tree branches. But when released in the open air, flies tend to disappear rather than reaching numbers large enough to control the adelgids. However, says Rhea, flies are “the best bet we know of right now.”

Forest Service researchers also found that storks in full sun grow faster than in the shade, even with the adelgids that feed on them. Rhea hopes that growing storks in forest clearings and deploying beetles and flies will eventually allow the trees to reach maturity and reproduce before the deltagida kills them.

For Preisser, however, the ongoing struggle to establish a population of sustainable adelgid predators in the forest suggests that biocontrol could be a dead end. It supports the search for schools of adelgid resistant trees adapted to different positions, growing them in large-scale demonstration plots to determine which genes provide the most adelgid resistance and raising the best performing trees together, an effort that Preisser admits will be expensive and time-consuming .

Even with modern practices, it may be necessary to grow a hemlock for seven or ten years before its seeds can be harvested, says Ben Smith, a forestry researcher at North Carolina State University’s Mountain Research Station in Waynesville who is developing a program. of hemlock breeding there. “To grow farmers, it’s an eternity,” says Smith. “For tree breeders, it’s not terrible.”

Preisser doesn’t stick to find out. Frustrated by the difficulties in persuading lenders to support his research, he goes on to other things. “Over the past 10 years it has become clear to me that science doesn’t make a difference,” he says.

The idea that the genomes of besieged trees could hold the keys to their salvation has actually met with resistance from scientists, says Jennifer Koch, a biologist at the Forest Service research station in Delaware, Ohio.

It was “a huge obstacle to getting people to believe that resistance existed,” he says. “Especially with hemlock, that’s what people have really struggled to believe in.”

Research by Preisser et al laid the groundwork for breeders to traverse partially resistant trees to produce those that can survive the reproductive age in the forest, says Koch. He is working to launch a hemlock breeding program with the American Forests nonprofit conservation organization; it will be hosted at the Holden Arboretum in Kirtland, Ohio, and will also include ash and beech trees. New Jersey is organizing its own hemlock breeding program.

Success will not require research or production of a fully resistant tree, says Koch. Beetles and flies can also play a role in helping to keep adelgid numbers low and extend the life of trees.

“We are not seeking immunity. We are seeking this balance. [The tree] may still be a host for this insect, but. . . the insect population will not reach a level that will kill the tree, “he says.” Imitate what happens in nature. “

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