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In order to save hemlock, scientists turned to genetics, natural enemies

Then, beginning in the 1970s, a tiny aphid-like insect called Hemlock Fleece Adejid, native to Japan, caused a pandemic in the hemlock forests of the United States. Adelgid can be identified by the cotton-like fluff produced when eating hemlock needles, which has killed millions of trees and ecosystems in the eastern United States. The insect has turned much of Appalachia and New England into a cemetery for trees, and reached the east shore of Lake Michigan in 2016 and threatened to continue its death march across the Midwest.

Many scientists and foresters wrote off hemlock as the reason for the failure. But some people want to know whether the rare combination of anti-Adjid genes lurks in the trees. These scientists searched, reproduced and planted saplings from trees that remained green after their neighbors turned into gray ghosts.

Researchers report that these trees survive better and grow faster than non-resistant trees. The result may mark a stepping stone towards a potential hemlock comeback.

When hemlocks began to die in large numbers in the late 1

970s and early 1980s, forest experts knew they had problems. There are dozens of species of insects and birds on this tree, such as blue-headed budgies and reclusive thrush, and its year-round shade makes the mountain stream cool enough to hold trout. Scientists began to look for ways to maintain hemlock.

One strategy involves looking for rare hemlocks that seem to tolerate Adjid. In the mid-2000s, a New Jersey entomologist conducted a survey near the Delaware Water Gap and found that there were dense green hemlocks in the gray skeleton. University scientists cloned cuttings from what they call “bullet-proof” trees and inserted them in test fields near other hemlock-infected hemlocks in 2015. Four years later, the researchers returned to each plot and evaluated the trees.

96% of clones from cuttings survived, and 48% of other hemlocks. Researchers reported in a paper published in the journal Forest in May that bulletproof clones are higher and have more leaves and insect-resistant chemicals called terpenes. Evan Preisser, an ecologist at the University of Rhode Island who was in charge of the research, said that this is even more impressive because the scientists did not help the trees in these four years.

Due to the small sample size of the study (only eight resistant trees and four non-resistant trees per site), Preisser called it a “proof of concept” that could discover and spread trees resistant to Adjid.

But some scientists are skeptical, thinking that genetics can save hemlock.

Rusty Rhea, an entomologist with the U.S. Forest Service in Asheville, North Carolina, believes that the growth of bulletproof trees may be better due to environmental factors, and Preisser’s research period is too short to prove that trees can survive in the forest. .

“I am a little hesitant to make people hope that there will be resistance based on … propagules that are only four to five years old,” Rhea said.

Rhea’s institution is the main source of funding for forest-related research in the United States, and it takes a different approach.

This service supports a long-term effort to identify predatory insects that can permanently limit the number of Adjid insects, such as how wolves control elk populations in Yellowstone National Park. This “biological control” strategy was not applicable before, but if done carefully, it can reduce the number of harmful pests. For example, the introduced insects have helped the citrus industry in California. But biological control is rarely successful in natural forests.

Beginning in the 1980s, scientists searched Japan where adelgid is located and the west coast of North America where adelgids live, looking for insects that are muted on them. Researchers have spent more than 20 years testing a product called Black squirrel, Native to British Columbia. In April, scientists reported that at the site where Laricobius was released in the eastern United States, beetles destroyed a third of the adelgid oocysts laid in winter. As a result of eating, the number of bollworms in these places in spring decreases.

But this is hardly worth celebrating, because adelgid has two life cycles per year. A follow-up paper published by the author in June reported that due to the high reproduction rate of insects, the number of Adjid in the study site rebounded in summer. An adult female can breed up to 500 offspring.

Biological control researchers are now pinning their hopes on two small silver flies native to the western United States to control the Adjid generation in summer. It turns out that fruit flies reduce the number of Adjid in the bag tied to the branch. But when released in the wild, the flies tend to disappear instead of getting enough numbers to control the flies. Rhea said that flies are still “the best option we know so far.”

Researchers from the Forest Service Department also found that hemlock grows faster in the sun than in the shade, even when mallards are feeding. Rhea hopes to plant hemlock in deforestation and deploy beetles and flies, which will eventually mature and reproduce the trees before they are killed.

However, for Preisser, continued efforts to establish a sustainable population of Adjid predators in the forest show that biological control may be a dead end. He advocated looking for stands suitable for Adjid-resistant trees in different locations and planted them in large-scale demonstration plots to determine which genes provide the most Adjid-resistant genes and reproduce them together with the best performance. Good trees, this is an effort that Preisser admits, will be expensive and time-consuming. Consumption.

Ben Smith, a forestry researcher at the Mountain Research Station at North Carolina State University Waynesville, said that even with modern methods, hemlocks may need to grow for 7 to 10 years to collect seeds. “For crop breeders, this is eternal,” Smith said. “For tree breeders, this is not terrible.”

Preisser will not look around. Frustrated by difficulties in persuading funders to support his research, he is turning to other things. He said: “In the past 10 years, I know that science has not changed.”

Jennifer Koch, a biologist at the Forest Service Research Station in Delaware, Ohio, said that the idea that the genomes of troubled trees can be the key to their own salvation has indeed met with resistance from scientists.

She said, “This is a huge obstacle to make people believe that resistance exists.” “Especially hemlock, this is something that people really can’t believe.

The research of Preisser et al. laid the foundation for breeders to cross some resistant tree species, thereby cultivating tree species that can survive to reproductive age in the forest. She is launching a hemlock breeding program with the non-profit conservation organization American Forests; it will be held at the Holden Botanical Garden in Kirtland, Ohio, and will also include ash and beech trees. New Jersey is establishing its own hemlock breeding program.

Koch says that success does not require the discovery or production of a completely resistant tree. Beetles and flies can also work by helping to reduce the number of Adjid and extend the life of trees.

“We are not looking for immunity. We are looking for balance. [The tree] Can still be the host of this insect, however. . . She said: “The number of insects will not reach the level that kills trees.” “You imitate what happens in nature.”

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