Think of long dense boughs of hemlocks straddling brooks. Then think of their needles falling off their branches. This is a long-term view of the damage of encroaching invasive insect hemlock woolly adelgid (HWA), which are the size of a pepper grain. They are parthenogenetic — able to reproduce using unfertilized eggs — so a single individual can alone produce offspring. HWA need a live hemlock to survive.
“They feed and kill all sizes and ages of hemlock,” said Dr. David Orwig of Harvard University’s Harvard Forest, who spoke during the Invasive Insect Certification Program for Landscape, Nursery, and Urban Forest Pests provided by UMass Extension in February.
“Hemlocks are definitely disappearing. They make up a large component of old growth forests,” he said. “There are 500-year-old hemlocks in Mohawk State Forest. They provide deep shade that other trees can’t and provide full life-cycle homes for birds such as the Blackburnian Warbler. Porcupines chew on their bark and deer prefer the thick cover of hemlock stands to sleep under because it intercepts rain and snow.”
The biggest issue is that there are no native predators for HWA and there is questionable hemlock resistance against them.
“Cold winter temperatures are one of the things that can stop HWA.” Studies in cold labs in 1998 to 1999, temperatures of -35 C (-31 F) was 100 percent lethal to HWA. There was some survival rate at -30 C (-22 F). With two cold winters in a row, such as in 2003 and 2004, HWA mortality rate was 82 to 100 percent.
In a 2016 study by Elkinton et al., it was shown that adelgids can acclimate to cold temperatures. HWA from cold interior regions could survive colder temperatures than those in the south and coastal populations. Sudden decline in temperatures following warm periods, like current weather patterns, can also cause HWA decline. However, because of their ability to reproduce without requiring a mate and their ability to lay large numbers of eggs, populations can quickly rebound.
They can be jumpstarted by warmer temperatures. After a very warm winter in Tennessee, HWA started laying their eggs in November, months earlier than normal.
They are called woolly due to the strands of waxy white substance they shed from their skin, visible underneath hemlock twigs, near the base of the needles. HWA extracts the trees’ nutrients, causing their needles to fall off.
In February, you can look underneath twigs and find the woolly presence of adults that have been there since July. They have a feeding tube, or stylet, that is inserted into the twig and begin feeding on warm winter days, laying 100 to 250 eggs per year per adult. In March and April, the eggs hatch as nymphs. This spring generation is called progrediens.
There are nine hemlock species worldwide. In a study of HWA recent genetics, they have been found in the south and low elevations in Japan. “They are a serious pest only in Eastern U.S. The Southern Appalachians have succumbed to this pest. When I first started, there was nothing above Massachusetts.” Now, HWA has made its way throughout Massachusetts and into coastal areas further north, including coastal Maine. They entered Connecticut in the early 80s and were seen in Forest Park, Springfield, MA in 1989.
Orwig was hired in 1995 to find out what HWA is going to do to the hemlock. Of eight long term sites he has studied in Connecticut, 90 percent of the hemlock has been lost and is still experiencing decline.
The HWA’s continuous feeding causes needle loss. The most typical and readily noticeable damage are the needles on the interior branches fall off first. The HWA doesn’t leave the forest; but varies in density. Studying Buffian Brook near Amherst, MA since 2003, hemlocks there have been defoliated.
Red maple, red oak, black birch and white pine will replace hemlock. Without hemlocks, there is a more rapid snowmelt and a moister ground surface, plus the replacement trees drink more water, effecting headwaters and stream interfacing.
Spraying of soaps and oils is still widely used — effective on ornamentals and a cheaper treatment than chemical applications which offer short-term protection. “Imidacloprid is an equal opportunity killer that kills most insects feeding on that tree. The good news is that it gives multiple years of protection.” It can be administered as a tree IV, drilled into the trunk to ensure that most of the chemical is drawn up by the tree, reducing risk of drift into water supplies.
Some agencies apply certain neonicotinoids, such as dinotefuran, as sprays on tree trunk bark. This chemical is quickly absorbed by the hemlock and can quickly kill elongate hemlock scale, another invasive insect. Imidacloprid, another neonicotinoid, can provide multiple years of protection from HWA. “The good news is we can protect hemlocks.”
If chemicals are ineffective, biological controls could and have been considered, but Orwig was not so sure how well the ladybeetle, Sasajiscymnus tsugae (S.tsugae) will do in controlling HWA. “Biological control is a slippery slope; you don’t know if they’ll hybridize,” he said. Many other biological control organisms have been introduced for HWA, including Laricobius nigrinus, a beetle native to the American Northwest that shows promise.
Answering an audience question about whether or not other Hemlock species can be used to help replace Eastern Hemlock in our forests, he said, “Western Hemlock can survive here. Do they look like they will do well? No, in my opinion.” Western Hemlock can’t be crossed with Eastern Hemlock, either. Substituting spruce along streambeds doesn’t provide the same cover as Hemlocks. “You can’t really replace Hemlock plantings along stream beds,” Orwig said.