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Hemlock Woolly Adelgid Resistance (9 Replies)

Posted January 5, 2019 at 11:51 am

Has anyone in the NE Region had enough experience with any of the tsugas other than canadensis to say that they are truly HWA resistant? I volunteer in a local park where the tsuga canadensis population has been decimated by HWA. We’re wondering if it’s worth trying to plant any of the other tsuga species.

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Posted January 16, 2019 at 12:58 pm

I work across New England and have been observing HWA-related decline in hemlocks for over 20 years. If your hemlocks are not dead, have you tried dormant oil spraying, pesticide stem injection, or root drenches?

One of the main papers documenting a replacement Tsuga for managed landscapes was written by Del Tredici and Kitajima and published in Arnoldia (63/2). I attach two tables from the paper, below.

In case you are interested, there are good articles on biocontrols vs. HWA here:

You can buy predatory Sasajiscymnus tsugae beetles here:

I am doing some beetle trials in NW CT this year in an old-growth hemlock stand. I want to use two colonies to start as they are expensive because it is challenging to rear them.

Good luck!

Tables from Del Tredici and Kitajima (2004):

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Posted January 16, 2019 at 1:11 pm

I dug this out of my archives of HWA data, posted by Paul Jost to the Eastern Native Tree Society’s forums in 2007:

Date: Wed, Nov 7 2007 6:57 pm
From: “Paul Jost”

The following is a list of potentially suitable hemlock species in order of
reported cold hardiness. The number on the left of the species name is it’s
ranking of adelgid resistance. The numbers to the right of the species
names are the ranges of USDA hardiness, with multiple range minimums and
maximums if numerous values are reported by different sources, possibly due
to varied seed sources? They are from my crude notes and are not
presentation grade, but have value enough to be shared.

Tsuga canadensis, Eastern Hemlock, 3-7

*2 Tsuga diversifolia, Northern Japanese hemlock, 3a(4)5-7 to -30 or -35C
and more. (I lost my reference to a lower minimum value.)

Tsuga caroliniana, Carolina Hemlock, 4-9

*3 Tsuga mertensiana, Mountain hemlock, 4b(5)-9 Idaho, Montana, British

*5 Tsuga heterophylla, Western hemlock, 6-8/10

*1 Tsuga chinensis (or oblongisquamata or tchekiangensis or formosana),
Taiwan hemlock, 6

*4 Tsuga sieboldii, Southern Japanese hemlock, Araragi hemlock, (4/5)6-9

Tsuga forrestii, Forrest’s or Lijang hemlock, dumosa x chinensis?

Tsuga dumosa (or yunnanensis or brunoniana or longibracteata), Himalayan
hemlock, Yunnan tieshan 7

Good reading on hemlock replacement species can be found at:

Paul Jost

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Posted January 21, 2019 at 3:05 pm

Harry – thanks for your responses. I’m wondering about the HWA resistance ranking from the Paul Jost presentation. What is the range what value represents highest resistance? I’ve tried poking around the Us Forest Service site for the proceedings he suggested. However that site seems to be severely impacted by the govt. shutdown. I went to the Arnoldia site and got the complete article you mentioned. Going to try to track down some of those references.
Mark Whitmore from Cornell release laricobius ngrinus beetles in the park and surrounding area in 2013. I participated in follow up scouting activities in 2014 and 2015. After that I gave up. The release area is very steep sandy loam and I was spending most of my time trying to remain upright. There was at least one more scouting trip in 2016. To the best of my knowledge no beetles were ever found. The last I knew, Mark was focusing on “silver flies” but as far as I know, none were released in this area. The local DEC Regional Forester didn’t know much more than that. I’d be interested hearing about you experience with these other beetles.
The local parks department did treat some hemlocks in the park with a trunk spray last fall – strangely none in the arboretum. I’m going to take that up with them in the spring. We still have a few that might be saved. It’s really difficult to do anything in public parks.

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Posted January 21, 2019 at 5:23 pm

I agree with the ratings for tsuga diversifolia. We have had ours for 25 years and have never had a problem. It is growing next a Tsuga Canadensis which I have to spray every year. It is also near a tsuga caroliniana which I usually have to spray as well. We live in zone 5B in CT. I have had good luck using insecticidal soap. It is not as effective as the other methods but it is easy to apply and you do not have some of the timing issues that the other methods have.

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Posted January 21, 2019 at 5:30 pm


The above list is ranked in order of cold hardiness. Here is the list ranked by most resistant to least resistant to the adelgid, again, according to Jost:

*1 Tsuga chinensis (or oblongisquamata or chekiangensis or formosana), Taiwan hemlock, Zone 6
*2 Tsuga diversifolia, Northern Japanese hemlock, Zones 3a(4)5-7 to -30 or -35C and more.
*3 Tsuga mertensiana, Mountain hemlock, Zones 4b(5)-9 Idaho, Montana, British Columbia
*4 Tsuga sieboldii, Southern Japanese hemlock, Araragi hemlock, Zones (4/5)6-9
*5 Tsuga heterophylla, Western hemlock, Zones 6-8/10

Resistance not described by Jost:
Tsuga caroliniana, Carolina Hemlock, Zones 4-9
Tsuga forrestii, Forrest’s or Lijang hemlock, dumosa x chinensis?
Tsuga dumosa (or yunnanensis or brunoniana or longibracteata), Himalayan hemlock, Z 7

I ended up reserving 4 colonies of Sasajiscymnus tsugae beetles for this year’s trial in a NW CT old-growth stand. According to Jayme at Tree Savers, she has had excellent results with them and the treated hemlocks not only recover quickly but also gain the strength necessary to knock off loopers and other secondary parasitic agents. No doubt that they are expensive, but they are terribly difficult to rear. She said that they have excellent cold hardiness with many released and re-captured in northern Maine studies. They only dine on A. tsugae so they perish once their prey is gone.

If you are going to use pesticides, you should be careful about timing – bees are particularly vulnerable to neonics and other broad spectrum chemicals.

I also ran into the government shutdown with USFS servers being off-line today – I was working on a project in northern VT and needed stand data and got the bad-address returns. I also can’t get my USDA EQIP grant applications completed as the Farm Service Agency is shut down and with a February 1 deadline, I’m looking at losing a whole year of restoration work. Not cool, Uncle Sammy.

Good luck!

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Posted January 21, 2019 at 5:56 pm

Hi Ed,
I’ve used dormant oil w/garlic on my large T. canadensis up here in Colebrook. My arborist has a sprayer with 90 vertical feet capacity and he sprays about 2.5 acres of our streamside property which is pretty much adelgid-free. Across the brook and in Algonquin State Forest, the riparian hemlocks are absolutely decimated. I wish that the State of Connecticut would release Sasajiscymnus tsugae beetles at least in the stream corridors in the state forests because the ecological implications of their loss in these habitats is stunning. But they’re more interested in cutting down forests for rabbits than protecting them. But I digress.
Kind regards,

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Posted January 25, 2019 at 2:27 pm

So Ed, you’ve never had to treat the tsuga diversifolia for adelgid? We only want to consider a tsuga that won’t need to be treated for the arboretum because we know they won’t get treated. We are sadly watching our hemlocks fading away. They were used extensively in Durand along with white pine and a few other natives to serve as the backdrop for more exotic specimens. We are losing two unique Scots pines – Beauvrensis (hope I spelled that correctly) and another ? cultivar because they won’t treat them.

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Posted January 30, 2019 at 4:50 pm

Hi Linda, No I have never treated the diversifolia. I just took a look at it and it even touches the tsuga canadensis in some spots. The first wave of adelgid was many years ago. These trees were much smaller then and it has been manageable. Both trees are about 25 years old and around 20 feet tall. I have topped the tsuga canadensis and kept it very open so I could monitor it easily and do a good job spraying.

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Posted February 4, 2019 at 12:09 pm

Harry – I’d be interested to hear what results you experience with your beetle test. I know that’s a long-term thing, but I’d like to be kept in the loop if you think of it. Linda

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