Conifer Pests in Canadian Hemlock

By Frank Goodhart

Read about the challengs of growing the hemlock, a dwarf conifer native to the US. This is part 3 of the Canadian Hemlock series. Click here to read part 1 and part 2.

Tsuga canadensis ‘Sargentii’ (Sargent’s weeping hemlock) interior
Tsuga canadensis ‘Sargentii’ (Sargent’s weeping hemlock) interior

The Canadian hemlock is distinctive among other hemlock, since so many different forms were found and propagated. The tendency for it to produce atypical forms exceeds all other species in the Tsuga genus, as well as many other ornamental conifer genera, used in the gardens of today. Early named cultivars were found as wild seedlings in the forest. Later on, nurserymen intentionally planted hemlock seeds and looked for unusual forms.

New and different forms and growth rates were noted in these conifer experiments, resulting in the naming of some. It was concluded that the variations consistently found among Canadian hemlock were of a peculiar genetic property. It may be that the mutant selections of hemlock could have come from seeds of trees and seedlings of witch’s brooms.

It appears that cultivars from witch’s brooms of Canadian hemlock were identified during the later phase of cultivar selection. John Swartley reports seeing many witch’s brooms in the forests as well as in gardens. The monographs for the cultivars sometimes list the source as a seedling or a witch’s broom, but the sources of many are unknown.

Tsuga canadensis ‘Abbott’s Pygmy’ (Abbott’s Pygmy Canadian hemlock)
Tsuga canadensis ‘Abbott’s Pygmy’ (Abbott’s Pygmy Canadian hemlock)

Conifer-Growing Challenges

The production of Canadian hemlock cultivars by some nurseries diminished starting about 25 years ago because of the insect and disease problems infecting the conifer species. In eastern U.S., cultivars were available from a number of small nurseries which no longer exist. Fortunately, many cultivars are now available from some West Coast specialty nurseries. It appears that Canadian hemlock cultivars may be making a comeback in areas, where the tree had been severely affected by several insect and disease problems.

Landscape use of Canadian hemlock is very limited today because of hemlock scale and woolly adelgid. Scale was identified in the Philadelphia area in the 1970’s and was very prevalent in northern New Jersey about 50 years ago. Trees died slowly over a period of time. The hedgerow of hemlock at the Watnong Nursery was removed in the 1980’s due to infestation. Soon thereafter, woolly adelgid appeared and killed off the rest of the trees in the Northeast.

Fear was put into the hearts of dwarf conifer collectors, who abandoned collecting and using dwarf hemlock. After a few years, it was found that these diseases are not prevalent in cultivars, and it appears that there is no well-defined reason for this. It has been hinted that the disease is spread by birds, but not on smaller plants.

Conifer Scale Infestation

Hemlock scale was introduced into the U.S. in 1908. It is commonly known as the elongate hemlock scale or as the fiorinia scale (Fiorinia externa). Infected branches have flat, waxy, elongated deposits under the needles. Female eggs hatch to form a type of nymph which crawls to the undersides of unaffected leaves. The mouth parts of the nymphs are inserted into the needles and suck out the fluids of the plant, while injecting a toxin. This toxin causes the needles to yellow and die. This cycle repeats itself several times during the growing season. This yellowing progresses throughout the tree in succeeding years and disfigures the tree as more and more branches die.

An even more serious disease of Canadian hemlock is the woolly adelgid (Adelges tsugae). It first appeared in western states of North America in 1924 and then much later in the area of Richmond, Virginia in 1951. The insect has been traced to southern Japan, where it has not affected the native conifers, either due to natural predators or the development of resistantance over time.

Hemlock woolly adelgid (HWA) is easily identifiable as it produces white, foamy-looking, egg masses that are cottony in appearance on the undersides of the leaves. Larvae hatch in the spring and feed on the phloem sap of young, tender twigs on the outer part of the branches. HWA asexually reproduces, and there are frequently two generations per year. The conifer branches die back each year, and if once infected and not treated, die within 4 to 10 years.

There seems to be a correlation between the cold hardiness of HWA versus the ability of the trees to be unaffected. Recently, I have seen Canadian hemlock in the Allegheny Mountains of West Virginia and in New Hampshire, which are free of woolly adelgid.

A hemlock hedge, formerly common, before identification problems
A hemlock hedge, formerly common, before identification problems

Smart Pest Management in Conifers

The insect can be partially controlled by application of horticultural oil. The timing of treatment is important, but this can be overcome by more frequent spraying and by using an integrated pest management (IPM) approach.

Trunk and soil pesticide injections are also effective via licensed professionals. More recently, it has been discovered that the black lady bug (Pseudoscymnus tsugae) from Japan has been an effective biological control. It has a life cycle similar to HWA and has been shown to be 47% to 88% effective in 5 months at sites in New Jersey, Connecticut, and Virginia.

Two other predators have been evaluated for the control of HWA. These are Laricobius nigrinus (tooth-necked beetle), native to the Pacific Northwest, and Laricobius osakensis (a species of derodontid beetle) native to Japan. Laricobius nigrinus beetles prey naturally on the HWA and have been released in a hemlock grove near Lansing, New York. It is hoped that it will be established after 2 to 3 years. No pesticides will be used in the area, and final evaluation will be made after 10 years.

L. osakensis, a relative of L. nigrinus, has also shown promise in field trials. It was first evaluated at Virginia Tech, Blacksburg for several years before obtaining approval from the USDA for release for evaluation in some natural forest sites in Virginia. It has been effective in reducing HWA infestations and has survived and reproduced naturally in the forest.

A different approach has been taken by the Daniel B. Warnell School of Forestry and Natural Resources, University of Georgia, Athens. Instead of seeking a predator for HWA, researchers created in vitro cultures from Canadian and Carolina hemlock not affected by HWA. Using cryopreservation, germ plasm was frozen, extracted, thawed, and then injected into the trees. Success was attained from all 3 samples of Carolina hemlock and 1 of 2 samples of Canadian hemlock.

Hemlock Hope

With several means to arrest the decline of Canadian hemlock showing promise, there is now optimism that someday the conifer species will regenerate itself naturally in the forest. Perhaps Canadian hemlock will once again be planted in landscapes, and the cultivars will regain a place in the garden as in former times.

However, the availability of hemlock cultivars is much less than it was 25 to 30 years ago. Aside from the insect and disease problems, many nurseries formerly growing hemlock are now closed. It appears that the wide range of cultivars will no longer be available except from collectors and small local nurseries. One may refer to the websites of Iseli Nursery, Stanley and Sons Nursery, and others to see what cultivars are in their catalogs. Generally, the listed cultivars are available on a rotating basis, depending on propagation schedules.

Photographs by Frank Goodhart.

This article was originally published in the Spring 2019 issue of Conifer Quarterly. Join the American Conifer Society to access our extensive library of conifer-related articles and connect to a nationwide group of plant lovers! Become a member for only $40 a year and get discounts with our growing list of participating nurseries in our Nursery Discount Program.