How, When & Where was Hemlock Woolly Adelgid Introduced to Eastern North America?
Not much attention has been given to the historical introduction of Hemlock Woolly Adelgid to the Eastern US. We know that HWA was first identified from a 1951 sample collected in Richmond VA. But was Richmond the only HWA introduction site in Eastern North America? What was the mechanism for HWA introduction? And did that introduction really occur as late as 1951?
There are two important lines of scientific research on HWA that can help address this issue. One line of research has applied statistical analysis of DNA from all known world Hemlock Woolly Adelgid populations, to identify the place of origin of the HWA introduced to the eastern US. The other area of scientific research has produced a historical map, based on early USDA monitoring records for HWA, that can be used to examine the HWA introduction process for the eastern US. And combining these two resources provides a scientific foundation for examining the introduction of Hemlock Woolly Adelgid into eastern North America.
The HWA genetic research published by Nathan Havill and associates (in 2006 & 2016) has transformed our understanding of world Hemlock Woolly Adelgid diversity! This research has not only documented biological differences between HWA lineages in the US and multiple Asian countries. It also has definitively identified the HWA found in the Eastern US as originating in Southern Japan. Below is the 2006 chart identifying five biologically distinct HWA lineages. (The 2016 analysis identified three additional HWA lineages located in China, Taiwan and Ullung Island.)
Where was Hemlock Woolly Adelgid Introduced?
This DNA analysis tells us that “our” Hemlock Woolly Adelgid came from Southern Japan, but how did this introduction occur – at a single site or at multiple locations in the Eastern US? We can use USDA historical data on HWA identification sites during the 30+ year period from 1971-2002 to consider this question. But please note that these USDA-collected data do not represent dates for the “First Introduction” of HWA (as labeled), instead they are dates for the “First Observation” of HWA by USDA representatives in the mid-20th century.
The historical map below, prepared by the US Forest Service, uses county-level data representing the first year in which HWA was confirmed present in each county in the Eastern US. Note that the sites of earliest HWA observations (1971-1984) are marked with the darkest red color, the next set of HWA observations (1985-1990) are marked a tan color. And later HWA observation sites (1991-2002) are marked bright red.
To interpret these data, we can ask: Where are the early Hemlock Woolly Adelgid introduction sites in the Eastern North America? And what is the pattern of HWA movement over time? For the first question we can note 5 geographically distinct sites where early HWA infestations were reported, as indicated by dark red coloring for Central Virginia mountains, Richmond, coastal Virginia, Philadelphia and Long Island. For the pattern of movement question, we need to note the transition on the map from dark red areas to tan areas to bright red areas, where the HWA are observed moving out to counties with previously uninfested hemlock areas.
My objective here is to use these historical USFS data on HWA distribution and dispersion to test “HWA introduction hypotheses” about single vs multiple HWA introduction sites. And again a cautionary reminder that these USFS data do not represent dates for the “First Introduction” of HWA (as labeled), instead they represent dates for the “First Observation” of HWA. And I will provide evidence below that the actual introduction dates for the Hemlock Woolly Adelgid from Southern Japan are much earlier than those presented in the map.
The first map (below) depicts the single-HWA-introduction-in-Richmond hypothesis which has dominated most USDA interpretations. The ovals represent hypothesized spread from a specified point, in this case Richmond, VA. Here, the hypothesis of outward distribution from a single site in Richmond works reasonably well in accounting for Hemlock Woolly Adelgid outflow in the Southeast, but it does not account well for the observed HWA distributions in the northeastern US.
Single Site Introduction Hypothesis for Hemlock Woolly Adelgid
The second map (below) presents the competing multiple-HWA-introduction-sites hypothesis, using four early Hemlock Woolly Adelgid introduction sites suggested by the USFS data: Central Virginia mountains, Richmond VA, Philadelphia PA and Long Island NY.
Multiple Site Introduction Hypothesis for Hemlock Woolly Adelgid
This multi-site hypothesis seems to work much better in accounting for Hemlock Woolly Adelgid dispersion throughout the eastern US – north as well as south. But what could be the explanation for specific HWA introduction sites proposed?
How and When Was Hemlock Woolly Adelgid Introduced?
This question was first raised in 2008 by USDA researchers Nathan Havill and Michael Montgomery. They followed up the DNA research evidence to search for a “Japanese connection” in historical documents for Maymont Estate in Richmond, Virginia – the site of the original HWA discovery. And this search led to reports on a 1911 Japanese Garden construction project at Maymont, involving international Japanese garden designer, Y Muto.
HWA biological research suggests that cross-continental transfers of Hemlock Woolly Adelgid from Japan would require “host plants” from one of the two species of live plants native to southern Japan on which HWA can survive. These are Southern Japanese Hemlock (Tsuga seiboldii), which hosts the HWA asexual reproductive cycle, and the Tigertail Spruce (Picea torano, aka Picea polita) which hosts the HWA sexual reproductive cycle. And Muto’s affiliation with the world-famous Yokohama Nursery provides a commercial source for both these Japan-origin landscape plants.
In 2017 Patrick Horan extended the search for additional Muto-affiliated ‘Gilded Age Garden’ sites in the eastern US. This extended search has located documentary evidence for three more early 20th century Japanese gardens involving Mr. Muto – in addition to that at Maymont in Richmond. And these locations correspond to the early HWA introduction sites identified in the USFS map. So this suggests a Muto-affiliated ‘Gilded Age Japanese Garden’ site for each of the Hemlock Woolly Adelgid introduction sites identified in the USFS monitoring data!
While this evidence for Muto-affiliated garden sites in Long Island, Philadelphia, Richmond and the central Virginia mountains is suggestive, it doesn’t prove that HWA was introduced at these sites. However, it does provide a plausible hypothesis that is consistent with both USFS monitoring data and existing archival documentary evidence for these early 20th Gilded Age Garden sites. This documentary evidence includes 1) Muto’s participation in the construction of these Japanese gardens, 2) the presence of Muto-era Japanese Hemlock and Spruce trees located at these garden sites and 3) documentation of purchase of such horticultural specimens from the Yokohama Nursery in Japan.)
Gilded Age Japanese Garden Sites
The map above presents the four Muto-affiliated Gilded Age Japanese Garden sites that have been identified to date. Three of these sites have some archival evidence as well as the involvement of internationally-oriented garden owners: John and Lydia Morris of Compton Estate in Philadelphia, and James and Sallie Dooley at both their Maymont Winter residence in Richmond and their Swannanoa Summer residence in the central Virginia mountains.
Both John Morris and Sallie Dooley were noted for their participation in international horticulture in the early 20th century. Morris had employed Mr Muto for multiple turn-of-the-century Philadelphia area Japanese garden projects, including several on his own Compton estate. And James Dooley described his wife Sallie’s garden orientation as involving the “purchase [of] the most costly evergreens from all parts of the world” – a budgetary topic on which he was presumably well-informed!
But for Long Island, beyond a report of Muto’s 1910 participation in a Japanese garden project, there is no documentary evidence indicating the specific site of this garden. As a large Japanese garden project pictured in 1915, the Marshfield Estate fits the profile and time frame. But there is no further evidence connecting this site to Muto or Japan-origin plants. So this should be considered only as a “possible” Long Island garden candidate, pending further historical research on Long Island gardens of that era.
Finally, the Virginia coastal site, indicated on the USFS map, is more likely a later garden HWA transfer from Richmond rather than an introduction from Japan.
Directions for Further Research
There are important differences between scientific applications to contemporary topics, as opposed to historical topics. For example, it is often possible to collect additional data to address different hypotheses that arise for contemporary topics, whereas it is rarely possible to go back in time to collect additional data to address historical topics. But here are some suggestions about possible additional historical data that would be relevant to this inquiry.
Further archival research will be needed to evaluate and elaborate the multi-site HWA introduction hypothesis proposed above. I welcome the participation of others in this research effort. And I acknowledge the helpful contributions of Curators Dale Wheary at Maymont and both Leslie Crane-Smith and Anthony Aiello at the Morris Arboretum to my own documentary research efforts
Another valuable research extension would involve the original HWA observation reports collected by USDA, which were used to construct the USFS historical county-level map utilized here. (The county-level data for the 1968-2002 period are available from USFS, but not the individual site reports from which these county-level data were derived.) So there remain some important unknowns about these original observational report data. For example, who were the USDA representatives reporting on Hemlock Woolly Adelgid sightings and where are the resulting site reports located?
My current hypothesis is that these data were collected by USDA County Extension offices in each state. And that would mean that these historical data for local HWA identifications are probably stored in USDA Extension archives at the land-grant universities in the respective states. But I have not yet been able to confirm the existence/location of these individual site records with officials at USFS or USDA.