Biological Control of the Hemlock Woolly Adelgid
The Hemlock Woolly Adelgid (HWA) is a non-native insect from Asia that is threatening the
existence of native hemlock trees throughout the eastern US. In Asia and the Pacific Northwest,
hemlocks and HWA coexist “peacefully” and HWA is not considered a plant pest. But in
Virginia’s Shenandoah Valley, where our current HWA infestation began, hemlock mortality is
over 80%. Why the difference? What all these other areas have, and the eastern US does not, is the
presence of one or more specialized predator beetles that feed, more or less exclusively, on HWA
and thus control its population.
Biological control of HWA utilizes insect predators that feed on the HWA. The objective is to
create self-sustaining populations of these predator beetles, so as to reduce adelgid populations
below levels that threaten the health of their hemlock hosts. Such a natural balance between
hemlocks, HWA and predator beetles is present in all other hemlock areas. And there are a number
of different “specialist” predators for the different HWA populations.
USDA efforts to identify and study HWA predator beetles began in 1992. Possible predators were
studied both in their home environments and in US quarantine labs to assess their suitability for
release in the US. Selection criteria included dietary specialization on and life-cycle synchrony
with HWA, and compatibility with climate conditions in the eastern hemlock zone (running from
NE Georgia to Maine). All beetles approved for release require an adelgid food supply to complete
their reproductive cycle.
The first beetle that was approved by USDA for release in the US was a small (poppy-seed size)
black beetle from Japan (Pseudoscymnus tsugae, recently renamed Sasajiscymnus tsugae). This
beetle is the native predator of the HWA strain that has infested the eastern US. Releases of
laboratory-reared beetles began in Connecticut in 1995, in adjacent states in 1999 and in western
NC in 2003. Over 2 million Sasajiscymnus tsugae (Sassie) have been released in the US.
Insect-rearing laboratories have been established at an NC lab in Cary, Clemson, UT Knoxville,
and in GA at UGA, Young Harris & NGCSU to produce Sassie and other approved predator
beetles for release on National Forest lands in the Southeast. Currently, private labs in PA and NC
are producing Sassie beetles that are available for use by private landowners and public interest
groups – with a Feb-April “window” for release. For more information, see Beetle Sources below.
Common Misconceptions about Bio-Control of HWA:
Biological control will not eliminate all hemlock woolly adelgids. It just reduces the number
of adelgids, to a level that will not cause damage to hemlocks.
All USDA approved predator beetles are “specialists”, feeding on HWA & other adelgids
and/or aphids. The beetles’ reproductive cycle requires adelgid food sources. They are not a
threat to other beneficial or non-pest insects.
Biological control is not limited to large woodland tracts or to trees that are still healthy. It
can be used effectively in residential areas that have a population of hemlocks and with trees
that are seriously HWA-defoliated.
Saving Hemlocks
Landowner Services for Biological Control of HWA
Contact Patrick Horan at (828) 884-5097 or phoran@uga.edu
Research Results (See links below or go to http://na.fs.fed.us/wv/hwa/ Bibliography and enter
name)
Research on field releases of Sassie (Sasajiscymnus tsugae) have established its ability to
successfully reproduce and overwinter in North America, surviving subzero temperatures.
(McClure, 2001)
Biological Control of the Hemlock Woolly Adelgid
Followups on Sassie field releases have identified significant declines in HWA concentrations,
and significant improvements in hemlock health, both sort-term (McClure 2001) and longer-term
(Cheah et al 2005).
The beetles have shown the ability to spread rapidly. Conway (2005) reports collecting beetles
400’ from release sites after 6 months. I have seen beetle movements over 2000’ along waterways
in a year.
DNA research on HWA has identified the origin of our HWA “import” (brought in on Japanese
hemlocks) as Tsuga seiboldii on Honshu Island, Japan. And these are the same trees and location
where Sassie (Sasajiscymnus tsugae) was collected (in 1992).
Reference Sources for Control of Hemlock Woolly Adelgid
http://www.invasive.org/hwa/ This is a 2004 Forest Service report on HWA biological control
strategies with good information on the USDA-approved predator beetles, including Sassie
(Sasajiscymnus tsugae).
http://na.fs.fed.us/fhp/hwa/index.shtm This is a good USDA forest service source on HWA in the
northeastern US, including information on both chemical and biological control. It provides links
to beetle release maps and #’s, as well as numerous professional papers and publications.
http://www.na.fs.fed.us/fhp/hwa/news7/hwa7.pdf This 2006 HWA Newsletter contains a report
on recent HWA research efforts, including DNA testing to differentiate HWA populations.
http://www.saveourhemlocks.org/ This is a good “local” source on HWA control issues in the
Southeast. It includes detailed information on timing and utilization of chemical control strategies.
Research Reports on Sassie beetle effectiveness
http://na.fs.fed.us/fhp/hwa/pub/hwapub2001.pdf This is a comprehensive report on CT field
research on Sasajiscymnus tsugae (then known as Pseudoscymnus tsugae) – it documents the
reproductive and predatory capabilities of the most effective HWA predator beetle currently
available!
http://www.na.fs.fed.us/fhp/hwa/pub/2005_proceedings/cheah.pdf This study compares Sassie
release sites in CT and NJ with comparable hemlock areas where no beetles were released. Four
years after release, significant differences were observed between treatment and control sites.
Beetle Sources (Prices may be out-of-date … check with source for current info)
Ecoscientific Solutions: Accepting Orders for 2008; (500 min) 1000 beetles @$2000 (+shipping)
PA (570)-335-7925 www.sasajiscymnustsugae.com
Conservation Concepts: May be Accepting Orders for 2008; (1000 min) 1000 beetles @$2290
NC (828)-221-2142 www.conservationconcepts.org