Biological Control of HWA with
Larval and Adult Sasajiscymnus tsugae feeding on HWA ovisacs
(Photos: courtesy of Carole Cheah, Ph.D., CAES)
How Does Biocontrol Work?
Biological control of the Hemlock Woolly Adelgid (HWA) involves introducing predator organisms that can reduce HWA populations to a level that does not harm our native hemlocks. The Sasajiscymnus tsugae predator beetle (aka, Sasi or St), which is the native predator for our HWA from Japan, is the most effective and readily available HWA biocontrol option for private property owners with hemlocks.
Over 4 million Sasi/St beetles produced by taxpayer-supported, USDA labs have been released by USDA/USFS in national and state parks and forests. But most state and municipal hemlock areas and all private hemlock areas are dependent on private individuals and groups (and purchases from private predator rearing labs) to implement the biological control strategies needed to protect our hemlock ecosystems.
The hemlock woolly adelgid kills by overrunning and defoliating our native hemlocks. Each feeding adelgid desiccates only a single hemlock needle. But heavy HWA infestation not only destroys existing foliage, it also suppresses the production of new hemlock foliage. So heavily HWA-infested hemlocks die from the loss of foliage needed for water take-up, respiration and food production. Sasajiscymnus tsugae predator beetles interrupt this destructive cycle by dramatically reducing adelgid populations, allowing hemlocks to resume producing new foliage and gradually recover. Both adult Sasi beetles and their larvae are voracious HWA predators. And because these specialized beetles cannot reproduce without an adelgid food source, they have been judged (by USDA/APHIS) to not be a threat to other native US insect or plant populations.
What does Biocontrol do?
Biocontrol establishes a balance between predator (Sasi/St) and prey (HWA), it does not eliminate HWA completely! Maintaining a biological balance means that HWA will continue to exist – to support the predation and reproduction of the Sasi predator beetles. However, these remaining HWA will not be a threat to normal hemlock growth and recovery. A successful release will restore a natural balance in which predator beetles feed on Hemlock Woolly Adelgid populations that are limited to allow normal health and foliage growth in our native hemlocks.
Once the predator beetles are established, it is easy to monitor Sasi predator beetle activity and its effects on Hemlock Woolly Adelgid. Each of the two HWA reproductive cycles (October-April, May-July) begin with new white round “cotton-ball” ovisacs at the base of hemlock needles. So if you turn over an infested branch and see all round balls, then the beetles have not yet begun feeding at that site. But in feeding, the beetles tear open the HWA ovisacs to consume HWA and eggs, leaving shreds of white residue. Below is a photo showing the before and after of Sasi predator beetle predation. The upper twig – not yet visited by the predators – has lots of round HWA ovisacs, each containing a single HWA and its many eggs. The lower twig (from a nearby branch) has had all HWA ovisacs recently destroyed by Sasi, leaving only white residues where the HWA ovisacs were previously attached. Yum Yum!
Where did “our” Hemlock Woolly Adelgid come from?
The HWA that is devastating our eastern hemlocks was introduced from Southern Japan around 1911 on horticultural nursery stock (Tsuga sieboldii) that were delivered for newly created Japanese Gardens at several Gilded Age Estates in the eastern US. (Perhaps an instance of “keeping up with the Vanderbilt’s”? For more on biological control of HWA at Biltmore Estate, go to Biltmore.)
This insect was first identified as Hemlock Woolly Adelgid from samples collected at Maymont Estate in 1951, in urban Richmond, (located outside the native Eastern hemlock range). But historical research suggests that this HWA population had been growing and spreading for 40 years. And it was another 40 years before HWA biocontrol efforts were initiated by a CT researcher in 1991. The effect of this 80 year delay (1911-1991) in mounting an HWA control effort was to allow a massive HWA population buildup in eastern US hemlock areas, which then plunged down across the Southern Appalachians in the form of a biological “HWA avalanche”, destroying most hemlocks in its path. (See animated USFS map.)
Where did our HWA Predator come from?
Biocontrol efforts for Hemlock Woolly Adelgid in the eastern US were initiated by entomologists at the Connecticut Agricultural Experiment Station (CAES) in the early 1990’s and were supported (but not initiated) by USDA. The objective was to locate and test (under quarantine) predator organisms that could control HWA population growth. Curiously, the first CAES quest for an HWA predator went to southern Japan – which 15 years later would be identified, by DNA analysis, as the origin of the HWA introduced to the eastern US!! How did that early ID happen?
In the 1980’s, CAES entomologist Mark McClure published several articles documenting Japanese garden sites in which both Eastern (T. canadensis) and Southern Japanese (T. seiboldii) hemlocks existed in good health with varying levels of HWA infestation. So these healthy Eastern hemlocks with HWA present in Osaka, Japan, attracted CAES explorations for HWA predators in the early 1990’s. As a result, the native predator for our HWA “import” had already been researched and approved for release in the Eastern US, well before the Southern Japan origin of this HWA was identified.
Predation and Resistance in HWA control
In its Southern Japanese homeland area, the presence of Hemlock Woolly Adelgid does not damage the native hemlocks –Tsuga sieboldii, even though this hemlock species is very similar to our Eastern and Carolina hemlocks in having low levels of biological resistance to this HWA. (See 2008 Research Report on Resistance of Hemlock Species to HWA) So this tells us that biological control is critical for hemlock survival in southern Japan. And the native predator beetle that controls this HWA in Japan is a small, black ladybird beetle by the name of Sasajiscymnus tsugae (Sasi or St for short … until 2004 it was called Pseudoscymnus tsugae or Pt – same beetle, different name.)
Thanks to Nathan Havill’s ground-breaking DNA-mapping research on HWA, we now recognize that the Hemlock Woolly Adelgid found in the Pacific Northwest, once thought to be the source for our HWA infestation in the eastern US, is a different biological organism. And both western and eastern hemlock species show considerable natural resistance to this western HWA strain, which has been present for centuries in the Pacific Northwest. The western HWA is rarely found on wild hemlocks in the Pacific NW, and is not considered a plant pest by state authorities. Instead it is typically found on hemlocks in urban or residential areas that have been weakened or stressed by human activities. So this suggests a limited biocontrol role for the western HWA predator beetle, Laricobius nigrinus as well as other possible HWA predators from the Pacific Northwest.How effective is Sasasajiscymnus tsugae as an HWA predator ?
In 1995 USDA approved Sasi (St) beetles for CT field release, and careful CAES field research activities have established the effectiveness of Sasi as an Hemlock Woolly Adelgid predator. (See the 2011 Research Report on Sasajiscymnus tsugae). These field research results include up to 80% adelgid reductions in a single season, a 2 generation-per-year (bivoltine) reproductive cycle that allows Sasi to match the exponential growth capability of Hemlock Woolly Adelgid, an ability to match HWA in over-wintering in cold winter climates, and an ability to move (and expand coverage) considerable distances over the course of a single season.
CAES field research has progressed far beyond USFS assessment practices by including hemlock recovery as a critical biocontrol outcome. For example, CAES research assessments comparing treatment and control sites 4-6 years after Sasi releases, found that hemlock foliage densities were significantly improved and HWA concentrations were significantly reduced where beetles were released. (See Assessments of Biological Control of HWA with Sasajiscymnus tsugae)
A grass-roots community effort in the Brevard NC area has involved city and county governments, conservation groups, summer camps, neighborhood associations and individual landowners in an HWA biocontrol effort that should safeguard the future health of hemlocks in the “greater Brevard area”. Eight years after multiple Sasi releases in Brevard and surrounding mountain woodland areas, we can see hemlock restoration underway – with increasing new foliage production and reduced adelgid populations in both urban and wild hemlock areas. See hemlock restoration in action in western North Carolina.
Recovering Hemlock Hedge in Brevard
So the success of the Brevard experience offers an urban “proof of concept” that once established, Sasi can maintain HWA populations at levels that can be tolerated by our Eastern hemlocks. A release of Sasi beetles in 2007, funded by Transylvania County and executed by Ecology students at Brevard College has created a protective zone for hemlock recovery and growth in central Brevard, originating at the Silvermont Mansion release site and extending over the surrounding urban areas. And the actions of numerous individual property owners and neighborhood groups have extended this protective covering over much of central Brevard, as well as outlying areas.
More recently, the Carolina Mountain Land Conservancy (CMLC) has encouraged and assisted its conservation easement property owners in initiating HWA biocontrol efforts in hemlock areas throughout CMLC’s multi-county area in western North Carolina. But it has placed special emphasis on protecting Carolina Hemlock colonies on Land Trust conservation lands in the Hickory Nut Gorge area. Below are CMLC Stewardship Director Julia Brockman and CMLC volunteer Patrick Horan releasing Sasi beetles on a large (31″ DBH) Carolina Hemlock on CMLC’s Weed Patch property in the NC Hickory Nut Gorge. Often the hardest part of a release on a large tree like this is fastening the coffee filter to the trunk! But how polite would it be to wish your little predators “Bon appetit” and then dump them on the ground? (For more information on CMLC’s hemlock restoration efforts, involving both Carolina hemlocks (Tsuga caroliniana) and eastern hemlocks (Tsuga canadensis), go to Community page.)