Most southern hemlock areas of the eastern US have already been “buried” under an avalanche of Hemlock Woolly Adelgid (HWA), generated by exponential growth from multiple horticultural HWA introductions in the early 20th century. But there are extensive hemlock areas in Michigan and the upper Midwest that face a threat from recent HWA introductions, while northern areas of Vermont, New Hampshire, Maine, & upstate New York are just beginning to experience the HWA influx (due to the colder winter temperatures that have slowed HWA movement). And these are the targets for the early intervention strategy proposed here.
Below is the latest (2015) USDA Map documenting HWA Infestations in the eastern US, although this is already a bit out of date. For example, the HWA infestation area in western Michigan now encompasses four counties and is no longer considered to be “eradicable” by chemical means. But this at least provides baseline information about the proximity of northern counties to the HWA advance.Until recently, the upper Northeast has been protected from HWA by extreme winter temperatures while Michigan has been protected by geographic separation from early HWA introduction areas further east. But now these areas with newly or soon-to-be HWA-infested hemlocks have a special opportunity to initiate biological control intervention strategies that can prevent or limit HWA damage to native hemlock populations.
Such “early intervention efforts” can draw on over 20 years of USDA-funded scientific research and knowledge about the multiple strains of hemlock woolly adelgid – including HWA origins, predators, and ecology. For example, we know that our”imported” HWA (in the eastern US) originated in Southern Japan, where it exists in “peaceful coexistence” with both the native Southern Japanese Hemlock (Tsuga seiboldii) and imported hemlocks (Tsuga canadensis) from the eastern US. And the key to this coexistence is the activity of a “specialized predator” beetle (Sasajiscymnus tsugae) that feeds only on HWA and other adelgids, (and is not a threat to other native insects or plants).
Hemlock areas that are early in the HWA infestation cycle – i.e., not yet inundated with massive HWA infestations – have the option of introducing this USDA-approved, Japan-origin HWA predator –Sasajiscymnus tsugae (aka St or Sasi) before significant damage to hemlocks occurs. USDA-funded research suggests that such biological control interventions are capable of extending protection to larger hemlock areas and entire ecosystems. (Here is the commercial source for this predator.)
Early Intervention Strategy
It is even possible to establish a predator population to control Hemlock Woolly Adelgid before HWA arrives in a hemlock forest area! This early intervention strategy involves using a native woolly adelgid as a food source to “grow” and establish the predator beetle population. This will create a “predator welcome commitee” to greet advancing HWA arrivals! And this in turn should minimize HWA damage to hemlocks in these areas.
The prime adelgid food candidate for such an early intervention strategy is the Pine Bark Adelgid (Pineus strobi) which is present on most White Pine (Pinus strobus) in the eastern US and which reproduces May through September. White Pines should be present in most of the hemlock areas that will experience an HWA influx in the next decade (in Michigan, Vermont, New Hampshire, Maine, & Upstate New York).
It is important to note that introduction of this Sasajiscymnus tsugae adelgid predator will not destroy the existing ecological balance between White Pines, the Pine Bark Adelgid (PBA), and our native eastern PBA predator beetle, Laricobius rubidus. Adding a second predator will extend PBA predation through the Summer, but it will not eliminate this native adelgid population in our woodlands. And research indicates that the newly introduced Sasajiscymnus tsugae predator will favor HWA as a food source, when it becomes “locally available”.
It is also useful to note that the other, occasionally available, USDA-approved HWA predator beetle – Laricobius nigrinus – is not suitable for this early intervention strategy utilizing White Pines, even if it were compatible with our eastern Winter and Spring temperature extremes. This Laricobius beetle from the Pacific Northwest cannot reproduce on the Pine Bark Adelgid. Plus, this predator has been shown to hybridize with our native Laricobius rubidus beetle – with unknown consequences for this native species!
Nonprofit Participation Welcome
Nonprofit (501c3) groups working with qualified “early HWA intervention” sites may be eligible for charitable donations of Sasajiscymnus tsugae predator beetles. Qualified sites will be those in the geographic areas designated above with either:
recent, low or moderate density infestations of hemlock woolly adelgid on hemlocks
pre-HWA-infestation sites containing both white pines and eastern hemlocks (or Balsam Firs and Eastern Hemlocks).