Western PA Conservancy Works To Introduce A Natural Predator To The Hemlock Woolly Adelgid

There are dozens of Western PA Conservancy properties where eastern hemlocks, the state tree, dominate forests as key ecological features. 
The invasive Hemlock Woolly Adelgid that feeds on and kills hemlocks was first detected on WPC lands along Sideling Hill Creek in Bedford County in 2008.

The HWA has now been detected at many Conservancy properties, except in the northwestern counties of the state.

Although there are no means to eliminate the Hemlock Woolly Adelgid at this time, there are several options for its treatment and control. Control methods include chemicals like insecticides or biological methods, such as predators or pathogen treatments.

In order to save some hemlock trees on certain Conservancy lands, limited applications of insecticide are being used on select trees and groves of hemlocks.

In an effort to use a non-chemical option, the Conservancy is testing a biological approach that may potentially have wider and long lasting benefits.

It’s possible to introduce predatory species into an environment as a potential solution to an invasive species. However, the introduction of a non-native predator as a potential solution to an invasive pest should only be attempted after careful evaluation and testing.

Researchers must be sure the intended solution is not another invasive species problem in disguise.

In the case of HWA, there is a non-native predatory insect that preys on the invasive adelgid. A species of tooth-necked fungus beetle is an aggressive predator of hemlock woolly adelgid.

The small, 2-3 mm long beetle, called Laricobius nigrinus (Ln), is native to the northern Rocky Mountains and Pacific Northwest, where it feeds on an insect identical to the invasive HWA that is infesting eastern hemlocks.

Ln was one of a few insects selected for release at HWA infestation sites in some eastern states beginning in the early 2000s. Unfortunately, Ln has never been given a common name for us to use.

The Conservancy is working closely with the DCNR Bureau of Forestry to fight the spread of HWA using the Ln beetle.

A challenge for such an effort is to establish the predatory beetle population in an area before HWA can kill the hemlock trees.

Although Ln already lives in far eastern Pennsylvania and some states to the south, it is unlikely to make its way on its own to western Pennsylvania in time to save the trees.

Acquiring an adequate supply of Ln beetles to release into the wild isn’t an easy task. It takes many individual beetles and several attempted releases to be successful at a particular location.

Common options for acquiring the Ln beetle include:

-- Rearing the predatory insects in a lab;

-- Collecting them in the field and transporting them to the infested area; or

-- Purchasing them from a supplier.

Instead, the Conservancy and the Bureau of Forestry decided to create a nursery habitat that is suitable for both HWA and the predatory beetle. This allows the Ln beetles to naturally feed on HWA and reproduce in their intended environment. This habitat is referred to as an insectary.

WPC is establishing two Ln insectaries at Bear Run Nature Reserve in Fayettee and Somerset counties using the following method:

-- Plant 12 young hemlock trees in a linear hedge-like configuration at two open locations, and care for them with water and fertilizer.

-- Fence in the area so that the local deer population will not damage the young trees, while also cutting surrounding vegetation to reduce competition for sunlight and water.

-- Trim the tree tops and longest branches to encourage a bushy growth form with lots of twigs that are within reach.

-- Introduce HWA into the insectaries by collecting HWA infected hemlock twigs and placing these along the branches of the young insectary hemlocks.

-- Wait at least one year to allow the HWA to spread and form a thriving colony within the hemlock hedge.

-- Release Ln beetles into the hemlock hedge when the HWA is abundant enough to supply the food necessary to feed a Ln colony.

-- Collect Ln beetles once a healthy colony has formed in the insectary and release these at locations where eastern hemlock trees are already infected.

-- Check the HWA-infected hemlock trees in the landscape surrounding the insectary to determine if Ln beetles from the insectary have dispersed on their own to prey on HWA elsewhere.

The Conservancy anticipates the insectaries will be producing Ln beetles to fight the HWA by 2021.

If the Ln beetle becomes established within the newly expanding HWA range, the hope is HWA numbers will remain low enough to allow most hemlock trees to survive.

HWA is an invasive species that is most likely here to stay. Although there is no current solution to completely eliminate HWA, efforts are underway to control this invasive species and lessen its damage to our forest.

For more information on the Conservancy’s efforts, visit its Conservation Science - Hemlock Woolly Adelgid webpage.

Visit DCNR’s Hemlock Woolly Adelgid webpage to find out how Pennsylvania is fighting this invasive species.
(Photo: The Ln beetle.)