Conifer Pests and Their Control

By Gerry Donaldson

Discover the resource in detecting and managing common pests in conifers.

A conifer pest, the hemlock woolly adelgid (HWA), Adelges tsugae
A conifer pest, the hemlock woolly adelgid (HWA), Adelges tsugae

Imagine your landscape with seventy percent, or more, of the plants dead! Imagine all of your community similarly affected! Imagine all of your state, all of your USDA growing zone, possibly all of your country affected!

No, this is not an introduction to an episode of Twilight Zone, and granted, this may sound alarmist, but such loses are a very real possibility if invasive insects and/or diseases have their way with our natural and built landscapes.

The Effects of Conifer Invasive Pests

Each loss in our landscapes results in changes in the ecosystem. All of the components of an ecosystem work together to clean water and to re-charge aquifers. They clean air and provide oxygen, moderate atmospheric temperatures and provide habitat for the complex web of life. All of this facilitates human survival.

Invasive pests wreak havoc on those ecosystems at a cost greater than 25 billion dollars in damaged or lost crops and forest products annually. Reduced property values and increased maintenance costs are direct costs to homeowners and all taxpayers, adding billions more.

The sawfly larvae from the suborder Symphyta, another conifer pest
The sawfly larvae from the suborder Symphyta, another conifer pest

The U.S. Government, through the USDA’s Animal and Plant Health Inspection Service (APHIS), works to keep invasive pests out of our country. We all know of examples where that effort has fallen short. Think about chestnut blight, which devastated the American chestnut in the late 1800’s and early 1900’s. In some areas, the American chestnut was up to 20% of the trees in forests and provided a significant food source for Native Americans, early settlers, and the animals of the forest.

In the 1920’s, Dutch elm disease was identified in the United States. By the 1960’s, it had virtually wiped out the American elm throughout North America, dramatically changing our forests and city streets.

More recently, in 2002, emerald ash borer was discovered in the US. Since that time, it has devastated the ash trees of the Great Lakes region and has spread to 30 states, killing millions of trees and costing billions of dollars in loss and damage.

Early Detection of Conifer Pests

The importance of early detection is exemplified by the emerald ash borer experience. Initial homeowner questions about what was happening with their ash trees were met with confusion on the part of arborists and in extension, university personnel, due to lack of knowledge of the insect. Little, if any prescriptive information was available. By the time the insect was identified and researchers had identified controls, the area infested had become so large that elimination was ruled out as a possibility.

Early detection of invasive species is critical if we intend to prevent future disasters. The longer an invasive goes undetected, the more difficult, and more expensive it is to control (see Invasive Introduction Curve). Fortunately, a plan to combat invasive species is in place and is beginning to have a positive effect.

The Invasive Introduction Curve for Conifer Pests
The Invasive Introduction Curve for Conifer Pests

The Sentinel Plant Network for Conifer Pests

With funding support from APHIS, the American Public Gardens Association has partnered with the National Plant Diagnostic Network to form the Sentinel Plant Network. Since its inception in 2011, the program has grown to include over 225 public gardens as members. Staff of the Sentinel Plant Network participating gardens are trained to look for signs and symptoms of potential and/or emerging threats in their geographic area.

Those same staff can aide in differentiating between indigenous insects and diseases and those that are new or unknown. They can then make referrals to the state or regional diagnostic centers when appropriate. The National Plant Diagnostic Centers work with federal regulators and local governments to form a rapid response to stop the spread of an invasive species and to minimize impact.

Helping to Curb the Threat of Conifer Pests

You can help with this vital effort. Go to and create an account. Please indicate that you learned about the program through the Sentinel Plant Network and the CONIFER QUARTERLY. There you can use the e-learning modules to learn about threats in your region, and how to report plant problems in your area.

Call your local public garden and talk with them about the Sentinel Plant Network and any classes they may offer to aide in invasive pest detection, prevention, reporting, and control. You can also help by monitoring your garden and keeping an eye on the flora wherever you travel. If you see something that does not look right, please take photos and then pass that information along to a Sentinel Plant Network garden. Together we can make a difference in our future.

Click here to read more about other conifer pests like the parasitic wasp, the Siberian silk moth, the hemlock woolly adelgid and the Asian longhorned beetle.

Photographs by Gerry Donaldson.

This article was originally published in the Fall 2017 issue of Conifer Quarterly. Join the American Conifer Society to access our extensive library of conifer-related articles and connect to a nationwide group of plant lovers! Become a member for only $40 a year and get discounts with our growing list of participating nurseries in our Nursery Discount Program.