What can I do about invasive species?

“Invasive species” are “alien” or “exotic” species whose introduction does or is likely to cause economic or environmental harm or harm to human health.  They are usually introduced by people either accidentally or on purpose and into a region far from their native habitat.  In their natural range, these species are limited by environmental conditions, pests or diseases, keeping these species in balance within their ecosystem. When introduced into an area where these limitations are absent, some species have the ability to become invasive. These are the species we are concerned about in conservation.

Invasive plants grow quickly and aggressively, spreading and displacing other plants. The known ecological impacts of invasive species include loss of threatened and endangered species, altered structure and composition of terrestrial and aquatic communities, and reduction in overall species diversity.  Some invasive species are hazardous to human health.  Giant hogweed (Heracleum mantegazzianum), a species threatening Pennsylvania lands, is currently on the Pennsylvania and Federal Noxious weed lists.  Its sap, in combination with moisture and sunlight, can cause severe skin and eye irritation, painful blistering, permanent scarring and blindness. Contact between the skin and the sap of this plant occurs either through brushing against the bristles on the stem or breaking the stem or leaves.  Invasive plants tend to appear on or near disturbed ground like hiking trails, rights-of-ways, and roadsides. The most aggressive can actually invade existing ecosystems.  Invasive plants are generally undesirable because they are difficult to control, can escape from cultivation, and can dominate large areas.

Insects and diseases are the most destructive agents affecting forest and shade trees in Pennsylvania. Tree roots, stems, limbs, needles, leaves of healthy or weakened trees, or logs waiting to be sawed into lumber are all subject to attack. Insects are by far the most numerous animal life inhabiting the forest. They have become well adapted to their surroundings and occupy a wide variety of ecological niches. Although the majority of insect species are either beneficial or innocuous, some are exceedingly harmful. Insect outbreaks that cause economic damage to forests vary greatly in frequency, size and duration. Fortunately, most outbreaks are small and short-lived, and usually consist of one or a few spots in a stand or region. Others, however, may expand and encompass hundreds or thousands of acres and can last for several years.

Some of the more far reaching ecological and economic impacts of invasive forest pests and diseases include:

  • Chestnut blight fungus (Cryphonectria parasitica) virtually eliminated the American chestnut (Castanea dentata) in the early 1900’s.
  • Currenlty destruction of the eastern hemlock (Tsuga canadensis) by the hemlock woolly adelgid (Adelges tsugae).
  • Imminent demise of ash species (Fraxinus spp.) caused by the advancing front of the emerald ash borer (Agrilus planipennis).

Managers can reduce the risks and incidence of insect attack by maintaining healthy and vigorously growing stands and trees. Research is being conducted to determine what conditions are conducive to forest insect outbreaks. This research may lead to improved control measures.

Recognition of the problem of invasive species are growing at the same time as damage to native ecosystems is mounting. Identifying invasive plants, pests, and diseases and understanding the potential damage they can cause is essential to slowing their spread and protecting native vegetation. In short, invasive species infestations can be extremely expensive to control, as well as environmentally destructive.

Identifying invasive species in your woods has become an unfortunate part of owning Pennsylvania forest land.  Your local county extension office or your DCNR Service Forester can help you with species identification.  An excellent federal publication is the Invasive Plants Field and Reference Guide: An Ecological Perspective of Plant Invaders of Forests and Woodlands by Huebner, Cynthia; Olson, Cassandra; Smith, Heather (2005), USDA Forest Service, State and Private Forestry, Northeastern Area and available online at .  While there are many field guides available about invasive plants and their identification, the purpose of this particular field guide is to give a scientific synthesis of what is known about the behavior of such species in managed, disturbed, and pristine forested systems in addition to key information for accurate identification. Such information will be helpful when choosing the best control strategies.  You may also view the Invasive Plant Atlas of the United States to find helpful species information, photos, and additional resources.  To view photos, information, and additional resources about invasive pests and diseases, view the National Agricultural Pest Information System (NAPIS).

PA DCNR has an excellent webpages with links about invasive plants and pests and diseases and their control.  Penn State Extension has additional Pennsylvania Invasive Species sheets.


There are a number of ways to manage undesirable vegetation: mechanical, cultural, biological and chemical. Integrated vegetation management (IVM) and Integrated Pest Management (IPM) use a combination of these techniques.  Unfortunately, many of mechanical or cultural techniques like cutting, pulling, and tilling don’t do enough to slow the spread of many invasive plant species.  Biological control is defined as “the reduction of pest populations by natural enemies and typically involves an active human role.” Generally, biological control is very expensive and extensive research must be conducted before a new organism or natural enemy can be introduced into the United States.  Chemical control is usually the most desirable form of control, but can be costly and a certified herbicide applicator is suggested for the application of many chemicals required to effectively control invasive plants.

If you discover that your woods have been invaded by undesirable species, you may want to download the Penn State Natural Resource Extension publication: Herbicides and Forest Vegetation Management, Controlling Unwanted Trees, Brush, and Other Competing Forest Vegetation.

Please consult your local service forester or a certified herbicide applicator before exposing yourself to dangerous chemicals.