Exotic invasive plant species are not native plants. Most did not originate in the United States or North America. The majority of these pestiferous plants which are native to the temperate regions of Asia and Europe have become naturalized in our native landscapes. These nonnative, aggressive species exhibit tree, shrub or vine forms; some are spreading herbaceous ground covers and others are popular ornamental garden plants. Unfortunately, many of these aggressively growing plant species were intentionally introduced for specific purposes in agriculture, horticulture, medicine, forestry, and wildlife enhancement programs, soil erosion control projects, or for botanical display in home gardens, estates, and arboretums. A few were introduced by accident and became established in the surrounding landscape before anyone really noticed their potential harm.
The goal of this paper is to introduce landscapers and homeowners to some of the more common exotic invasive plant species that are troublesome on residential and other small to medium sized properties and provide some general background information about these pests in our native landscapes, their invasive characteristics, and their control using Integrated Pest Management (IPM) techniques.
Thirty years ago when I was an undergraduate studying forestry at UMass, Amherst, I learned that weedy plants in woodlands were naturalized foreign species and were termed “opportunistic” plants that were competing with our native plant species. At that time the term invasive was not used. Back then there were very few people who were concerned about exotic invasive plants in the woods. As I think back to those days I realize that my ecology teachers also taught us about another “sleeping giant,” the perils of the “greenhouse effect” which a decade later or so was termed “Global Warming.”
Over the past few years, I have heard and read from various media outlets that global warming is a result of natural climate cycles and not from the burning of fossil fuels, and therefore is more kindly referred to as “climate change” by global warming naysayers. These skeptics believe that if global warming gets worse it will be a problem in the future and by that time they won’t care because they’ll be dead. Similarly, invasive species have been touted by some to be a minor problem to no problem at all. Invasive plant skeptics seem to see only plant pathogens and insect pests as threats. As descriptive terms for environmental assaults change over the years, the increase and spread of environmental disruption/degradation does not change. In fact these assaults, whether they are biological (invasive plant and animal species) or weather related or weather mediated, are getting worse and influencing each other more or less over time. It is my belief that now the time has come to learn, to understand, and to do something physically constructive to save our native landscapes. My intention is to stimulate concerned citizens to continue the dialog, but go out and physically rid these troublesome plants from our personal properties and local conservation areas.
The New Group of Pests Differs from Insects and Diseases
Hiding in Plain Sight
Exotic invasive plant species have proven to be an insidious group of naturalized landscape and garden plants that have infested local and larger ecosystems. They truly are the silent and secret invaders of our neighborhoods, larger communities and nation. These plant species have gained notoriety over the past several decades among the many people working in the conservation and horticulture professions as destructive agents of change (i.e. pushing out or reducing co-existing native plant species) (Quentin, Fuller and Fuller, 1995; Zimdahl, 1999).
Historically, the horticulture trade has worked in concert with specific government and private interests in the exploration, collection, distribution, planting and sale of known and unknown invasive plant species. For example, some exotic invasive plants such as crown vetch (Coronilla varia), Giant reed (Arundo donax), Scotch broom (Cytisus scoparius), and Russian olive (Elaeagnus angustifolia), were used in government and private erosion control projects before their destructive invasive traits were known.
Similarly, certain wildlife enhancement programs used multiflora rose (Rosa multiflora), Russian olive, common buckthorn (Rhamnus cathartica), and glossy buckthorn (Frangula alnus = R. frangula), to attract birds and other berry feeding animal populations before the destructive impact of their invasive traits to the surrounding environment were fully realized.
Many of the more common exotic invasive plants available to the public have been and still are sold at some nurseries and garden centers. Many exotic invasive species, because of their desired botanical characteristics, have become integrated into creative aesthetic appeal projects as presented in various public and private gardens and arboretums for the sole purpose of enhancing landscape design.
However, to most people exotic invasive plants frequently go unnoticed because they “blend in” with the surrounding native vegetation. Simply put, they are green in color! In fact, it is because of this camouflage-like characteristic of “blending in” or “hiding in plain sight” that these insidious plant species have been largely ignored for decades by the general public.
Exotic Invasive Plants are Pests Too!
After World War II the continuous decline of open space, due to the increase in urban and suburban development, began to accelerate. During these decades of middle class expansion the native landscape became more fragmented, contributing in no small way to the ever increasing and insidious expansion of many established exotic invasive plants.
Current research has exposed the dangers of these plants as causing subtle yet powerful degrading effects on local native plant ecology and biodiversity. Fortunately, more and more people in the horticulture and conservation professions have acknowledged that exotic invasive plants are actual pests of the native landscape. The viewpoint emerging is that exotic invasive plants are slow moving continuous threats to the local and larger ecosystem and should not be viewed as a group of plants in a finite or limited space with limited movement like weeds in a vegetable garden.
Ecological and Control Concepts Must Work Together
In the natural world of plants and animals there is the respected and accepted concept of species diversity (i.e. biodiversity). For native plant and animal species that have co-evolved over time, this concept of species diversity is inherently linked to the ecosystem concept of sustainability (Zimdahl, 1999; Milius, 2010). These two concepts are not mutually exclusive! Thankfully, for the sake of ecological landscaping practitioners and for the sake of the newly emerging profession of ecological restoration, the seeding, planting, cultivation and/or allowing of the influx of exotic invasive plants into native landscapes does not contribute to native plant diversity and ecosystem sustainability. In the same breath, the concept of exotic invasive species management as practiced by ecological restorationists is also deeply rooted in the concepts of biodiversity and ecosystem sustainability. Exotic invasive species living in our native ecosystems may increase biodiversity, but do not contribute to native ecosystem sustainability. Practitioners employing the pest management philosophy of Integrated Pest Management (IPM) embrace these concepts when developing pest management strategies aimed at specific arthropod, microbial, and weed pests.
Exotic invasive plant management involves very long-term control efforts employing various IPM strategies that are the least disruptive to the site for the purpose of reducing or temporarily eliminating exotic invasive plant populations. Please keep in mind that most pest control actions in the landscape are reactionary or short-term and do not involve true IPM concepts and practices.
Therefore, before one can develop an IPM control program for a targeted exotic invasive plant species, one must develop an understanding of the characteristics that define an exotic invasive plant and of how such plants damage the native components of the local landscape and larger ecosystem.
What Makes an Exotic Plant Invasive?
Many plant biologists and ecological restorationists have identified a constellation of traits or characteristics that better define what the term “invasive plant” really means. These traits are shared by most exotic invasive plants with some species expressing more of these traits than others.
To provide a better understanding of exotic invasive plant strategies, I will review the eight invasive ecological traits that were presented by Boston area ecological restorationist, Josh Ellsworth at the Ecological Landscape Alliance’s 2005 Winter Conference, Marlborough, Massachusetts (www.ecolandscaping.org).
According to Ellsworth (2005), the eight invasive ecological traits or characteristics have been categorized for a better description of the exotic invasive plant strategies that give these plants a better competitive edge over our native plant species. Again, please keep in mind that some of these exotic invasive species share more invasive traits than others. Where possible, I have taken the liberty of adding more examples to raise awareness for the novice.
Trait 1: High seed production and good seed viability. According to Ellsworth (2005), the exotic invasive Asiatic bittersweet vine (Celastrus orbiculatus) produces a tremendous amount of seed annually. Only two-thirds of the seed crop will actually germinate, but the sheer number of seeds produced ensures that this species will spread rapidly. Several privet plants (Ligustrum species) are also prolific seed producers as well as common buckthorn and glossy buckthorn.
However, not all seeds germinate as soon as they contact soil. Delayed germination may occur, and according to Ellsworth (2005), such seed becomes a member of the seed bank. The term seed bank refers to the collection of dormant seeds in the soil of the infested site that is capable of germinating at a later time when environmental conditions are right. Seed bank seeds that are capable of delayed germination a year later or more exhibit high seed viability. The abovementioned species as well as garlic mustard (Alliaria officinalis) and many other exotic invasive species exhibit good seed viability. As a side note, there are many weed seeds typical of agricultural soils that exhibit seed viability from five to forty years depending on soil type.
Trait 2: Vectors. Seed is dispersed by wind, water, and/or animals. For most exotic invasive plants to be naturally successful at invading into new areas, they must rely on a vector (i.e. carrier or transporting agent). For example, birds feed on the berries/seeds and then sporadically deposit them over the landscape. The seeds of Asiatic bittersweet vine, glossy buckthorn, common buckthorn, bush honeysuckle (Lonicera species), privet, English holly (Ilex aquiflium), multi flora rose, winged euonymous (Euonymous alatus), and porcelain berry vine (Ampelopsis brevipedunculata) are primarily dispersed by birds over great distances. The small seed of common reed (Phragmites australis) is carried by wind and water. The vector trait characterizes the most successful exotic invasive plant species.
Trait 3: Sexual reproduction breeding system. Most exotic invasive plants reproduce sexually. Sexual reproduction involves pollen produced by a male (staminate) flower contacting and fertilizing a female (pistillate) flower resulting in the production of seed. Sexual reproduction occurs two different ways depending on the location of the reproductive organs on the plants in question.
The first way plants reproduce is referred to as dioecious reproduction where male and female flowers are located on separate sex plants (e.g. Asiatic bittersweet vine). If you see a single Asiatic bittersweet vine in the woods that has not produced berries it could be either a lone male or female vine just waiting to “hook up” with the opposite sex. Only the female vine produces fruit and seed. The second way is referred to as monoecious reproduction. In this case both male and female flowers are located on the same plant (e.g. Birch Betula, Oak Quercus, Pachysandra terminalis). Either dioecious or monoecious sexual reproduction is better referred to as a breeding system.
Another monoecious arrangement is when both male and female reproductive parts are located (consolidated) in the same flower and always on the same plant. When this arrangement occurs the resulting flower is referred to as being perfect. (Perfect flowers in botanical literature are occasionally referred to as bisexual or hermaphroditic). Glossy buckthorn has perfect flowers.
Exotic invasive plants that are either monoecious or have perfect flowers can produce fruit and seed on the same plant. Therefore, invasive plants expressing the monoecious breeding system need only a single plant to start an invasion! This is one of the major reasons why unmanaged monoecious exotic invasive plant populations are so successful at rapidly colonizing vulnerable new landscapes and larger ecosystems.
When landscapes support such populations for decades it becomes very costly to control them, especially to reduce the population by 90%. Ellsworth (2005) states that “you should always think about the timing of seed production when undertaking an exotic invasive plant control project.” You should equally consider the species breeding system. In other words, no matter what exotic invasive species you are faced with you need to be aware of its breeding system and, as Ellsworth stressed in his lecture, at what point in its life cycle it produces flowers, fruit and seed.
Trait 4: Vegetative or asexual reproduction (i.e. sprouting). Vegetative or asexual reproduction produces new shoots without the benefit of male (pollen) and female (ovary) contact required for sexual reproduction (see above). The new shoots are genetically identical to the parent from which they arise except where mutations occur.
Many exotic invasive woody plants sprout or send up shoots after the main stem or branch has been cut or broken. Some species such as Asiatic bittersweet vine and black swallow-wort (Vincetoxicum nigrum) can send up new vine shoots from large existing roots (e.g. root sprouts). The cut stumps of glossy buckthorn, Asiatic bittersweet vine, multi-flora rose, common buckthorn, Norway maple (Acer platanoides), and others will all send up new shoots if an appropriate herbicide is not readily applied.
I have observed glossy buckthorn producing new shoots and roots from branch tips held to the ground by fallen branches of large over story trees. However, you should keep in mind that any woody exotic invasive plant exhibiting the monoecious breeding system, will give rise to flowers on new shoots (i.e. asexual reproduction) that will self-fertilize and produce more berries (seeds) than the original uncut stem. A planned management method of either herbicide application or repeated mowing or cutting over many years is necessary for optimal control.
Trait 5: Predator avoidance and/or deterrence. Some exotic invasive plants have physical structures such as thorns, or tiny sharp spines that deter grazing animals and humans from touching them. Ellsworth (2005) mentioned that Japanese barberry (Berberis thunbergii) is the most common landscape example that comes to mind. I have seen Japanese barberry growing over vast areas of spruce-fir, pine forest in Acadia Park, Maine. It escaped cultivation years ago and is not fed upon by deer and other grazing wild animals.
Ellsworth (2005) and other practitioners warned of specific chemicals produced by some exotic invasive plants that also make them unpalatable to plant feeding animals. For example, Ellsworth (2005) stated that Japanese or Asian stiltgrass (Microstegium vimineum) and garlic mustard have unpalatable compounds that repel white-tailed deer (Odocoileus virginianus) and other grazing animals.
Trait 6: The timing of leaf out and of leaf loss. Several exotic invasive plants acquire an edge over our native plants by leafing out earlier in the spring and holding onto their leaves a little longer in the fall. This trait allows certain invasive plants to carry out photosynthesis longer than most native plant species, thus allowing more time for the invasive plants to store a greater quantity of carbohydrates in their roots for more efficient growth and development when environmental conditions are right. Glossy buckthorn, honeysuckles, garlic mustard, and Japanese barberry are a few species that exhibit this trait.
Trait 7: Shade/Sun tolerance. There are some exotic invasive plants that exhibit the ability to germinate and become established in shade. This is known as being shade tolerant, and it is an extremely good trait to have when competing with our native plants. Shade tolerant species such as glossy buckthorn and Asiatic bittersweet vine will not produce a large increase in growth or seed on a yearly basis when growing under shady conditions, but will not necessarily succumb to shade as a limiting factor either. However, don’t be fooled; shade-tolerant species can grow rapidly when shade opens up due to over story branch die-back, breakage, or fallen trees. Take notice of shade-tolerant species when studying and observing exotic invasive plants. Urban and suburban woodlands generally differ in light levels from mature forests (especially evergreen forests) by having more light reaching the woodland floor in the spring and early summer and thus increasing the probability of exotic invasive plant germination and establishment.
Trait 8: Time of year of fruiting. Different exotic invasive plants set fruit at different times during the growing season. They all do not set fruit and seed in the fall. Ellsworth (2005) stressed that to get the maximum control for the least amount of effort one should observe when your targeted exotic invasive plant sets fruit (seed) and remove that plant before seeds mature. He strongly suggests focusing control efforts in July for glossy buckthorn and black swallow-wort, August for autumn olive (Elaeagnus umbellate) and purple loosestrife (Lythrum salicaria), and September for Asiatic bittersweet vine.
Native Soils = Susceptible Host
In addition to the above invasive ecological traits there is one very important ecological characteristic worth mentioning. That is the ability of most exotic invasive plant species to germinate and become established in a wide range of our native site and soil conditions.
The climate, site, and soil conditions here in New England closely approximate the climate, site, and soil conditions of the exotic invasive plants’ native home ranges in Asia, Europe, and other exotic (or North American) locales. In other words, the concept of compatible cultural growing conditions between their native (home) ranges and our native soils is working!
Therefore, it is safe to say that exotic invasive plants are adaptable landscape pests to our variable native soils. Our native soils serve as the susceptible host to these foreign invaders! The continued increase in exotic invasive plant germination, growth and establishment is partially due to the lack of evolved native soil resistance. The lack of this resistance contributes to native soil susceptibility to these exotic invaders.
Note: Soil resistance is the natural interplay between the physical, chemical, and biological components of our variable native soils at inhibiting exotic invasive plant germination and/or establishment.
Different Vectors and Susceptible Hosts: A resemblance of function
In pest management many analogies can be made between the destructive action of insect and disease organisms and their host plants. For the purpose of illustrating the concept of susceptible host, I have presented in Table 1 the analogies between two fungal pathogens and two exotic invasive plants using the major events in a plant disease cycle presented by Cloyd, Nixon and Pataky (2004). I have adapted it to include the two exotic invasive plant examples.
Table 1. The comparison of fungal pathogens and exotic invasive plants using the disease cycle scenario (adapted from Cloyd, Nixon and Pataky, 2004).
|Causal agent (pest organism)
|Vector (transporting agent)
|Susceptible host (substrate for causal agent)
|Disease or ecological disruption
|Ceratocystis fagacearum (fungal pathogen)
|Sap beetles and bark beetles; underground root grafts spread infective spores.
|Oak group; somewhat pathogenic in the white-bur oak group.
|Oak wilt, a systemic fungal disease.
|Ophiostoma ulmi (fungal pathogen)
|Native elm bark beetle and European elm bark beetle spread infective spores.
|Dutch Elm disease, a systemic fungal disease.
|Glossy buckthorn (exotic invasive shrub)
|Birds and other animals spread viable seeds.
|Variety of native soils and growing sites.
|Exotic invasive shrub; out competes native plants for water, sunlight, nutrients and space contributing to ecosystem and local habitat disruption/degradation.
|Asiatic bittersweet (exotic invasive vine)
|Birds and other animals spread viable seeds.
|Variety of native soils and growing sites.
|Exotic invasive vine; out competes native plants for water, sunlight, nutrients and space contributing to ecosystem and local habitat disruption/degradation.
The comparison displayed in Table 1 between the systemic fungal tree pathogens and the exotic invasive woody plants was constructed to illustrate the resemblance of function between these two pest groups that are very different from each other. Both groups rely on vectors to colonize a susceptible substrate which serves as a host for each pest’s continued life cycle success. The reproductive units vectored are the spores of the pathogenic fungi and the seeds of the woody exotic invasive plants.
Exotic invasive plants can act in ways very similar to many plant pathogens. For example, plant pathogenic fungi must persist in their host plant tissues in order to continue their life cycles (i.e. increase in numbers) and cause disease. Similarly, exotic invasive plants must persist by way of seeds in their hosts (i.e. our native soils) to increase in their numbers (i.e. germinate and become established at a later time when environmental conditions are right) to cause ecosystem disruption/degradation (e.g. loss of space for native plant reproduction; reduction in native plant diversity; weakening native ecosystem sustainability, etc).
Not all Susceptible Hosts are Valued Equally
In the disciplines of plant pathology and entomology, the concept of susceptible host applies to all plants regardless of value. However in the landscape pest control industry, this view is very different. The susceptible host is limited to only plants prized for their high economic and/or aesthetic value. Not all trees, shrubs, and herbaceous plants are treated for insect and disease problems. Only plants with the highest economic and/or aesthetic value get protection.
In my opinion this reasoning is partly why we are resistant to seeing exotic invasive plants as actual pests; they occupy the same sites as our valued residential and woodland specimens, but unlike traditional insect pests and disease pathogens, they do not directly impact our native plants in the short-term. Most are green and just ‘blend in’ with the surrounding native vegetation. I agree with many ecological restorationists that the time has come when landscaping professionals and homeowners must look beyond the individual “prized plants” and be concerned with the actual habitat where all plants are growing.
The continued invasion of diverse, native habitats by exotic invasive plants is forcing us to acknowledge that native landscapes and larger ecosystems are susceptible sites (i.e. hosts) and these sites or habitats, regardless of real estate value, need protection on a regular basis from these pestiferous plant invaders.
The borders of countries, sanctuaries, parks, waste lots, low value and high value real estate lands, and backyards are not effective boundaries to any kind of invading pest when our native vegetation and soils serve as susceptible hosts (see Table 1). As you read more about the exotic invasive plant species you encounter when volunteering to remove such species on conservation lands or in your own backyard, you will begin to see life cycle analogies between plant and pest species even when they are very different from each other as presented in Table 1. Your own acquired knowledge and experience may give you a better holistic appreciation for native biodiversity and a deeper understanding of the many species of pest organisms, their life cycles, and their interactions with each other and the environment.
From Theory to Practice
Not every exotic plant becomes a naturalized invasive. Wagner (1993) reports that there are over 100 species of buckthorn shrubs, but only two (glossy buckthorn and common buckthorn) are exotic invasive plants in North America. The same holds true for honeysuckles; with over 200 species, only amur honeysuckle (Lonicera maackii), morrow honeysuckle (L.morrowii), tatarian honeysuckle (L.tatarica), and Japanese honeysuckle (L. japonica) are pestiferous invasive plants in North America (Wagner, 1993; Munger, 2005). In fact, the majority of exotic plants introduced into the United States have not become landscape or larger ecosystem pests.
According to Quentin et al (1995), the general theories proposed by many researchers about predicting exotic plant invasions have little practical value and the development of predictive invasion models with their accompanying theories may not serve land managers adequately for all exotic invasive species in all susceptible habitats. Predicting invasive plant behavior has greater reliability when specific invasive traits are matched to susceptible ecosystems and/or site situations (Quentin et al, 1995).
Therefore, the eight traits mentioned above have proven value as reliable predictors of exotic invasive plant success and warrant an integrated control approach for long-term management. I and other practitioners strongly agree with Wagner (1993) that there is little evidence that exotic invasive plant species will adapt to our native ecosystems and become less aggressive over time.
The IPM Concept in Exotic Invasive Plant Control
Integrated Pest Management or IPM is a pest management philosophy that employs either a single pest management strategy or a combination of strategies to control or suppress plant pest populations from reaching damaging levels (Cloyd et al, 2004). Plant pests include insects, mites, fungi, bacteria, other microbes, vertebrates, and parasitic plants. Garden, lawn, and agricultural weeds are included because they compete with desired plants for water, nutrients, light, and space. We can now add exotic invasive plants to the weed list as being “weeds of the woods that have colonized and re-colonized residential and other suburban properties.”
Exotic invasive plants cause slow and long-term damage to the surrounding native plant areas. They compete with native plants for water, nutrients, light, and space. As they compete for these resources for growth native plant succession is impacted slowly over many years, sometimes decades, contributing to landscape and larger ecosystem disruption and/or degradation. When native landscapes are colonized by these invaders, IPM control strategies need to be defined for each exotic invasive and employed for continuous control. See the IPM fact sheets at the end of this article.
The goals of conventional IPM programs in landscape and agricultural systems are (1) to reduce, and in some cases eliminate, chemical pesticides and inorganic fertilizers; and (2) reduce plant pest populations to manageable levels, but not necessarily eliminate those populations (Cloyd et al, 2004).
The four IPM control strategies used are (1) cultural (environmental), (2) mechanical (physical), (3) biological, and as a last resort (4) chemical (Cloyd et al, 2004). IPM incorporates the principles of ecology in which individual plants, small plant and animal assemblages, and larger plant and animal communities interact with each other and with the physical environment (Clark and Matheny, 1994).
The IPM approach to any pest problem is most receptive to using the least disruptive control product or procedure specific to each crop system including specific plants needing protection in landscapes and agricultural systems. Different pest threshold levels have been developed for particular crop systems to help determine when IPM treatments are needed. For example in agricultural systems, very specific science-based IPM monitoring programs have been developed to determine the amount of insect and/or disease tolerance levels for many kinds of crops before pest control actions (treatments) are warranted (the action threshold) (Dreistadt, 1994).
According to Flint (1990, p.11), “Control action guidelines for insects and other invertebrate pests are often expressed as numerical thresholds indicating the population levels that will cause economic damage. Guidelines for weeds, diseases, nematodes, and vertebrates are usually based on the history of a field or region, the stage of crop development, weather, pest distribution, and other field observations. Control action guidelines are helpful only when used with accurate pest identification and careful field monitoring.”
However, such monitoring programs with specific action thresholds have not been developed for most gardening and landscaping situations because the values of landscape plants do not equal the economic criteria of agricultural crops (Dreistadt, 1994). In landscape pest situations, it is the opinion or attitude of the plant pest evaluator that determines what is or what is not acceptable plant damage before control actions are employed. This subjective view is referred to as the aesthetic threshold (Dreistadt, 1994).
When it comes to aesthetic appeal of landscape plants, the aesthetic threshold or visual tolerance level for particular pest populations can vary widely. For example, some people see a few aphids on their roses or weeds in their lawns and immediately decide in favor of a chemical control option. Other people may tolerate a larger pest population under these same circumstances before they opt for chemical or even non-chemical control measures. Some folks may do nothing at all, allow nature to take its course, and accept the damaged roses or weedy lawn because their visual tolerance level or aesthetic threshold is very high. In gardening and landscaping, where ornamental plants are the focus, research has shown that people view plants with about ten percent defoliation or discoloration as damaged (Davidson and Raupp, 2010).
These differences in attitude of visual tolerances also apply to land managers who encounter exotic invasive plants in the course of their work. Some land managers are alarmed to see a small stand of garlic mustard starting to grow under the shade of an oak tree, but they are not too concerned seeing the established Asiatic bittersweet vine growing up into the canopy of that same oak tree. This kind of scenario helps motivate land managers to control efforts because what they were planning to control (the vine) has now reached actionable levels due to the newly established exotic invasive plant (the garlic mustard). To the land manager, the addition of the new exotic has threatened his or her domain of responsibility. I know this from personal experience! In addition, it is not uncommon to have well planned exotic invasive control projects delayed due to a lack of time, money and available labor (more about this below).
Science based or formal action thresholds (common to agricultural systems) and pest control guidelines do not exist for terrestrial exotic invasive plant species because there are too many biotic (living) and abiotic (non-living) ecosystem variables to contend with for developing such protocol. In addition, the land infested is mostly viewed in terms of real estate value and not conservation value. Most suburban public and private owned exotic invasive plant infested yards, woodlots, waste lots, woodlands, fields, and the like are not of our personal concern; they are not part of our domain or responsibility. Such infestations are viewed as being the responsibility of other people (i.e. that’s your neighbor’s responsibility, not yours).
Unfortunately, most people are not “conservation minded” and will tolerate exotic invasive plants in their own yards, local landscapes, and larger ecosystems, but they may have no tolerance for aphids on their roses or weeds in their lawns. I have heard many times over the past fifteen or so years that exotic invasive plants are just “part of the woods. What’s the big deal?” More education is needed about these pestiferous invaders for the general public. These exotic invasive plants need to be taken more seriously because they are spreading great distances from your yard and your neighbor’s yard as well. Therefore, the eight ecological traits that help define and characterize exotic invasive plant species are more than sufficient to warrant IPM control options for the conservation minded person, group, and organization.
Indecision = Inaction
For many years I have witnessed many misinformed, uninformed, and even well-informed eco-friendly people and organizations exhibit indecision about implementing exotic invasive plant control programs on their home properties or on properties owned by conservation non-profits. Some excuses revolved around waiting for pest experts and researchers to develop formal guidelines including action thresholds and/or aesthetic thresholds as are commonly used in agricultural IPM programs. Again, it will be a very long time before such protocols are developed for exotic invasive plants infesting suburban landscapes and larger ecosystems. A more realistic approach is to follow good “work guidelines” as proposed by Dr. Julie Richburg, Regional Ecologist for The Trustees of Reservations, and described in her article “Resource-based Invasive Plant Management.”
Other excuses have revolved around the “wait and see” or “let’s see what other institutions are doing” paradigm. These attitudes reflect a fear of possibly doing something wrong or creating a bigger problem that is not economically feasible to correct or complete. Sound land stewardship cannot include inaction. In the near future land managers must include a new economic infrastructure at their organizations specifically for exotic invasive plant management programs.
Money and Labor: the Dilemma of Large-scale Control Programs
Private conservation organizations and federal and state parks, agencies, and departments are excellent sources of information about exotic invasive plants but are restricted by a lack of adequate funding to carry out effective, long-term control programs on all lands under their management.
These private and governmental organizations are ecologically minded and would love to rid their lands of all exotic invasive species that plague them. However, they can only select specific invasive work projects that are economically feasible (i.e. not beyond their standard operating expenditures). These institutions have the ecological knowhow to rid a land of these troublesome plants, but due to their infrastructure they are forced to employ a business resolution approach when designing and implementing large-scale exotic invasive plant control projects.
The priorities they embrace are not for reducing all exotic invasive plant populations by 90 percent (the ideal goal), but for developing economically feasible restoration and/or control projects that minimize labor and equipment costs. In other words, they must stay within their existing or dwindling operating budgets and direct their efforts to specific areas or specific exotic invasive species. Thus long-term control success is contingent upon extra funding from budget increases, long-term grants, donations, etc. that would allow for well-planned control projects to include continuous on site follow-up control tasks of routine monitoring, maintenance, and evolving restoration plans.
This is why you may see Department of Transportation (DOT) restoration projects, including exotic invasive plant control efforts along certain sections of a public roadway, but not along the entire stretch of road. The same parameters hold true for federal and state parks and private conservation organizations. These groups will reduce or temporarily eradicate a particular exotic invasive plant or group of plants from a critical ecological habitat, but not from their entire property, or infrequently encourage such work beyond their property line.
This kind of business approach to exotic invasive species control seems to be a “square peg in the round hole.” The continued influx of exotic invasive plants to such lands is a major, long-term threat to native landscape biodiversity and sustainability. Current control plans and projects are too costly and not sufficient to curtail the continued invasion of these species on a regional scale.
Can Homeowners and Private Landowners Come to the Rescue?
Homeowners and other private landowners could be the “hero” at effectively controlling exotic invasive plants at the neighborhood and larger community level if they incorporated into their own landscaping maintenance plans exotic invasive plant removal. However, to have a positive impact neighborhood-wide or in the larger surrounding community, the effort would have to be done along specific guidelines under the support of local city or town ordinances. This would ensure that all households would cooperate and that all invasive plant refuse generated would be properly disposed as is done with municipal composting, recycling, and green waste programs.
It is beyond the scope of this paper, but the criteria, development, and installation of such a citizen-based, municipality-supported program would need additional funding generated on an annual basis (possibly) from a special lottery game and/or tax incentive. This is just an idea to get people thinking about developing solutions for coordinating regional support, one community at a time that could augment larger control programs already in existence on government and non-profit conservation lands. If something like this were to happen, all efforts would be linked in the service of increasing regional native biodiversity while decreasing the growth and spread of exotic invasive plants.
In an ideal world, we all should be cooperative, participating eco-citizens working to develop solutions to further native plant and animal conservation efforts. However, in the real world we can start controlling these pestiferous plants in our own backyards by hiring a landscaper or doing the work ourselves or volunteering at a local conservation organization removing these plants. Let’s get started!
IPM Control Strategies for Exotic Invasive Plants
The following four control strategies are in accordance with the IPM framework of landscape pest control and are specific to controlling the continued spread of exotic invasive plants. I refer to this as Conservation IPM because it involves the removal or reduction of exotic invasive plants from small as well as large habitats of native plant assemblages which have co-evolved with each other and other native organisms over thousands of years.
1. Cultural controls. Visually inspect your property for any exotic invasive plants. This action is referred to as monitoring. As you carry out the monitoring process you must be able to correctly identify the exotic invasive plants targeted for removal. (There is more information about the importance of monitoring below). Prevention is another cultural control. Do not plant or encourage the planting of any exotic invasive plant. Educating others about the dangers of such plants to the native ecosystem is a cultural strategy that has enormous value. By communicating to others about the dangers of these plants, you are raising awareness of the problem so other people or organizations may follow your example.
2. Mechanical controls. This strategy is also referred to as physical controls in IPM circles. Mechanical control strategies are the most widely used control methods and are the most laborious. They include pulling or digging and/or cutting, mowing, or controlled burning of targeted exotic invasive plants to temporarily eradicate them or reduce their spreading potential. Mechanical control methods are the first used with a sound monitoring program and must be ongoing, year after year, to prevent any exotic invasive plant population from going to seed or otherwise increasing to unmanageable levels.
3. Biological controls. Biological control incorporates the implementation and long-term use of beneficial living agents (insects, mites, microbes, etc.) to control pest populations (Cloyd et al, 2004). Biological control for all exotic invasive plants is in its infancy. In other words, almost all exotic invasive plants currently in the U.S. have no effective biological control agents. Not all arthropods and fungi associated with specific exotic invasive plants prove to be good bio-control agents (Zheng, Wu, Ding, Binion, Fu, and Reardon, 2004). More funding for research is desperately needed in this area.
The promise of biological control is to integrate beneficial living agents into the ecosystem for the purpose of slow and effective control that paid workers and volunteers may not be able to effectively carry out (Hight, 1993; Dreistadt, 1994; Quentin et al, 1995). The well-designed and field-tested biological control projects aim to selectively reduce the targeted pest of concern and reduce the labor and chemical costs of conventional control programs (see side note about purple loosestrife). This very laborious “trial and error” research needs more support at the university and government levels to work properly where it is ecologically feasible (see side note about Klamathweed and rose rosette disease, two examples that needed more time to evaluate in the field).
For the majority of exotic invasive plants, biological control agents are the least funded, researched, and utilized as control agents. Biological control must work only for the targeted exotic invasive plant in question and absolutely no others. Again, more funding for research is desperately needed in this area.
Side note: Purple loosestrife (Lythrum salicaria) is a wetland exotic invasive plant colonizing many wetlands in the northeastern United States. It was introduced to the U.S. about 200 years ago from Europe (Hight, 1993). Purple loosestrife flowers between June and September creating a showy display of tall spike-like purple flowers (Blossey, 1996). Most everyone living in New England and New York has seen this plant growing in large and expanding clumps in many rivers, pond edges, and other wetlands. Stands can become so dense that they displace native aquatic plants and animals.
Three European beetles have been tested for host specificity to purple loosestrife and two are being used as effective biological control agents in parts of Massachusetts and elsewhere. The beetles are the root-feeding weevil, Hylobius transversovittatus (larvae feed on roots; adults feed on leaves) and two leaf and stem-feeding beetles, Galerucella calmariensis and G. pusilla (Hight, 1993). G. calmariensis and G. pusilla are employed as biocontrol agents for purple loosestrife. As with other biological control projects, the purple loosestrife projects aim to provide inexpensive control over many years to reduce purple loosestrife 70 to 80% while causing minimal to no damage to the native ecosystem (Hight, 1993).
Many biological control researchers, IPM practitioners, ecologists, and concerned citizens share the fear that not all biocontrol projects stay on target. The biocontrol organism may switch its host (i.e. targeted plant) to a native plant (Wagner, 1993). This event occurred with the Klamathweed beetle (Chrysolina quadrigemina). In the 1940’s, Klamathweed beetle was introduced into California to control the pestiferous Klamathweed (Hypericum perforatum), a toxic rangeland plant that poisoned livestock (Dreistadt, 1994). The beetle introduction proved very successful at reducing Klamathweed for ranchers; however, the beetle switched to a closely related ornamental ground cover (H. calycinum) causing severe defoliation (Dreistadt, 1994). The beetles imported to control purple loosestrife have not permanently switched to native hosts at this writing and are effectively and slowly showing signs of being a biocontrol success.
There are times when the biocontrol agent controls the targeted plant and the desired ornamental landscape plants used in many garden displays making true biocontrol efforts difficult if not impossible. For example, Rose Rosette Disease (RRD) is a virus-like plant disease made up of a double-stranded ribonucleic acid (RNA) which is unorthodox for a plant virus (Amrine and Stasny, 1993; Cloyd et al, 2004). The exotic invasive multiflora rose is the most common host of this disease (Cloyd et al, 2004). In woodland and pasture settings the disease can take about two years to kill multiflora rose (Amrine and Stasny, 1993). RRD is vectored by the eriophyid mite, Phyllocoptes fructiphilus (Amrine and Stasny, 1993). This very tiny native mite was first discovered and described in California in the 1940’s feeding on Rosa californica (Armine and Stasny, 1993). Here lies the problem; RRD can also spread via the mite to desired, and highly valued ornamental roses such as hybrid teas, climbing roses, miniatures and floribundas making RRD a very poor biocontrol agent in urban areas for multiflora rose control (Cloyd et al, 2004). Infected ornamental roses need to be immediately destroyed to stop the vector (mite) from transmitting the disease to uninfected ornamental roses (Cloyd et al, 2004).
4. Chemical controls. True IPM programs rely on chemical controls as the least used component for pest control. Ideally, this is true whether the chemical is an insecticide to kill insect pests, fungicides to kill fungal pathogens, or herbicides to kill weeds. In true IPM circles, the chemical control component is used only as a last resort and it must be the least toxic to the environment.
Herbicides are chemical compounds that are formulated to kill living plants. Glyphosate, better known as Roundup® (manufactured by Monsanto), is the herbicide choice most frequently used for controlling exotic invasive plants growing in residential landscapes. Both homeowners and licensed pesticide applicators (e.g. landscapers and arborists) can purchase Roundup®.
All herbicides must be used in accordance with state and federal pesticide laws and guidelines, and all applicators (professional and non-professional) must follow all product directions and precautions stated on the herbicide label. Roundup® ‘Poison Ivy Killer’ is an excellent IPM component choice for both herbaceous and woody exotic invasive plants. Brush-B Gon (active ingredient Triclopyr, manufactured by Ortho) is also an effective herbicide for homeowners and landscapers.
Regulations: The Massachusetts Prohibited Plant List (January 1, 2009) lists specific exotic invasive plants as being banned for importation, trade, and/or sale in the Commonwealth of Massachusetts. Visit www.mass.gov/agr/farmproducts/prohibitedplantlist.htm for more exotic invasive plant status updates and additions. On the website, the Invasive Plant Prohibition list is located alphabetically under the “I” heading in the “A-Z Index.”
Click to continue with Part III of this article.
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About the Author
Bruce Wenning has univeristy degrees in plant pathology and entomolgy and is an ELA Board member and regular contributor to the ELA Newsletter. Bruce also spearheads the effort to expand ELA’s website content. Watch for his upcoming articles with information about individual invasive species. He is a horticulturist at The Country Club, Brookline, MA where he continues his battle with exotic invasive plant species.