Controlling an invasive plant without herbicides

Invasive species are plants, animals, fungi or microorganisms that spread rapidly and cause harm to other species. They are introduced species that can thrive in areas beyond their natural range of dispersal.

Characteristically, invasive plant species are adaptable, aggressive and usually lacking natural enemies that can limit their growth and populations. They have a high reproductive capability; growing rapidly in short life cycles and producing abundant amounts of seed. They aggressively compete with native plants and plant communities and often displace them, thereby disrupting the normal functioning of ecosystems and threatening biodiversity and already endangered native plant species.

Purple loosestrife is a perfect example of an introduced plant species that has become a serious and widespread threat to native species, natural communities and ecosystem processes. It was brought to North America by the European colonists as an herbal remedy for dysentery, diarrhea and other digestive ailments and introduced in the 1800s as an ornamental. It was well-established in New England by the 1830s, and spread along canals and other waterways. Supposedly sterile species were offered for sale for many years, but researchers later found that those cultivars were fully capable of cross-pollinating with plants growing in the wild.

Mature purple loosestrife plants are also capable of reproducing through a very extensive system of rhizomes. A single mature plant may contain as many as thirty flowering stems capable of producing more than two million seeds per year.

Established populations of purple loosestrife threaten entire ecosystems. They persist for decades and can be extremely difficult to control. With the exception of Florida and Hawaii, purple loosestrife is now present throughout the United States and all of Canada. Some researchers have called it a “purple plague.”

According to Cornell University Professor Bernd Blossey, purple loosestrife can degrade prime wetlands, resulting in large, monotypic (containing only one species) stands that lack native plant species. Monotypic stands of purple loosestrife may inhibit nesting by native waterfowl and other birds. Other aquatic wildlife, such as amphibians and turtles, may be similarly affected. The plant itself is of little to no value for wildlife, either as a viable habitat or as a food source.

Established populations can endure for decades, and are very difficult to control using conventional (chemical, physical, mechanical) techniques. Professor Blossey, who currently directs the University’s Ecology and Management of Invasive Plants Program (and is also the chair of the Cornell Deer Management Committee), was among the earliest researchers studying the survival, performance and impacts of two non-native, leaf-feeding beetles as a potential biocontrol for purple loosestrife.

In 1992, after six years of study, Galerucella beetles were introduced to North America as part of a purple loosestrife control program. Fortunately, the beetles, which are native to Eurasia, were proven to be trusted biological control agents for purple loosestrife and, after six years of intensive studies, were introduced in several North American locations as part of an international purple loosestrife control program. In places where the natural defoliators were released, colonization of introduced beetle populations was enormously successful.

Releasing thousands of non-native leaf-eating beetles into native wetlands may sound unwise, but initial Galerucella beetle-release projects had demonstrated just how successful this approach can be. In fact, Galerucella beetle releases have consistently been among the most-successful biocontrol programs ever initiated. The beetles prefer purple loosestrife as a food (or host) plant over any native wetland species.

Once Galerucella beetles have been released, it will generally take 3-5 years to see a significant difference in purple loosestrife populations. And, although complete eradiation generally cannot be achieved, maintaining a population of Galerucella beetles can prevent purple loosestrife from spreading and overtaking large areas. The process offers effective and environmentally sound control of purple loosestrife without herbicides.

On July 12, the environment division of the Saint Regis Mohawk Tribe, in collaboration with the SUNY Potsdam Department of Environmental Studies, conducted a beetle release to combat local populations of purple loosestrife. In all, 1000 beetles were released — 250 along the St. Regis River, 250 along the Racquette River and 500 at various other locations within the community. Similar releases have been administered in Akwesasne in the past, and this small release was performed to help maintain area beetle population.

A single female Galerucella beetle can lay as many as 400 eggs in a lifetime. With large enough populations, the beetles can, over time, completely defoliate large groupings of loosestrife plants. Larvae feed on the underside of leaves, eating the photosynthetic tissue creating what’s known as a “window-pane” effect. Adults eat straight through the leaves and shoot tips creating a ‘shot hole’ effect.

We know that purple loosestrife can devastate native ecosystems. In some places, the lovely flowering plants have replaced 50% of the native species. What’s more, their leaves decompose faster than most native species, which changes the natural water chemistry of the wetlands it inhabits. So, if you see a little brown beetle on a purple loosestrife leaf, know that they are part of a much bigger mission and a true invasive species success story.


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