Illinois Natural History Survey - University of Illinois

Frequently Asked Questions about Biological Control of Purple Loosestrife in Illinois

1. IS PURPLE LOOSESTRIFE REALLY A PROBLEM? IT IS SO PRETTY WHEN IT FLOWERS!

Pretty it may be, but the bright purple color is deceiving. It is not so much what you see as what you DO NOT. When purple loosestrife (Lythrum salicaria (pronounced LITH - rum sal - ih - KAR - ee - uh) invades a wetland, it can take over, outcompete and displace the native species. The solid purple you see from the road has been turned into a virtual monoculture, devoid of many of the species that usually are found in biologically rich wetlands. In many cases, wetlands have been designated natural areas to conserve rare or threatened plants; the invasion and dominance of loosestrife in these wetlands further endangers these rare organisms. Because wetlands are so modified by purple loosestrife, there are major influences on the animals that normally occupy the habitats. Birds depend on insects as a food source and since loosestrife has few insects that feed on it there are few insects for native birds (e.g., wrens or blackbirds). Ducks will not use dense loosestrife infested areas to nest in and muskrats do not use these sites.

2. DOES PURPLE LOOSESTRIFE NATURALLY OCCUR IN ILLINOIS?

No. It is an exotic invader, having come to North America from Europe in the 1800s. Apparently, it arrived in Illinois in the middle of this century, but it really only has become a serious pest in the last 20-25 years. There are several native loosestrife species, such as winged loosestrife (Lythrum alatum) and swamp loosestrife (Decodon verticillatus).

3. WHAT CAN BE DONE ABOUT PURPLE LOOSESTRIFE? WHY IS BIOLOGICAL CONTROL BEING USED AGAINST PURPLE LOOSESTRIFE?

Three control methods have been tried -- cultural control (hand pulling/digging), chemical control (herbicides), and biological control (insects are being grown and released against serious infestations of the weed. Other methods of controlling purple loosestrife -- hand-pulling/digging or herbicides -- are the method of choice when the infestation is just beginning or on a very small scale in a wetland. However, these methods are not appropriate when more than about a hundred plants are present. Also, because the loosestrife seeds remain viable for a long time, these cultural or chemical methods will not eliminate the problem -- only the plants that are present right now. Several areas around Chicago have had herbicides used for more than a decade, with no progress in reducing the loosestrife infestation.

4. WHAT KINDS OF INSECTS ARE USED FOR BIOLOGICAL CONTROL?

Three beetle species have been grown and released in Illinois since 1994. Two of these species feed as both adults and immature larvae on the leaves and growing tips. These two areGalerucella calmariensis (pronounced Gal - err - ooh - SELL - uh cal - mary - ENN - sis) andGalerucella pusilla (pronounced pew - SILL - uh). They have one generation per year and overwinter as adults. Eggs are laid in clusters of 5-20 on the leaf surface, often where the leaf joins the stem. The feeding causes holes in the leaves, reducing the amount of leaf area, weakening the plant. Also, larvae feed in the growing tips, which can reduce or prevent flowering -- and no flowers means no seeds. A third species is a weevil, named Hylobius transversovittatus(pronounced High - LOW - bee - us trans - verse - o - vih - TAH - tus). Adult female Hylobius lay eggs into a stem of purple loosestrife; the egg hatches and the larva tunnels into the root, where it feeds for several years before emerging as a new adult. Adults live for several years. Root feeding is extensive enough to kill the plant. As of January, 1998, more than a half-million Galerucella adults (both species) have been released at approximately 40-50 sites in Illinois -- primarily in the greater Chicagoland area. Approximately 1,000 eggs ofHylobius have been released at about 20 sites.

5. WILL THESE INSECTS FEED ON OTHER PLANTS, ESPECIALLY THE ONES IN MY YARD AND GARDEN?

Before the insects were allowed into the U.S. in the early 1990s, they were tested against 49 species of plants to determine on which ones what they would feed. This was done by looking first at the closest relatives, then more-distantly related species including many of our important agricultural crops. The Galerucella beetles fed slightly on the native loosestrife species (winged loosestrife, Lythrum alatum and swamp loosestrife, Decodon verticillatus), when they were given NO CHOICE. When they were given a simultaneous choice of purple loosestrife or the native species, the beetles fed very little on the two native species, but did NOT lay eggs on them -- meaning there would be no damage from larval feeding and no new generation of beetles on these species of plants. With the Hylobius weevils, a few laid eggs on winged loosestrife and swamp loosestrife, but larvae developed completely only on swamp loosestrife. So, there was a little feeding and egg-laying on a few other wetland plants, but only the closest relatives to purple loosestrife (and not at all on the distant relatives). And even then, whenever there was a choice (as would occur in the real world), the insects chose the exotic purple loosestrife. This is why we are pretty confident that the beetles will have little or no damage to non-target plant species. We say "pretty confident" but there are no guarantees. There have been a few cases where biological control agents have fed on non-target species to some extent. However, in all but one of these few cases, the non-target plants were known beforehand to be part of the diet of the weed control insects -- insects intentionally released for biological control have not suddenly shifted or broadened their diet to feed on plants that they would not touch originally. So, your roses, garden vegetables and landscape plants are safe from these insects.

6. WHO IS INVOLVED IN THE BIOLOGICAL CONTROL PROJECT AGAINST PURPLE LOOSESTRIFE?

The project is a partnership between the Illinois Natural History Survey (INHS), Illinois Department of Natural Resources, the Forest Preserve Districts of Cook, Lake, DuPage and Kane Counties, the McHenry County Conservation District, the U. S. Fish and Wildlife Service and several city park districts. The Illinois Natural History Survey grows the insects and provides them to the city, county or state agencies for release and monitoring. Financial support for this project has come primarily from the U. S. Army Corp of Engineers, with additional funding provided by many of the partnership organizations listed above.

7. WILL BIOLOGICAL CONTROL WORK?

There are no guarantees of success, but there are several beginning signs of promise. At one site near Savanna, Illinois, (on the Mississippi River), a total of 1,000 Galerucella beetles were released at three discrete places in 1994. In the summer of 1997, there was severe damage to the loosestrife -- many hundreds of dead plants, many thousands more that did not flower, even though other loosestrife in nearby areas was flowering. This same result has been seen in Ontario, Minnesota, and New York, and seems to take 3-5 years after the first releases were made. At other sites, less-severe -- but just as important -- effects have been seen. In this case, a direct reduction of flowering plants, without massive numbers of beetles, has been seen near Zion, Illinois. From the release of a few hundred Galerucella in 1994, flowering at the site was reduced, by 1997, to only 3% of the plants sampled -- a number small enough that native plants can return to the area.

8. WHY CAN I STILL BUY PURPLE LOOSESTRIFE AT GARDEN CENTERS?

The Illinois Exotic Weed act prohibits the sale or distribution of purple loosestrife without a permit. Further, this permit can only be issued by state agencies for experiments in controlling this exotic species. Other species of loosestrife are sold at garden centers, they are supposed to be sterile but apparently can cross-fertilize purple loosestrife. Purple loosestrife is not defined as a noxious weed. However, purple loosestrife, multiflora rose and Japanese honeysuckle are listed under the Illinois Exotic Weed Act. The noxious weed law is a different law. The primary difference with the Exotic Weed Act is that is controls the sale of the plant, but does not require the control of a listed plant on private property.

 

Information on this page maintained by David Voegtlin.

Translated into Belorussian 21 February 2011 by Patric Conrad



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