Biological Control of Purple Loosestrife in Rhode Island

Purple loosestrife (Lythrum salicaria) is an introduced aquatic plant, which has become a serious wetland weed.  It is native to Europe, but was introduced into North America in the 1800’s both accidentally (in ship’s ballast) and on purpose for use as a medicinal herb and for ornamental purposes.   Purple loosestrife is a tall plant with a distinctive purple flowering spike.  It possesses a strong rootstock that serves as a storage organ, allowing the plant to grow quickly in the spring and to dominate the native herbaceous canopy.  Mature plants can produce over 2.5 million seeds annually, which are long-lived and easily dispersed.  With no native herbivores or pathogens in the USA, loosestrife has spread rapidly and is currently distributed throughout the entire northern USA and southern Canada.  Although it has an attractive flower, loosestrife is considered a noxious weed because of its tendency to displace other types of aquatic vegetation.  Dense stands of loosestrife eliminate the food and cover necessary for wetland wildlife such as ducks, geese, bog turtles, and muskrats.  Loosestrife displaces native vegetation and eliminates the shallow sandbars needed by many of these species.

Control measures generally include mowing, hand-pulling, flaming, chemical treatments, and water level management.  The relative lack of success of these measures has led to the current use of biological control against purple loosestrife.  Biological control of an introduced weed like purple loosestrife involves introducing natural enemies of the weed from its place of origin into its new home.  The search for insect natural enemies began in Europe in the mid-1980’s.  One hundred and twenty insect species were found to feed on loosestrife in Europe.  Of these insects, 14 were found to be host-specific to Lythrum species and 6 were recommended for further study as potential biological control agents in the USA.  Following years of laboratory and field research on host specificity, five species were cleared for use in the USA.

In 1994 the University of Rhode Island became involved in biological control of purple loosestrife because Roger Williams Zoo was interested in controlling the loosestrife in its wetland.  Anyone who is familiar with the wetland walk at the zoo will recall the dense fields of purple on both sides of the walk when loosestrife blooms in July and August.  The zoo wanted to remove the loosestrife and encourage the growth of native wetland plants, but their hand-pulling efforts had been unsuccessful.  From 1994 to 1996 we released three species of insects at the zoo: Galerucella calmariensis, Galerucella pusilla, and Hylobius transversovittatus.  Galerucella beetles were also released at three other locations in Rhode Island.  

The two Galerucella species are difficult to tell apart, and are generally reared and released together.  They are small beetles (3-5 mm), which emerge from overwintering in May, lay eggs in May and June, and die in June.  Each female can lay 3-400 eggs on stems and leaf axils.  The larvae feed on buds, leaves, and stems.  They pupate in the soil and groundcover near loosestrife plants.  There is only one generation per year, and the newly emerged adults overwinter in leaf litter.  These insects have very specific host preferences; only two plant species in Rhode Island other than purple loosestrife will sustain limited feeding by Galerucella: winged loosestrife, Lythrum alatum, a plant native to North America, and naturalized in Rhode Island, and swamp loosestrife, Decodon verticillatus, which is native to Rhode Island.  If given a choice Galerucella avoid both winged loosestrife and swamp loosestrife.

Galerucella adult

Galerucella eggs

Galerucella larva

Galerucella larvae

Galerucella beetles overwintered successfully in the zoo the first year after release, the population has increased each year, and in 1998 the beetles truly showed what they could do.  The entire stand of purple loosestrife on the wetland walk was completely defoliated.  Many visitors to the zoo probably did not realize that something very positive had happened to turn the purple field of flowers into a brown expanse.  The beetles prevented acres of loosestrife from setting seed, allowing many native plants to regain a foodhold.  Of course, purple loosestrife is a remarkably resilient plant and new shoots were already beginning to sprout by the end of the summer.  Galerucella beetles will also be back next summer to again defoliate the new stems that emerge.   The reduced growth of purple loosestrife will afford native plants a competitive advantage and we anticipate a sharp decline in the loosestrife population over the next few years, due to Galerucella, and increased diversity of native plants. 

Roger Williams Zoo: July 1997

Roger Williams Zoo: July 1998

One of the most common questions that is asked at this point in a discussion on biological control, is “now what?” and “If the beetles have eaten all of the loosestrife what will they eat now?”   Beetle populations in the defoliated area will begin to migrate in search of new patches of loosestrife.  This was already evident at the zoo in 1998.  The loosestrife along the wetland walk was totally defoliated but other patches of loosestrife in other areas of the zoo and elsewhere in Roger Williams park were just beginning to show feeding damage.  Rather than starve to death the beetles search for nearby loosestrife.  This natural spread of the Galerucella population will benefit other areas of Rhode Island infested with purple loosestrife.  As purple loosestrife populations decline, the insect populations will die of starvation because of the reduced food supply. 

 Hylobius transversovittatus was also released at the zoo.  Hylobius is a large (13mm) weevil; Hylobius larval feeding on the roots can kill purple loosestrife plants.  Hylobius weevils have the same degree of host specificity as the Galerucella beetles; only purple loosestrife, winged loosestrife, and swamp loosestrife are potential hosts.  If given a choice of plants all three insect species avoid winged and swamp loosestrife.  We have not yet confirmed that Hylobius is established at the zoo.  The results of the introduction of Galerucella beetles into Roger Williams zoo are very encouraging.  In an area of Roger Williams Park we found purple loosestrife growing among a large stand of swamp loosestrife.  We were pleased to see that while the purple loosestrife was showing clear signs of defoliation from Galerucella beetles, the surrounding Swamp Loosestrife had only received a few bites.  We will continue to keep track of both Galerucella and Hylobius populations and document the anticipated decline of Purple Loosestrife and resurgence of native plants in the zoo.

 Lisa Tewksbury: Manager, URI Biological Control Lab