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Purple loosestrife (Lythrum salicaria) is a perennial wetland plant that is native to Europe and capable of producing two to three million seeds a year. It was introduced to North America in the early 19th century for ornamental and medicinal uses and is still widely sold as an ornamental, except in states such as Minnesota, Wisconsin, and Illinois where regulations now prohibit its sale, purchase, and/or distribution (Swearingen, 2009). Past attempts to control purple loosestrife have included hand pulling or cutting, herbicidal treatments, water level manipulation, and burning and have been relatively unsuccessful. Over 100 insects feed on this plant in its native land and in the mid-80's biologists began testing several species of phytophagous insects and many have been identified as having sufficient host-specificity to serve as effective control agents for this invasive plant (Blossey et al., 1994). These insects include two leaf feeding beetles in the Galerucella genus (the black-margined loosestrife beetle and the golden loosestrife beetle) and one species of weevil whose larvae feed on the roots of purple loosestrife (Hylobius transversovittatus). Over 100 insects feed on this plant in its native land and in the mid-80's biologists began testing these species for importation to the United States for biocontrol of the plant. The three species of beetles that were chosen and imported are currently controlling purple loosestrife in almost 90% of all release spots and have established reproducing populations able to find infestations on their own. This type of biocontrol was researched thoroughly and 50 species of plants from the United States including native wetland plants, important commercially grown plants, and agricultural species were all tested. This paper will review establishment of the insects and the diverse interactions between the different species as well as the native plant competition and recovery seen when the insects are introduced into purple loosestrife infested wetlands.
Because there are more than one insect species that have been introduced to control purple loosestrife researchers have begun to study the interactions between the insects and the level of plant control they can deliver when competing with each other. Recent studies have suggested that interactions of above- and below-ground herbivores can affect plant resource allocation and physiology, seed set and plant fitness, how a plant allocates its defensive chemicals, as well as indirectly affecting decomposers and soil processes by altering the quality and quantity of resources entering the soil (Masters et al., 1993, Proveda et al., 2003). Researchers Hunt-Joshi and Blossey tested predictions that root herbivore fitness would be negatively affected by leaf herbivory, and that leaf herbivore fitness would be positively affected by root herbivory. They used both potted plants and large field cages set up in purple loosestrife-dominated wetlands. The two insect species used in the experiment were Hylobius transversovittatus (the loosestrife weevil), a root feeder and Galerucella calmariensis (the black-margined loosestrife beetle), a leaf herbivore. The researchers found that weed biocontrol programs targeting perennial plants appear unlikely to jeopardize their overall control success by introducing root and above-ground herbivores simultaneously (Hunt-Joshi et al., 2005).
Most information about plant feeders is done on above-ground herbivores which remove plant tissue resulting in reductions in growth rate, biomass, and plant height, all which can be visually investigated rather easily. Comparatively little is known about the effects of root feeders on plant performance despite the often profound effects on plant fitness (Nötzold et al., 1997) such as water stress, differing carbohydrate levels and changes in nitrogen concentrations within the plant. Nötzold et al. researched not only the destruction to purple loosestrife caused by the root feeder H. transversovittatus but also the impact the root feeding had on the ability of the plant to continue compete with other native plants such as Phleum pretense in the habitat. All their research was performed in pots in greenhouses and lasted over a two year period with dormancy (winter) season in between growing times. They found that root herbivory by H. transversovittatus and the interaction with plant competition will directly alter the competitive ability of purple loosestrife due to the removal of resources that may disrupt other important physiological functions of the plant (Nötzold et al., 1997).
Schooler et al. researched the relationship between G. pusilla (golden loosestrife beetle) and purple loosestrife and found that plant damage is an increasing function of insect density and that visual estimates of leaf area damaged can be used to estimate G. pusilla beetle density for field populations below carrying capacity. This research may help local governments by making beetle damage assessment and plant control easier than it is with actual beetle counts. If protection agencies can easily evaluate beetle density and active control in one release plot, they would be able to assess the correct number of beetle releases to control a specific area of land. Another group of researchers assessed the establishment of H. transversovittatus on loosestrife. This beetle was released in North America in 1992 and within two years was established in 18 of the 27 release sites in nine states and two Canadian provinces (Hight et al., 1995). Because H. transversovittatus larvae are root feeders and the adults are nocturnal this insect's impact on purple loosestrife receives less attention than more visual insect controls for L. salicaria. McAvoy et al. deduced that root feeders are generally more effective than foliage feeders in reducing plant vigor and their observations show that H. transversovittatus has the potential for a much greater impact on purple loosestrife than the foliage feeders and that this beetle is relatively easily established.
McAvoy and Kok researched competition between the two foliage feeders G. calmariensis and G. pusilla and found that G. calmariensis larvae consume more foliage when there is an abundance of food but are out-competed by G. pusilla when food is scarce. G. pusilla also has faster development but has a harder time getting established in colder northern climates than its sister species G. calmariensis. The researchers concluded that it is possible that these two Galerucella species can coexist and have little impact on each other when food is abundant due to their similar developmental rates (McAvoy et al., 2007) but that when food is limited G. pusilla completes larval development more quickly on less food and may favor its species survival once loosestrife is being controlled adequately.
Purple loosestrife's capacity to out compete native flora is also aided by its ability to exploit disturbances and produce over two million seeds per adult per year (Welling et al., 1990). In a recent study pertaining to native flora recovery after successful control of purple loosestrife Albright et al. worked with the two Galerucella species over a five year span. The first two to three years of the study consisted of establishing the beetle species, the last two years (2001-02) the beetles were able to control the loosestrife by reducing the above ground portions of the plant. This reduction led to the plant being unable to produce flowers and set seed and an estimated 81% of the plants biomass was reduced (Katovich et al., 1999) because of the energy the plant spent on trying to regrow its aboveground parts. The researchers found that the beetle abundance reduced the loosestrife and that by 2002 78% of the controlled area was now covered with species other than loosestrife with cattail (Typha spp.) and swamp candles (Lysimachia terrestris) becoming dominant (Albright et al., 2004).
Purple loosestrife is a very invasive plant that can easily devastate native flora and fauna if not controlled properly and the cultural controls such as hand pulling or cutting, herbicidal treatments, water level manipulation, and burning have been relatively unsuccessful. I feel that more research could be done on other possible interactions between the three types of beetles and the climate where the plants are growing. Because G. pusilla is a hardier species when food is scarce but is unable to fully establish in colder climates maybe it would be possible to select for a hardier biotype to be reared and released in these colder climates. Also, I believe that colonies of these insects should be kept either by governmental or private insectaries that could supplement established wild colonies incases of major floods, disturbances, or where pesticides may intentionally be used or drift into the environment. These insectaries could also work on other insects that could cooperate with these three more established beetles to eliminate this invasive plant even more. More work should also be done on the native species of flora that could better compete with purple loosestrife in native environments.
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- Swearingen, J.M. 2009. PCA Alien Plant Working Group - Purple Loosestrife. National Parks Service. http://www.nps.gov/plants/ALIEN/fact/lysa1.htm