Individual study: The effectiveness of a phytophagous beetle, Galerucella calmariensis, in the control of an invasive plant, purple loosestrife Lythrum salicaria, at Netley-Libau Marsh, Manitoba, Canada
Henne D.C., Lindgren C.J., Gabor T.S., Murkin H.R. & Roughley R.E. (2005) An integrated management strategy for the control of purple loosestrife Lythrum salicaria L. (Lythraceae) in the Netley-Libau Marsh, southern Manitoba. Biological Control, 32, 319-325
Purple loosestrife Lythrum salicaria, native to Eurasia, is a marshland perennial plant introduced to North America in the early 1800s. In some areas it forms dense single-species stands, displacing native plants and fauna, and is hence considered an invasive species.
Several methods have been used in an attempt to control purple loosestrife. Mowing or cutting plants decreases plant vigor and reduces (otherwise prolific) seed production but does not kill the rootstock. Cutting may reduce stem densities but many cuts are necessary and it may never be eliminated from a site using this technique. Cutting is also a costly, localized and short-term solution. Triclopyr amine and glyphosate have been trialed as potential herbicides controls. However, it has been found that following application, treated areas become dominated by purple loosestrife seedlings recruited from the seed bank. Thus, an effective method of controlling loosestrife seedlings after herbicide application is required. Since 1992, control efforts have focused on biological control agents. Those approved for release in Canada include two phytophagous beetles, Galerucella calmariensis and G.pusilla, a root-mining weevil Hylobius transversovittatus and the flower- and seed-feeding weevils Nanophyes marmoratus and Neolamprologus brevis.
Biological control may represent a long-term solution as it may take several years for agents to reach densities sufficient to control established stands of purple loosestrife. Herbicidal control may provide immediate control, but is costly if repeated applications are required. Therefore, it was considered that a solution to purple loosestrife control might be to integrate these two approaches. In Manitoba (Canada), purple loosestrife has invaded an estimated 5,575 ha of wetland habitat. To evaluate the potential benefits of combining herbicide and biological control for purple loosestrife management, a study was undertaken. The effectiveness of biological control, herbicide control and an integrated approach (biological and herbicide control combined) were examined. The biological control treatment is outlined here.
Study area: The study was conducted in 1996-1998 in field cages positioned on a 2 ha stand of purple loosestrife Lythrum salicaria in Netley-Libau Marsh (24,381 ha), Lake Winnipeg, southern Manitoba. A survey carried out in 1936 reported no purple loosestrife but in 1944, it was discovered upstream of the marsh near Lockport (on the Red River). By the 1970s it was considered common throughout the marsh.
Biological control agent: The beetle Galerucella calmariensis, was selected as the biological control agent as it is easily reared, has high reproductive potential, was already established in Manitoba, and studies had shown it to be compatible with the herbicides triclopyr amine and glyphosate (see Cases 218 & 219). Adult beetles emerge in late spring, feeding on young leaves and meristems of purple loosestrife. Females begin oviposition 7–10 days after emergence/feeding. Young-instar larvae feed on shoot tips while older ones feed on all plant parts. Mature larvae pupate in the soil and emerge as teneral adults in 7–10 days. In Manitoba, adult beetles emerge in late May to early June, oviposition begins in early June, and peak larval densities occur in late June to early July.
Lumite screen cages: Trials were conducted within 'walk-in' screen cages to reduce variability potentially arising from G.calmariensis dispersal and predation. Each cage was 8 m³ and covered with screening material (Lumite, style 50090000, 20 × 20 mesh, 15% shade, 1629 cfm porosity; Synthetic Industries Performance Fabrics Division, Gainesville, GA) on all sides and the top, with a zipper at one end to enable access. Cage frames were constructed of 6 × 6 cm kiln-dried spruce wood, reinforced with wooden cross-braces in each corner and anchored to the ground using 60 cm long by 10 mm diameter steel rods. Lumite screening was secured along the bottom of the frame using 25 mm roofing nails in combination with 10 mm washers. The screening was placed over the frames in early May of each year prior to purple loosestrife shoot emergence and removed in early October to allow snow accumulation, thereby providing adequate insulation for the over-wintering beetles.
Sampling design: Twenty-four Lumite cages were set out in three blocks of eight cages (see also Cases 218 and 219). Treatments were assigned within each block into a randomized complete block design to account for any variance within the study area. Each treatment was replicated three times.
Treatments: Adult G.calmariensis were collected from established populations in the area, as well as from populations in Minnesota, USA. Adult beetles were sexed and counted in the laboratory prior release into field cages. Six pairs (n = 12) were released into each of the three insect alone treatment cages on 18 June 1997. As a control, three Lumite cages received no herbicides or G.calmariensis introductions.
Sampling purple loosestrife performance: Within each cage, 15 purple loosestrife stems (n = 45 stems/treatment) were randomly selected, tagged at the base of the plant with orange tape, and numbered for identification. Stems were selected as the sampling unit as it is difficult to define an individual purple loosestrife plant without digging it up. Data on stem heights, main spike length (the flowering spike forming the apex of the plant), the number of flowering spikes, and seed capsules were recorded. In 1996, data were collected on 31 July (prior to herbicide applications in herbicide treatment cages), 14 August, and 9–10 September. In 1997, data were collected on 26 June, 29 July, and 15–19 September. In 1998, data were collected on 9 July, 28 August, and 30 September.
Beetle establishment: G.calmariensis established successfully in all of the cages to which they were introduced. A summary of the results is given in Table 1 (attached).
Purple loosestrife stem densities & height: By September 1997 and 1998, mean stem heights of purple loosestrife were significantly lower compared to the controls. In the G.calmariensis treatments, mean stem heights were reduced by as much as 70% by beetle herbivory. Stem densities remained more-or-less the same in both years in both beetle and control treatments.
Purple loosestrife inflorescences: In September 1997, the mean number of inflorescences produced by purple loosestrife plants in the G.calmariensis treatment was significantly higher than in the controls but the main inflorescences in the G.calmariensis treatment produced significantly fewer seed capsules. By September 1998, no infloresences at all were produced on the tagged plants, whilst the number in the controls had increased from 4.4 in 1997 to 6.1 in 1998.
Purple loosestrife stems in response to G.calmariensis larvae herbivory produced more lateral branches resulting in 'bushy' plants with many, smaller flowering spikes.
Purple loosestrife seedlings: In August 1998, G.calmariensis adults and larvae were observed feeding and ovipositing on purple loosestrife seedlings.
Conclusions: This and other recent studies indicate that G.calmariensis may be effective at controlling purple loosestrife. Beetle herbivory significantly reduced stem height by up to 70% within two years, and by the second year the number of inflorescences on tagged plants was reduced to zero. Importantly, young seedlings emerginging from the seed bank were also grazed by the beetles.
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