October 2009 Purple Loosestrife Biocontrol Project Update
As autumn slips over the Neponset wetland landscape, our little beetles with the big appetites lie dormant, waiting for spring. Beneath the rushes, the Sensitive fern and the goldenrods, under the leaf litter and along the cold mud of the wetlands, the new generation of Galerucella rests up, dreaming of warm weather and young Purple loosestrife shoots.
Interesting happenings have come to light in the site monitoring data gathered by Volunteers this autumn and last. Data indicate that after two consecutive summers of beetle releases, Purple loosestrife at each of our treatment sites (Brookwood Farm, Milton-Fowl Meadow and Boston-Fowl Meadow) indeed is showing the effects of the feeding beetles.
During the 2008 growing season, the Purple loosestrife (“PL”) plants grew shorter, fewer in number, and sprouted many fewer flowers (and shorter flower spires at that) than previously. How is this significant? To deal with the Galerucella damage to their leaves, the Purple loosestrife plants had to expend energy to “heal” themselves and re-leaf. They diverted energy from efforts to grow taller or to produce flowers. Result: Shorter plants with few to no flowers. Because the flowers on one PL plant can produce 2 million+ seeds, and taller PL plants block the sun from nourishing potential native plant competitors, this summer’s shorter, flower-less PL plants allowed more sunlight to slip into the wetland to nurture potential native competitors, and the PL plants added far fewer seeds to the wetland seedbank than normal. This is great news for our efforts to reduce the grip of Purple loosestrife on the wetlands and restore a diversity of native wetland plants.
Interestingly, Brookwood Farm showed the greatest impact from Galerucella, this year:
At left is Brookwood Farm’s wetland swale covered in tall Purple loosestrife flowers in August 2008.
The photograph at right depicts the same site, one year later (August 2009). In the foreground of the 2009 photo is a dusty-pink-flowered native plant called Joe Pye-weed. Just beyond the Joe-Pye weed, and in the distance, you see the yellow blooms of native goldenrod species. The lavender-gray color in the center of the photo consists of dead Purple loosestrife flower stalks – remnants of the 2008 growing season. No alive Purple loosestrife flowers are visible in the 2009 photo. In fact, hardly any Purple loosestrife plants at all flowered this season at Brookwood Farm. This season’s PL plants also were far shorter than the 2008 plants, whose height is indicated by the year-old flower stalk remnants.
The next greatest impact from the Galerucella beetles was evident at the Milton-Fowl Meadow wetland site, followed by the Boston-Fowl Meadow site.