Controlling weeds and restoring desired vegetation requires ecosystem level management to avoid replacing one weed with another. A biological control program using two leaf-feeding beetles Galerucella calmariensis and G. pusilla was implemented to control purple loosestrife in North America in 1992. At Baskett Slough National Wildlife Refuge in Oregon, strong suppression of purple loosestrife (Lythrum salicaria) led to an increase in reed canary grass (Phalaris arundinacea). Since reed canary grass aggressively excludes native organisms, this is an example of controlling one weed only to get a worse one in its place. Managers at Baskett Slough are faced with the delicate task of controlling reed canary grass without interfering with purple loosestrife control. They are concerned that if control of purple loosestrife is not maintained while controlling reed canary grass, the loosestrife population will rebound.
We conducted an observational study along a topographic moisture gradient to estimate the effect of water level manipulation (1) on the distribution of reed canary grass and purple loosestrife along the gradient, (2) the purple loosestrife-Galerucella interactions, and (3) to quantify the buried seed bank and actively growing vegetation. Buried seeds of purple loosestrife and reed canary grass were found across the entire gradient, though the number of seeds decreased with soil depth. Actively growing purple loosestrife was predominantly in the low end of the gradient and reed canary grass in the high end. Galerucella insects caused 100% defoliation across the entire gradient in 1998 and 2000 when beetle populations were high, but did more damage at the drier end of the gradient in 1999 when populations were low. The actively growing community had fewer species represented than the buried seed bank community. Raising water levels should reduce abundance of reed canary grass without interfering with purple loosestrife control.
Species 1: Lythraceae Lythrum salicaria (purple loosestrife)
Species 2: Poaceae Phalaris arundinacea (reed canary grass)
Species 3: Coleoptera Chrysomelidae Galerucella sp
Keywords: biological control
The ESA 2001 Annual Meeting - 2001: An Entomological Odyssey of ESA