Biological Control of Purple Loosestrife Using Gallerucella Beetles

Abstract

Since the early 1990’s, use of Galerucella beetles for biological control projects of purple loosestrife (Lythrum salicaria L.) has exhibited positive results in decreasing purple loosestrife stands and may aid in the re-establishment of native species communities. I looked at data taken over a nine year period from permanent research plots where Galerucella beetles were released at the University of Michigan’s Matthaei Botanical Gardens in Ann Arbor, Michigan. A path analysis was used to determine the relationship between beetle defoliation, purple loosestrife reproductive success, and native plant composition. I also compared data between years to determine where, in the course of the biocontrol program, purple loosestrife reaches its lowest density and native species begin to show signs of recovery. I found that purple loosestrife height mediates the effect of beetles on the number of plant inflorescences and that when inflorescence number and height decrease there is an increase in the floristic quality index of the native community. Sites with lower disturbance and less severe purple loosestrife invasions react positively to beetle introduction more quickly than sites with higher disturbance and purple loosestrife density. In the sites studied, approximately six years after beetle introduction, purple loosestrife reached an all time low, which suggests this as an ideal time to implement supplementary restoration techniques in order to further control the invasive population and aid in the restoration of native species.