The effect of shoot feeding by the biocontrol agents, Galerucella calmariensis and Galerucella pusilla (Coleoptera: Chrysomelidae) on purple loosestrife (Lythrum salicaria) seed production and seed germination was quantified in two Minnesota wetlands. In a wet meadow where Galerucella spp. were present on isolated plants, feeding by adults and larvae during shoot elongation resulted in stunting and malformation of shoot tips. There was a subsequent reduction in purple loosestrife inflorescence length and number of flower buds and seed capsules. As Galerucella spp. larvae preferentially fed on shoot meristems, even low levels of feeding on a whole-plant basis (approximately 10% defoliation) reduced seed production. In a sedge meadow wetland with severe feeding damage (a minimum of 70% leaf defoliation), few to no flower buds formed on plants, and subsequently, few to no seed capsules were produced on purple loosestrife plants. Of the few capsules that were produced, number of seeds per capsule and percent germination of seeds did not differ from control plants. In both wetlands, feeding on a main shoot of purple loosestrife did not result in a compensatory increase in the number of axillary inflorescences. Feeding by Galerucella spp. and the subsequent reduction in number of seeds produced on purple loosestrife plants will decrease the number of seeds available for dissemination to new sites. Fewer seeds will enter the seedbank, and over time, feeding by Galerucella spp. will decrease the number of seeds available for seedling recruitment. The benefit of leaf defoliation on purple loosestrife plants caused by Galerucella spp. feeding has been reported. In this study, we have quantified the additional benefits of reduced seed production from Galerucella spp. feeding on purple loosestrife in North America.
Nomenclature: Purple loosestrife, Lythrum salicaria LYTSA; Galerucella calmariensis; Galerucella pusilla.