Galerucella birmanica (Coleoptera: Chrysomelidae), a promising potential biological control agent of water chestnut,Trapa natans

Water chestnut, Trapa natans, has become a major invasive plant in shallow water bodies in the northeastern United States. The failure of chemical and mechanical means to provide long-term and economically sustainable suppression of the species resulted in interest in the development of biological control. Field surveys in Asia and Europe identified a number of potential biological control agents in the native range of T. natans. The most promising species appeared to be the leaf beetle Galerucella birmanica, which is considered a pest of farmed T. natans in China. However, initial attempts to develop biological control faltered when field observations in China suggested that G. birmanica may not be host specific. Of particular concern was attack on water shield, Brasenia schreberi, a species native to China and North America. We conducted a number of laboratory and field investigations in China to assess preference and performance of G. birmanica on T. natans and B. schreberi. Initial no-choice experiments using 19 different plant species in 13 different families demonstrated that G. birmanica oviposited and was able to complete development only on Trapa spp. and B. schreberi. In larval no-choice tests G. birmanica was able to complete development on B. schreberi, however, larvae showed a 20% increase in mortality and longer development time compared to larvae developing on T. natans. In laboratory and field choice tests adults strongly preferred T. natans and in the field only occasionally laid eggs on B. schreberi. In addition, adults emerging from larvae reared on B. schreberi were less fit with reduced feeding and a declining oviposition rate. Their strong preference for T. natans was maintained in the field, even when T. natans was completely defoliated and adults were forced to migrate. We found only occasional “spill-over” of beetles onto B. schreberi and our data indicate that G. birmanica is a more promising biological control agent of T. natans than previously thought, although additional host specificity tests with many more North American plant species need to be completed.