Natural enemies of Lemna minuta in its native range and their potential as biological control agents for Europe

Lemna minuta is native to North America but an invasive alien plant in Europe, where it poses significant threats to freshwater ecosystems. Explorations for biological control agents were conducted in two regions of the United States (Louisiana and California), revealing three candidate species. Subsequent laboratory investigations focused on the weevil Tanysphyrus lemnae and the fly Lemnaphila scotlandae as the most promising agents and confirmed that both the herbivorous insects attack L. minuta plants. To evaluate the host specificity of these two potential agents, insects were exposed to five species of duckweed commonly occurring in North America and Europe. Preliminary host-range testing indicated that T. lemnae develops on all evaluated duckweed species, including those from different genera (Lemna, Spirodela, Landoltia). Conversely, data revealed that feeding and development of the dipteran L. scotlandae are limited to species in the genus Lemna, but this includes L. minor, a native European species threatened in Europe due to the spread of L. minuta. No-choice and choice tests confirmed that neither larvae nor adults of L. scotlandae discriminate between the two Lemna species, except for pupation, which occurs more frequently in L. minor under choice conditions. We conclude that the broad host-range of T. lemnae and L. scotlandae render them unsuitable as biological control agents of L. minuta in Europe.[GRAPHICS].