Experimental evidence of the consumption of the invasive alien duckweed Lemna minuta by herbivorous larvae of the moth Cataclysta lemnata in Italy

Alien plant invasion is a serious threat for biodiversity conservation. Most theories on the mechanisms regulating the expansion of an alien species, agree that herbivory is one of the main factors affecting the success or failure of these species. One worrying example in Europe is the American duckweed Lemna minuta that, since its arrival in the 1940s, has become wide spread throughout many countries. This study focused on determining and quantifying the herbivorous nature of the larvae of the moth Cataclysta lemnata (native to Europe) with regards to L. minuta. On the premise that, if a native herbivore feeds on an alien plant species, it could help to contain its expansion. We tested the effectiveness of larvae at three different instars in consuming L. minuta under laboratory conditions. Laval preference for the alien L. minuta and the native L. minor was determined by quantifying the removal of monolayer mats of these two duckweeds. Firstly, we found that C. lemnata larvae was able to use L. minuta as a trophic resource and also to build protective cases. Moreover, they feed effectively, and seemingly without preference, on both the native and the alien species, contrary to the Enemy Release Hypothesis, which assumes that native consumers are better adapted to consume native species than alien ones. In addition, C. lemnata late-instar larvae were more efficient in Lemna consumption. This study suggests that where C. lemnata is a native herbivore the spread of L. minuta could be effectively contained.