Abstract
The submersed macrophyte Elodea nuttallii (Hydrocharitaceae) is invasive in
Europe and frequently found in aquatic plant communities. Many invertebrate herbivores, such
as larvae of the generalist aquatic moth, Acentria ephemerella (Lepidoptera, Pyralidae), avoid
feeding on E. nuttallii and preferably consume native species. First instar larvae exhibited a
high mortality on E. nuttallii compared to the native macrophyte Potamogeton perfoliatus.
Mortality of older larvae was also high when fed E. nuttallii exposed to high light intensities.
Growth of older larvae was strongly reduced on E. nuttallii compared to pondweeds
(Potamogeton lucens). Neither differences in nitrogen nor phosphorus content explained the
different performance on these submerged macrophytes, but plants differed in their flavonoid
content. To investigate whether plant-derived allelochemicals from E. nuttallii affect larval
performance in the same way as live plants, we developed a functional bioassay, in which
Acentria larvae were reared on artificial diets.We offered larvae Potamogeton leaf disks coated
with crude Elodea extracts and partially purified flavonoids. Elodea extracts deterred larvae
from feeding on otherwise preferred Potamogeton leaves, and yet, unknown compounds in
the extracts reduced growth and survival of Acentria. The flavonoid fraction containing
luteolin-7-O-diglucuronide, apigenin-7-O-diglucuronide, and chrysoeriol-7-O-diglucuronide
strongly reduced feeding of larvae, but did not increase mortality. The concentrations of these
compounds in our assays were 0.01–0.09% of plant dry mass, which is in the lower range of concentrations found in the field (0.02–1.2%). Chemical defense in E. nuttallii thus plays an ecologically relevant role in this aquatic plant–herbivore system.
