The effect of two herbivorous insect larvae on Eurasian watermilfoil

Larvae of the moth Acentria ephemerella (Denis and Schiffermuller) (=Acentria nivea (Olivier)) and the weevil Euhrychiopsis lecontei (Dietz) were associated with a population of Eurasian watermilfoil (Myriophyllum spicatum L.) that had declined. To determine if these herbivorous insect larvae played a role in the decline we conducted three experiments (Acentria alone, Euhrychiopsis alone, Acentria and Euhrychiopsis together) in outdoor pools which quantified their effects on watermilfoil growth. Acentria larvae significantly reduced watermilfoil growth in two experiments. Acentria damaged the plants by cutting the stem and removing leaves. Late instar Euhrychiopsis larvae significantly reduced watermilfoil growth in one experiment but not the other. Weevil larvae significantly reduced plant growth in the experiment in which watermilfoil exhibited a faster growth rate. Weevil larvae fed by burrowing through the stem and burrowed through approximately 6-8 mm of stem per day. These results suggest that these insect herbivores have potential as biological control agents for Eurasian watermilfoil in North America.