Publication Summary

A milfoil weevil augmentation project took place in Mattoon Lake in central Washington. We monitored aquatic plant growth, invertebrates associated with Eurasian watermilfoil, and the fish community. During the project Eurasian watermilfoil decreased significantly, and other aquatic plant species did not change or increased. The milfoil weevil took five years to establish, during which time a midge population controlled Eurasian milfoil growth. The fish community changed from domination by small sunfish to a balanced community of predator and prey fish. Fish diet analysis indicated that fish predation impacted herbivorous invertebrate populations. This study supports the theory that fish and herbivorous invertebrates influence lake food web interactions.

This study was undertaken to gain experience propagating the milfoil weevil (Euhrychiopsis lecontei Dietz) and to monitor the macrophyte, macroinvertebrate and fish communities at a milfoil weevil augmentation site between 2002 to 2008, in a small lake in central Washington State. The milfoil weevil propagation was time consuming, but not difficult. Over the course of the project, monitoring showed a significant decrease in the frequency and biomass of Eurasian watermilfoil (Myriophyllum spicatum L) and no change or a slight increase in frequency and biomass of other macrophytes at the augmentation site. The milfoil weevil took 5 years to establish in the lake, during which time a midge (Chironomidae) population started to control Eurasian milfoil growth. The fish community changed from one dominated by stunted pumpkinseed sunfish (Lepomis gibbosus L) to a more balanced community of predator and prey fish. Fish diet analysis indicated that fish predation likely influenced herbivorous macroinvertebrate populations. This study supports the theory that fish and macroinvertebrate herbivores influence lake trophic interactions, affecting primary productivity as macrophyte growth.