Eurasian water-milfoil (Myriophyllum spicatum L., henceforth “EWM”) is the most heavily managed nuisance submersed aquatic plant in the United States. EWM’s rapid spring growth and formation of dense surface mats inhibits native macrophyte communities, serves as poor-quality habitat for fish and macroinvertebrates, impacts recreation, and can clog water supply infrastructure. EWM invasions result in economic losses to revenues associated with recreational activities, decreased property values, and the cost of control. The milfoil weevil (Euhrychiopsis lecontei Dietz) has been associated with EWM declines in several states, though natural weevil densities are generally too small to effect control. Augmentative biocontrol has had varied success. Fish predation may be a primary factor influencing weevil populations. Weevils were augmented in 2 of 4 EWM beds in each of 4 northern Wisconsin lakes in summer 2013. To assess fish feeding habits and preference for particular prey items, we collected invertebrates associated with EWM (150 environmental samples) plus 442 diet samples from bluegill sunfish (Lepomis macrochirus Rafinesque) in the EWM beds.
Results/Conclusions
Captured bluegill ranged in size from 37 to 200 mm (mean 106 mm). A total of 40 invertebrate taxa were identified from diets and environmental samples. Overall, chironomids and oligochaetes were the dominant invertebrates associated with plants, while chironomids and Daphnia spp. constituted up to 27.2% and 24.0% of the fish diets, respectively. When compared to prey availability in the environment, bluegill appeared to avoid littoral cladoceran species, oligochaetes, and gastropods while preferring pelagic cladoceran species, in particular Daphnia spp. Milfoil weevils were rare in the diets, occurring in 2.9% of diet samples examined. Weevil larvae were preyed upon more frequently than adults (94.2% of weevils consumed) and sometimes occurred in high numbers within single diet samples. Since the larval stage contributes the most to EWM damage, selective predation on this stage may limit its use as a control agent.