Developing and evaluating new techniques to selectively control invasive plants

Project manager: Ray Newman

Funded by: Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources

Phase I Description: To improve approaches to selectively control invasive aquatic plants such as curly-leaf pondweed with integrated approaches including herbicides, we will work with the DNR to evaluate extant and new strategies to control submersed invasive plants selectively in ways that will also restore native plant communities. We will focus efforts on reanalysis and meta-analysis of data previously collected by the university, the DNR and other cooperators on the effects of curlyleaf pondweed herbicide treatments, including any data collected on the longer-term response after treatments are stopped.

To assess the potential of using milfoil weevils for the biological control of Eurasian watermilfoil, enclosure experiments will be conducted to determine if sunfish limit herbivore abundance and control of milfoil.  Previous research has shown that weevils can control water milfoil if sunfish do not consume the weevils. To determine the extent of sunfish consumption of herbivores and factors limiting herbivore abundance in lakes, twenty or more lakes will be surveyed for milfoil, herbivores and sunfish. Sunfish gut contents will be assessed in a subset of these lakes. Analysis of these data will then be used to propose further study.

Project start date: 2013

Project end date: 2016

Findings:

Curlyleaf pondweed (Potamogeton crispus) and Eurasian watermilfoil (Myriophylum spicatum) are the most widespread and problematic invasive aquatic plants in Minnesota. Approaches to improve their management are needed to reduce economic and ecological costs of invasive control. We collated and analyzed pre-existing data on curlyleaf pondweed from 60 lakes across Minnesota to provide an analysis of factors affecting curlyleaf abundance. For untreated lakes, productivity (prior summer Secchi depth) and over winter conditions were important with greater abundance in lakes with higher productivity and milder overwinter conditions (shorter duration of ice cover and lesser snow depth). For herbicide treated lakes, consecutive years of treatment was also important; abundance decreased with more years of treatment. There were diminishing returns from repeated treatment and populations can rebound quickly once treatment stops. Mild winters will likely result in more abundant populations that spring.

Potential biological controls are available for Eurasian watermilfoil and we focused on assessing factors liming the milfoil weevil and other herbivores. We conducted enclosure experiments to assess the effect of sunfish predation on herbivore and milfoil abundance. Enclosures were placed in two lakes and stocked with 0, 5 and 20 sunfish. Weevil populations developed in the enclosures but there were no differences in weevil abundance or milfoil biomass due to fish stocking. We were unable to recover stocked fish from the enclosures and suspect that predation by herons removed the fish. We assessed herbivore abundance in metro lakes and found milfoil weevils in 12 of the 19 lakes surveyed. Abundance was higher in 2015 than 2016 but abundance both years was lower than some prior years. Milfoil weevil abundance was negatively correlated (r=-0.44) with sunfish abundance but only 1 weevil was found in over 450 sunfish stomachs examined. Further work accounting for environmental variability is needed to identify factors limiting milfoil herbivores.

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