Introduction
In July 2021, McCloud Aquatics was chosen the preferred applicator to treat and monitor a zebra mussel (Dreissena polymorpha) population in a private, 26-acre lake in Glenview, IL. Zebra mussels are small, highly invasive mollusks that are native to Eurasia. D. polymorpha outcompete native mussels by filtering out algae that native species require for food, and they also incapacitate native mussels by attaching to them. (USGS)
Methods
400 zebra mussels were collected on July 12, 2021 from the targeted private lake in Glenview, IL. A total of 20 zebra mussel cages were placed throughout the water column of the lake in 10 different locations, with 2 cages at different depths for each location: one closer to the substrate, and one approximately a foot below the surface. 20 mussels were placed in each cage. Cages were constructed using 4” corrugated tubing and screen, and anchored with bricks dropped into the sediment, and held in the water column by foam floats. Locations were chosen to be distributed throughout the lake relatively evenly with different depths in an attempt to be representative of the entire system. (See fig. 1)
Application of EarthTec QZ was done by licensed pesticide applicators utilizing an on-boat spray system with drop hoses. The scheduled applications were as follows:
Figure 1 Locations of mussel cages and their relative depths
Application 1: 550 gallons of EarthTec QZ, 240 μg/L as copper
Application 2: 275 gallons, 120 μg/L as copper
Application 3: 550 gallons, 240 μg/L as copper
Application 4: 550 gallons, 240 μg/L as copper
Total: 840 μg/L as copper (0.84 mg/L as copper)
Prior to continuing applications, copper concentrations were recorded and mortality was observed to determine need for applications.
Results
Three days after the first application of 550 gallons, copper concentrations and zebra mussel mortality in the cages were observed. Beginning with 400 live zebra mussels, after the first application only 40 survived, located in the two deepest sections of the lake. Due to timing of last treatment and remaining copper concentration exceeding lethal levels of >100ppb Cu, no treatment was performed.
The next visit showed mortality of 99.7% and an average copper concentration of 180-190 ppb, so the decision to not add additional product was made. By our final visit in early August, 100% mortality was achieved, with only the first application of EarthTec QZ having been performed.
Discussion
Initial calculated application dosing was provided by the manufacturer of EarthTec QZ. We suspect copper concentrations remained high within the lake due to there being little outflow, allowing the product to remain within the intended treatment area. Some considerations were made regarding the two cages that showed no mortality at the first follow-up visit, as they had a depth of approximately 20ft, compared to the deepest of the other cages which were closer to 10ft. This could potentially be due to the buoyancy of the product and it needing more time for copper to sink to deeper depths.
Overall, we found EarthTec QZ a successful tool in treating zebra mussels in a closed system.
References
What are Zebra Mussels and Why Should We Care About Them? (n.d.). USGS.GOV. https://www.usgs.gov/faqs/what-are-zebra-mussels-and-why-should-we-care-about-them?qt- news_science_products=0#qt-news_science_products