Thanh-An Annie Voqui and Michael Brett, Department of Civil and Environmental Engineering, University of Washington
Aquatic invasive species like dreissenid mussels (Dreissena spp.), New Zealand mud snails (Potamopyrgus antipodarum), Eurasian watermilfoil (Myriophyllum spicatum), and the benthic diatom didymo or “rocksnot” (Didymosphenia geminate) can spread very rapidly. These invaders can harm the ecological functioning of Washington’s lakes and streams by competing with native species for resources and altering habitats. They can also damage civil and industrial infrastructure, in some cases costing many millions of dollars. Because controlling these species is nearly impossible once they have invaded an aquatic ecosystem, the most effective control strategy is to prevent their spread to new systems.
Invasives are often transported from one waterway to another by clinging to recreational equipment — boats and boat trailers, boots, waders, and fishing gear. We did this study to determine the most efficient method for cleaning recreational and scientific equipment to prevent these (and likely other) invasive species from spreading in this way. Methods must also be fast and relatively inexpensive to be most useful.
To determine the best methods, we studied the invasive species control literature as well as web pages for biological control agencies in New Zealand, Australia, and Canada, and New York, Missouri, Wisconsin, California and Washington. Below we summarize the control methods that have killed these organisms cost-effectively. (This report is a condensed version of Voqui (2012).)
Several commonly used disinfection methods — desiccation, freezing, soaking in salt water, and pressure washing — were only partially effective. On the other hand, soaking in hot (60 °C) water for at least 10 minutes was 100% effective for all the species tested. Combining hot water immersion with 1% solutions of commercial detergents appears to be a very cost-effective way to control the spread of many invasive aquatic biota.
Who are these invaders?
Showing how didymo (“rocksnot”) can overgrow aquatic environments under the right circumstances, this is a pile of didymo created by five people in five minutes in New Zealand (photo credit Haldre Rogers).
Zebra and quagga mussels are dime-sized mollusks that have already caused major damage to aquatic ecosystems and infrastructure throughout the American Midwest. Dreissenid mussels have not yet invaded the Pacific Northwest, but it is likely that they will: contaminated watercraft are detected coming into the Pacific Northwest region every year. Dreissenid mussels latch onto any hard surface they encounter and can survive out of water for a week or more, making it very easy for them to spread from one system to another. Once they invade an aquatic system, these mussels can build up very dense populations which can effectively cover all hard and even some soft surfaces. They are also aggressive feeders, and can out-compete other mollusks and invertebrates for food (mainly phytoplankton).
The feeding and nutrient excretion of dreissenids favor undesirable cyanobacteria and restructure aquatic food webs towards benthic production. Dreissenids foul boat hulls and docks, and can clog pipes and filters of drinking water treatment and water distribution systems.
New Zealand mud snails are very small (1-5 mm), reproduce very quickly and can establish populations of many thousands of individuals per square meter. Mud snails can out-compete other snails and insects for food, thus disrupting the food web for more desirable species, such as recreational and commercially important fishes. Mud snails can tolerate near-freezing temperatures and drying and thus can be transported very easily. New Zealand mud snails have already invaded Washington, but are currently found in only a small number of lakes and streams.
Eurasian watermilfoil is an aquatic plant that forms dense mats in lakes and streams, where it can cause oxygen deprivation by preventing wind from mixing oxygen into the water. These dense mats also greatly hinder boating, water skiing, swimming and fishing, and can clog pipes in irrigation and water distribution systems. Milfoil fragments easily and the fragments can survive several days out of water to start new populations in other aquatic systems. Eurasian milfoil is already widespread in Washington, but its spread within the state should be controlled wherever possible.
“Rocksnot” or didymo is a benthic diatom that can form thick, felt-like mats on stream bottoms and in the rocky shallows of lakes. Didymo occurs most commonly in nutrient-poor systems with stable flows. In more nutrient-rich systems, didymo gets overgrown by other benthic algae, and in flashy streams is easily sloughed off. Didymo mats are 2-3 cm thick and can suffocate invertebrates and overgrow other plants, which limits the amount of food available to invertebrates and fish. In streams invaded by didymo, the benthic invertebrate communities shift from dominance by mayflies, caddisflies and stoneflies to dominance by midge larvae (chironomids). Didymo can also clog hydropower and water distribution systems. Didymo occurs naturally throughout the western US, and is only a minor component of the benthic algae community in most systems. However, in New Zealand and parts of British Columbia, aggressive forms of didymo have completely overrun many streams and lakes. If the aggressive form of didymo were to invade Washington, it could severely impact recreational activities, hydropower generation and drinking water and irrigation infrastructure.
What treatments have been tried and to what effect?
Dreissenid mussels: Various studies have tested salt solutions, freezing, chlorine, detergents and various pesticides to control the spread of zebra and quagga mussels. Beyer et al. (2011) found immersion for five minutes at 43 °C resulted in 100% mortality. A study by McMahon et al. (1993) also found that freezing at -10 °C for 2 hours resulted in 100% mortality. Conversely, salt treatments up to 45 ppt did not result in 100% mortality even with very long exposure times. Chlorine, detergent or pesticide treatments only resulted in 100% mortality when very high concentrations and/or very long contact times were used.
New Zealand mud snails: Decontamination studies for New Zealand mud snails have included heat treatment, detergents, salt solutions, desiccation, and freezing. Schisler et al. (2008) found heat treatment at 45 °C for 10 minutes resulted in 100% mortality. These authors also found 50 to 100% concentrations of various detergents resulted in 100% mortality within 10 minutes, and treatment with 5% Sparquat® also resulted in 100% mortality after 10 minutes. New Zealand mud snails have wide salinity tolerances so salt treatments were not effective. Desiccation and freezing were only partially effective.
Eurasian milfoil: Hot water and pressure washing treatments have been tested as milfoil control measures. Blumer et al. (2009) found that 100% of milfoil was killed after 10 minutes of exposure at 60 °C. Pressure washing was not entirely effective at removing milfoil for any of the time frames tested.
Didymosphenia: High and low temperatures, salinity and various detergents have been tested for didymo disinfection. Kilroy et al. (2007) found a heat treatment at 45 °C for 20 minutes killed 100% of didymo. Cold treatment at -15 °C resulted in 100% mortality after several hours. Similarly, a 100% saltwater solution killed 100% of didymo after four hours. The algaecide 303 Clearall killed 100% of didymo after only one minute of exposure when using a 1.5% solution. Several detergents also resulted in 100% mortality after only a few minutes of exposure to 5% solutions.
Sources:
Beyer, Jessica; Moy, Phillip; De Stasio, Bart. 2011. “Acute Upper Thermal Limits of Three Aquatic Invasive Invertebrates: Hot Water Treatment to Prevent Upstream Transport on Invasive Species.” Environmental Management 47: 67-76.
Blumer, David L.; Newman, R. M.; Gleason, F. K. 2009. “Can Hot Water Be Used to Kill Eurasian Watermilfoil?” Journal of Aquatic Plant Management 47: 122-127.
Kilroy, Cathy; Lagerstedt, Amy; Davey, Andrew; Robinson, Karen. 2007. “Studies on the survivability of the invasive diatom Didymosphenia geminate under a range of environmental and chemical conditions Prepared for Biosecurity New Zealand.” NIWA Client Report CHC2006-116.
McMahon, Robert F.; Ussery, Thomas A.; Clarke, Michael. 1993. “Use of Emersion as a Zebra Mussel Control Method Prepared for Headquarters, U.S. Army Corps of Engineers.” Zebra Mussel Research Program.
Schisler, George J.; Vieira, Nicole K.; Walker, Peter G. 2008. “Application of Household Disinfectants to Control New Zealand Mudsnails.”North American Journal of Fisheries Management 28: 1172-1176.
Voqui, Thanh-An Annie. 2012. “Preventing the Spread of Invasive Species.” Independent Study Report in partial fulfillment of a MSCE degree in the Department of Civil and Environmental Engineering, University of Washington. Dec. 13th, 2012.
