The Zebra mussel

In the late 1980s, another important organism began to create problems in North America. The Zebra mussel, Dreseina polymorpha, appeared in the Great Lakes basin. Native to the Caspian Sea of Europe, where it has been a nuisance for many years, the Zebra mussel appears destined to be a major problem throughout North America.

Like the marine blue mussel and the Asiatic clam, Zebra mussels reproduce by releasing mobile larvae, which are so small they can pass through most screens or traps. In order to settle, they produce sticky byssal threads to anchor onto virtually any hard surface. Once attached, the mussels grow rapidly to a mature size of up to 1.5 inches. Their reproductive cycle is rapid and prolific, with each female capable of releasing 30,000-40,000 larvae per year. This, coupled to their habit of clinging to one another in thick mats or clumps, can rapidly lead to exceptionally dense populations. Up to 500,000 organisms per square meter have been reported, and they can attach to virtually any surface, including rocks, screens, piping, boat hulls and buoys.

The Zebra mussel prefers cooler water than the Asiatic clam (optimum range is 20°C to 25°C). They are susceptible to death by warm or hot water. As filter feeders, the continuous movement of water which they find in industrial plants, utility industrial or drinking water plant intake structures provides an ideal growth environment since oxygen and food are continuously replenished and wastes removed. In their typical four-year life span, they can grow up to two inches in length and produce up to 150,000 offspring.

Control methods for Zebra mussels appear to be similar to those of other bivalves. Continuous chlorination, the use of activated bromide salts or chlorine dioxide all appear to be effective. A few nonoxidizing biocides also appear to be effective, primarily because mussels cannot detect the presence of cationic biocides such as quaternary ammonium compounds. These interfere with oxygen transport in the gills of mussels leading to death.

A recently conducted test to evaluate chlorine dioxide as an agent to control zebra mussels indicated very positive results. Chlorine dioxide residuals ranging from 0.25 to 5.0 ppm produced one hundred percent mortality in 2.9 to 8.8 days with a mean water temperature of 14.3°C. The control mussels exhibited no mortality during the test period. The most efficient zebra mussel toxicity level was determined to be 1.0 ppm C1O2 residual for 102 hours to produce one hundred percent mortality.

Subsequent testing indicated that water temperature is an important variable. At a water temperature of 25.8°C, the most efficient zebra mussel toxicity level was determined to be 0.5 ppm C1O2 residual for 40 hours to produce one hundred percent mortality. This information is very useful in planning on-site applications. Actual field experience has verified these results and established chlorine dioxide as a cost-effective and easy-to-use molluscicidal agent. The use of chlorine dioxide for this application must be approved by all local regulatory authorities and may require deactivation with a reducing agent.