Mussel life cycle and behavior influence the strategies and tactics of chemical control, as well as choice of molluscicidal compound. Zebra mussels cannot survive in saline conditions but are well adapted to water temperatures (12 ºC to 32 ºC (55 ºF to 90 ºF)), pH range (6.5 to >8), and turbidity levels that can be found in the Great Lakes and many U.S. riverine environments (Claudi and Mackie 1994; Figure 1). Spawning occurs in spring when water temperatures rise above 12 ºC and can continue into October. Females release up to 30,000 planktonic (free-swimming) larvae, called veligers, which move with water currents and grow up to 1.3 cm (0.5 in.) in the first half year. These settle in colonies and attach to firm surfaces by means of secreted strands called byssal threads. Densities can reach 500,000 per square meter (46,500 or more per square foot), and individual life spans are 3 to 5 years. Zebra mussels are filter feeders, opening their shells to allow ingestion of particulates. When their sensitive chemoreceptors alert them to certain toxins in the environment, they have the ability to maintain shell closure for up to 2 weeks and thereby remain immune to certain biocide contact. Not all molluscicides evoke this response, however.
The application of chemical molluscicides in the field is limited by several considerations. Firstly, a method must be judged by how well it removes or kills the various life stages of the zebra mussel. Secondly, any chemical control method used must not be harmful to natural fisheries and aquatic ecosystems and must also be eventually compatible with possible potable water use. Thus, flow-through systems may require a different suite of chemicals than is possible in static or closed systems where there is no release to the environment. Since chemical control is most suitable for application to problems in closed systems and internal piping, it is much less effective in treatment of external surfaces where it may be impossible to maintain required treatment concentrations and contact times of the compound. Thus, current chemical options are not available for treating and reducing densities of zebra mussels in source waters, such as lakes, rivers, and streams. In these areas nonchemical methods are more suitable. Finally, use of the material must be cost-effective.
Chemicals identified for zebra mussel control have been derived mainly from water treatment compounds and antifouling biocides and biodispersants. Chlorine has been used for nearly a hundred years in drinking water disinfection, where its properties and behavior in effluent are well known, and it has been the primary chemical for zebra mussel control in Europe. In contrast, molluscicidal properties have been associated only recently with endothall, a compound used for several decades as an aquatic herbicide. Investigation of toxicity to both the target and nontarget organisms in the aquatic environment is the first step in the ongoing
Figure 1. Zebra mussel distribution (from Dreissena!, Vol 9(3), Summer 1998, 8-9, courtesy of New York Sea Grant) (for a current version of the map please see: http://www.cce.cornell.edu/seagrant/nansc/zmaps.htg)
effort to identify more compounds that will be effective against zebra mussel. While oxidizers, and particularly the various forms of chlorine, continue to be the most commonly used of the chemical controls, additional compounds have been registered; and more continue to be tested in the search for environmentally sound and effective treatment of this pest.
Chemical applications can be used for both proactive treatment, to ward off settlement of zebra mussels and subsequent fouling before they occur, and for reactive treatment, where clean-up measures are used to remove zebra mussels already at nuisance levels and disrupting system function. It has been suggested that reactive systems or procedures are adequate if 1 year’s worth of shell buildup and fouling can be tolerated by the system, allowing for the minimum of an annual purging (Claudi and Mackie 1994). Both oxidizing and nonoxidizing chemicals are suitable for this type of application. Where macrofouling buildup or the “legacy” problems of disposal of dead mussels and shells (Allen 1994) cannot be tolerated, however, proactive treatments of nonoxidizing chemical are more commonly used. These create environments hostile to the settlement stage of the zebra mussel larvae (the veliger) and maintain inviable conditions that prevent adult zebra mussel translocation and settlement. Both approaches can be combined into a single strategy.
The goal of any chemical control program is to choose chemicals that will be effective, work rapidly, and have a minimal environmental impact. Treatment chemicals can be categorized as oxidizing (electron acceptor) and nonoxidizing compounds, with different properties and requirements. Since these groupings also generally differentiate between nonproprietary versus proprietary and organic versus inorganic compounds, they are followed in this guide to describe the chemistry of molluscicide compounds and give directions for the use of each compound. Further guidance for designing a control program using chemical molluscicides for a facility or installation is given by Claudi and Mackie (1994). They provide a detailed description of chlorination strategies and outline criteria for effective chemical application in general.