Broadly defined, biological control is the use of one species for the suppression of another. Two groups of organisms have potential as Dreissena control agents: selectively toxic microbes and natural enemies. Selectively toxic microbes are naturally occurring soil and water microorganisms that simply by chance happen to be toxic to Dreissena. Applied at artificially high densities to water, the microbial cells per se or their released metabolites are selectively lethal to Dreissena. In contrast, natural enemies are organisms that, by their evolutionary nature, will debilitate or kill Dreissena, including predators, parasites (both multicellular and

microbial), and benthic competitors (organisms capable of competitively displacing Dreissena from substrates).

Selectively Toxic Microbes: In the short run, these microorganisms represent the most promising candidates as biological control agents, The use of highly-specific, toxic microbes has a clear record of commercial success and environmental safety in the control of invertebrate pests in North America. and strains lethal to Dreissena have been already isolated. Considering how quickly a selectively toxic microbe could move from the laboratory to commercialization, continued research to develop this microbial control strategy is a high priority.

Natural Enemies; In North America, as in Eurasia, there will likely be isolated field reports of major impacts by natural enemies, and, on the whole, we will likely see a cumulative effect of a complex of enemies having a constant, but limited, role in naturally suppressing Dreissena populations. In the majority of cases, Dreissena populations will cause economic and ecological effects at densities well below those that their enemies could naturally maintain. This does not mean, however. that certain natural enemies could not be artificially manipulated (e.g., mass produced and released in an inundative fashion) to cause major Dreissena reductions.

1. Parasites: These natural enemies would likely be the most environmentally-safe biocontrol agents, due primarily to their high host specificity. Comprehensive investigations to identify parasites that are strictly specific to Dreissena and that can be economically mass produced are a key research priority. Parasites from Dreissena’s native Eurasian range should be studied to identify species that would be “nearly risk free” candidates for importation into North America. Research to uncover parasites of Dreissena that are already present in North America should be intensified.

2. Predators: Although organisms, such as fish and birds, have sometimes been documented to consume Dreissena at high rates, this does not necessarily mean that they would be effective candidates for use in control programs. Predators are typically not specific enough in their prey choices. A predator introduced to a waterbody from outside its natural range may highly prefer Dreissena as a prey item, but will also consume other aquatic organisms. The consumption of such nontarget prey could potentially have serious, adverse, ecological impacts.

3. Benthic Competitors; In addition to being micro/macrofouling organisms themselves, the lack of specificity of species that can competitively displace Dreissena from substrates (e.g., sponges) significantly reduces their potential as biocontrol agents.