Natural enemies are organisms that debilitate or kill Dreissena, including predators, parasites (both multicellular and microbial), and benthic competitors (i.e., organisms capable of competitively displacing Dreissena from substrates).
In their review of the international literature on natural enemies of Dreissena, Molloy et al. (1997) cited 176 species involved in predation, 34 in parasitism, and 10 in competitive exclusion of Dreissena. The vast majority of species that are natural enemies in Eurasia are not present in North America, but ecologically similar organisms do exist, particularly among predators (Molloy et al. 1997). Dreissena is a novel and abundant organism for these North American species, and they have become the new natural enemies of Dreissena. The idea that these organisms might eliminate zebra mussel populations, even in limited areas of North America, however, is far more hopeful than realistic.
As zebra mussel populations attempt to expand in density and biomass within North American water bodies, natural enemies, particularly molluscivores, undoubtedly are exerting a suppressive influence, but to what extent is unclear. Although seasonal and localized reductions of Dreissena densities by natural enemies have been documented in both North America and Europe (Molloy et al. 1997), the high recruitment rate typical of Dreissena populations inherently makes them very difficult for natural enemies to control over the long term. One theory holds that Dreissena in North America, being a nonindigenous pest, quickly reached high population densities in the Great Lakes and elsewhere primarily due to the absence of an established natural enemy complex. The ability of natural enemies to regulate prey populations, however, depends upon the prey's rate of increase. If the net rate of increase is too great, then the prey population can escape control. i.e., no matter what functional and numerical responses are mounted by natural enemies, they simply cannot keep up with prey reproduction (Crawley 1992b). Molloy et al. (1997) therefore, concluded that in North America, as in Eurasia, there will likely be isolated reports of major impacts by natural enemies, and on the whole, a cumulative effect of a complex of enemies having a constant, but limited, role in suppressing Dreissena populations is likely.
For information on the potential use of natural enemies for the biological control of zebra mussels, see the section titled Natural Enemies as Biocontrol Agents.