Predators, parasites, and benthic competitors are well-documented for Dreissena. 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. As Dreissena increases its geographical range in North America, the number of species that can be listed as natural enemies will constantly increase as organisms make their initial contact with Dreissena; this will be particularly true for molluscivores.
Natural enemies in North America, particularly predators, undoubtedly are suppressing Dreissena population densities, but to what extent is unclear. While seasonal and localized reductions of Dreissena densities by natural enemies have been documented, 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, 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. 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) concluded that in North America, as in Eurasia, there will likely be isolated 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 suppressing Dreissena populations.
The classical biological control approach involves the release of a nonindigenous pest's imported natural enemy and assumes that this natural enemy will proliferate on its own and, in the long term, lower pest densities to levels below economic and/or ecological thresholds (Debach and Rosen 1991). Such a classical approach is not likely to work for the control of Dreissena in North America. This approach certainly has a successful track record for other nonindigenous pests (both animals and plants) -- specifically those that have little to no adverse ecological or economic impact in their native range since their densities there are kept "low" by their natural enemy complex. Evidence to date suggests that this is not the case with Dreissena spp. in their Eurasian range. Due to the relative lack of "complaints" from overseas raw-water users about Dreissena problems, there is a misconception among North Americans that Dreissena densities are naturally "low" in Eurasia. This is not the case. Complaints about Dreissena are infrequent in Eurasia because raw-water users have had over a century to find solutions to these macrofouling bivalves. In reality, Dreissena population dynamics, densities, and biomass in Eurasian water bodies are far more similar to those in North American waters than generally thought. Like in North America, when introduced into European freshwaters, it is not unusual for Dreissena to become enormously abundant in a short time, obtaining a biomass 10 times greater than that of all other native benthic invertebrates (Karatayev et al. 1997). Thus, in the vast majority of Eurasian water bodies, it does not appear that Dreissena's natural enemies keep their densities low enough to avoid ecological or industrial problems. Thus, until a natural enemy can be identified from Eurasia which naturally suppresses Dreissena populations to levels below economic or ecological impact thresholds, it would not be worthwhile to attempt a classical biocontrol release in North America. The question remains, however, as to whether certain natural enemies could, both in Europe and North America, be artificiallymanipulated to give satisfactory control of Dreissena.
Management and Control Contents
Management and Control Options
Biological Control Possibilities
Parasites as Biocontrol Agents