Selectively Toxic Microbes as Biocontrol Agents

 

These are not parasites of Dreissena, but rather naturally occurring soil and water microbes which simply happen to be toxic to Dreissena. Applied at artificially high densities to water, these microbial cells or their released metabolites are selectively lethal to Dreissena. Commercial products based on such toxic microbes have the potential to be effective irrespective of whether they contain dead or live cells (Gaertner et al. 1993).

 

Why would one look to naturally occurring, nonparasitic microbes as a novel strategy for Dreissena management? It is widely accepted that screening of the diverse biochemicals found in tropical plant species is a worthwhile activity due to the discovery of drugs that can prevent or cure animal diseases, particularly cancers. Production of these biochemicals, however, did not evolve in these plants for this purpose, and the effect of these plant substances on animal diseases is purely coincidental. Using the same logic, we can also look to microorganisms for unique biochemicals that have potential as highly selective pesticides. Selectively toxic microbes have a clear record of commercial success and environmental safety in the control of invertebrate pests in North America, as well as globally. Sales of the selectively toxic bacterium Bacillusthuringiensis for insect control represent over 90-percent of the current international biocontrol market (Rodgers 1993). Individual subspecies of B. thuringiensis are highly specific to targeted pests. One subspecies, B. thuringiensisisraelensis (Bti), has become the most environmentally safe and effective method ever developed for the control of two aquatic invertebrate pests black flies (Simuliidae) and mosquitoes (Culicidae) (Molloy 1990, Entwistle et al. 1993). The toxic activity of Bti to black fly and mosquito larvae is due to particulate, proteinaceous crystals that are formed during bacterial sporulation. Bti, although a highly lethal bacterium to larval black flies and mosquitoes, does not kill them in nature since it is very rare. Applied at artificially high densities to waters in which these fly larvae live, however, these filter-feeding invertebrates concentrate the bacterial protein particles from the water column and typically die within 24 hr. Like chemical pesticides, Bti and other selectively toxic microbes give only short-term control and require periodic reapplication. In stark contrast to chemical pesticides, however, selectively toxic microbes pose no problems with biomagnification of any toxic residues along the food chain and are significantly more difficult for pests to develop resistance to.

 

In North American black fly control programs, chemical pesticides have been essentially replaced by Bti, as for example, in the Adirondack Mountains of New York State, where Bti is annually applied to streams and rivers in over 3,000 km2. The State of Pennsylvania has the largest North American Bti-black fly control program, with an annual operational budget of ca. $4 million (D. Arbogast, personal communication). Both Dreissena and larval black flies are lotic, filter-feeding invertebrates, and thus, the protocols for the control of Dreissena with selectively toxic microbes could someday be adapted from those currently used for black fly control with Bti.

 

Unreliable and inconsistent performance of biological control agents has often resulted from insufficient knowledge of the parameters affecting their performance, including both environmental (e.g., pH, temperature, etc.) and ecological (e.g., relative susceptibility of pest life stages) variables. In this regard, the relative insensitivity of toxic-microbe preparations to these variables has been one of the keys to their commercial success (Rodgers 1993). Bti applications, for example, are capable of killing black flies of all larval instars and at all water temperatures and pH values.

 

Use of selectively toxic microbes is a small, but growing, portion of the pest control business. Sales of biocontrol agents (primarily selectively toxic microbes) account for about 1-percent ($120 million) of all global pesticide sales (Powell 1993), but are annually increasing at a rate of 10-25-percent while the world chemical pesticide market is static to shrinking (Rodgers 1993). One major reason is the relatively high expense of a chemical pesticides research and development. Due primarily to the costs of nontarget safety testing, the average budget of bringing a single chemical pesticide to market is about $20-40 million (Marrone and MacIntosh 1993) versus $2 million (Rodgers 1993) for a biocontrol agent. Thus, the relatively low cost of biocontrol research and development is a plus for commercial development.

 

Shortly after the discovery of Dreissena in North America, it was hypothesized that microorganisms existed in nature that could be selectively toxic to this new bivalve invader (Molloy 1991). Over the last 8 years, several North American laboratories have been actively involved in the screening of hundreds of microorganisms (primarily bacteria) to find strains useful for Dreissena control. Gu et al. (1997) have explored the incorporation of selectively toxic bacterial metabolites into coatings to deter attachment. Other North American scientists have focused more on applying such bacteria directly to water for Dreissena control (Molloy and Griffin 1992, Singer et al. 1997, Genthner et al. 1997). In this regard, recent trials with bacterial strain CL0145A, an isolate from a North American river, have been particularly encouraging (author, unpublished data). Treatment of Dreissena with strain CL0145A routinely achieved 80- to 100-percent kill in the laboratory, yet without any mortality to unionid mussels. To test for field effectiveness, a small-scale trial was conducted under once-through conditions within a hydroelectric station on the Mohawk River in New York State. This trial achieved a 94-percent kill and represented the first successful demonstration of the feasibility of using a biological agent for Dreissena control within a raw-water-dependent infrastructure. As a result of these successful laboratory and field trials, a patent application for the use of strain CL0145A has been filed. Because of the commercial success of using selectively toxic microbes for the environmentally safe control of other invertebrates, the potential of this approach for Dreissena control should not be underestimated.

 

Management and Control Contents

Management and Control Options

Biological Control Possibilities