killing, serve to encapsulate the toxin, thereby enhancing its persistence in the field. Some products using these strategies are currently commercially available, others are in the development stage. The success of Bt-toxin research raises optimism that other organisms are also potential sources of effective biological-control products.
Pathogens of diseases that afflict pests offer potential for use as biological controls. Unfortunately, the time lag between when the disease is introduced and when it becomes established in the pest population limits this method; crop damage during the incubation period often is too extensive to be economically acceptable. This is the problem with baculoviruses, which otherwise offer considerable potential for biological control because of their specificity and safety. Research to identify the initial steps of baculovirus pathogenesis can lead to the development of fast-acting baculoviruses. Other research is being directed at developing disabled ("suicide") recombinant baculoviruses which would eliminate only the arthropod that received the virus. Disabled viruses are attractive because there is only a remote possibility that a foreign gene introduced into the viral genome by recombinant DNA technology could enter the ecosystem. Research plans are also being developed to determine whether some microbial pathogens that have coevolved with arthropods could be manipulated to control specific arthropod pest populations.
A virus of the plant pathogenic fungus Cryphonectria parasitica is an important model to describe how naturally occurring or recombinant viruses can control fungal diseases of plants (Nuss, 1992). Work with this biological-control agent has shown the potential for use of viruses to control fungal diseases. Although use of viruses of fungal pathogens could result in a reduced virulence of the fungal pathogen population, much more information about viruses is needed before their potential for biological control can be exploited fully.
Perhaps the greatest potential for the use of diseases as biological controls is in weed management. Research is needed to enhance or conserve naturally occurring soil microbes that have weed suppressive abilities. Unfortunately weed management via microbial agents will not be viable until it is possible to (a) increase the speed and effectiveness of the control agent in the field and (b) ensure that the agent will not damage beneficial plants. Understanding the genetic basis of pathogenicity, virulence, and host-range of microbes that attack weeds could result in the selection and development of safer and more effective microbial weed-control agents.
To date, the major emphasis in genetic improvement of arthropod predators