At present, essentially all of the available information relating to use of mermithids for the control of insects has been generated from studies with R. culicivorax. Thus, R. culicivorax serves as a model system for other aquatic mermithid species as their mass production systems are developed. Studies with this species have shown the potential for either inundative control systems, that is, the release of infective stages in sufficient numbers to give immediate control of mosquito populations, or inoculative control, that is, the introduction of the postparasitic stage or reduced numbers in the preparasitic stage to establish the nematode population in the environment to give partial control for an indefinite period. Studies with R. culicivorax have also demonstrated the limitations associated with the use of agents of this type. As living organisms, a knowledge of limiting physical and chemical factors is essential if the parasite is to be effectively used. Without this knowledge potentially effective agents may be discarded because they were used in environments outside their limits of tolerance.
One attractive characteristic of mermithids is their safety for nontarget organisms, but, because of this, it is important to know the host specificity of the parasite and to be sure that the intended host system is compatible with the parasite. Mermithids may be species specific (i.e., P. c-ulicis), generically specific (i.e., S. peterseni), generally family specific (i.e., 0. muspratti, R. culicivorax) or may have a broad host range over several orders (i.e., M. nigrescens). However, as with R. (- ulicivorax which parasitizes many mosquito species, the host species still vary greatly in their susceptibility. Studies with R. culicivorax have also demonstrated the importance of timing of application, and that it is essential that the parasite-host systems be synchronized for effective control.
Hominick and Tingley (1984) discussed the theoretical problems associated with (1) density-dependent parasite reproduction (i.e, sex ratios, fecundity, nutritional stress, and postparasite densities), (2) distribution of parasite numbers per host, (3) density of host populations, (4) pathogenicity of the parasites, (5) reproduction of the parasites and their hosts, and (6) predation. They concluded that mermithid populations are controlled by such tight density-dependent constraints that they can cause at most only moderate depressions of their host populations, and that mermithids are unlikely to be useful in biological control programs aimed at providing effective long-term control of hosts with single introduction of the parasite. These works clearly point out the complexity of these biological control systems and the need for extensive background data for each parasite species in order to develop effective control strategies.
The key to success with any biological control agent lies first with sufficient material for study and field testing, and second with availability of quantities of the agent to permit host control beyond the research stage.
Conversely, many of the desired characteristics of these agents make them unattractive for commercial development. For example, host specificity, a characteristic that gives mermithids a unique advantage over chemical pesticides, also limts their usefulness, to at most, only a few target insects. This means that the total product volume sold per year will be considerably less than would be sold if the product were applicable to a range of pests. Also, other characteristics such as the potential for establishment and recycling further threaten to reduce the potential market for a mermithid product in the eyes of prospective producers. In addition, the problems of environmental limitations on the mermithid further reduce the potential effective use of a given mermithid species.
Additional factors make production of mermithids a risky commercial enterprise. First, mermithids are living organisms. To keep them alive, one must maintain favorable environmental conditions. Shipping and storage of biological agents are major problems for any producer. Second, the people who will use mermithids must be trained and develop some degree of skill in application. Biologicals cannot be frozen, cooked, ground to pieces, or stored on a shelf for years (Petersen and Cupello, 1981).
These problems are academic if economical methods of production cannot be developed for desired mermithid species. At present only R. culicivorax can be produced economically. In the past, commercial production of R. culicivorax was undertaken by two companies. The first company entered the market place before serious problems with handling and shipping had been adequately studied and they were forced to stop production for economic reasons. A second and larger company became interested in R. culicivorax because of its control potential and ease of production, and because it was exempt from costly regulatory registration. The company allocated 2 years of effort (6 person years) for development of this nematode into a commercial product. They developed efficient and less costly in uiuo rearing techniques and developed a shipping container that would keep the nematode eggs viable during transportation and storage (Cuppello et al., 1982). Just prior to test marketing of the new product the company changed direction and discontinued all marketing studies on this and other biological agents. Although this work established that a marketable product could be developed and that R. culicivorax had market potential, the limited market evidently puts production economics outside the interests of large manufacturers. Thus, the future for these types of biological control agents probably lies with small corporations or partnerships or with government-subsidized production systems.
If and when these mermithids are successfully brought to the state of the art presently held by R. culicivorax, they will undoubtedly confront the same road blocks to commercialization encountered with R. culicivorax. The characteristics of the true biologicals make them unattractive to commercial producers; coupled with the previously mentioned environmental limitations, special training needed by users and the lack of patent protections associated with these agents will continue to discourage the research and development needed to put these agents on the market.
As things now stand, we- still awaii the success of the first truly effective commercially available product that will demonstrate the feasibility of mermithids as biological control agents for insect control. It appears, that as mermithids are developed to the stage presently held by R. culicivorax, they will be placed "on the shelf " until such time as the need for such biological control provides small business with the incentive to make them available for general use.