Natural parasites and biological control

Abstract

Widespread concern about the hazards of pesticides to human health and the environment has led to considerable international efforts to reduce their use (Dinham, 1993; Zedan, 1994). In this regard, nematicides are no exception and this has stimulated the search for alternative approaches to nematode management (Thomason, 1987) of which biological control is one amongst others such as the development of resistant cultivars. The introduction of one organism to control another has a history going back to the 1920s and the first studies using natural enemies to control plant parasitic nematodes date from the mid-1930s (Linford, 1937; Linford, Yap and Oliveira, 1938). The research, since the mid-1930s, has been thoroughly reviewed by Stirling (1991). The results of using natural enemies to control nematodes has been inconsistent which has led many nematologists to be sceptical about their use to control plant parasitic nematodes; indeed, at present there are no reliable commercial products available. However, the increase and subsequent decline of cereal cyst nematodes in monocropped cereals reported between 1955 and 1968 by Gair, Mathias and Harvey (1969) has been attributed to the effects of two main species of nematophagous fungi Nematophthora gynophila and Verticillium chlamydosporium (Kerry, 1975). The evidence for the involvement of these fungi in suppressing nematode populations below the economic threshold comes from a number of studies in which: (1) H. avenue failed to increase where spores of V. chlamydosporium and N. gynophila were numerous (Crump and Kerry, 1981; Kerry, Crump and Mullen, 1982a), (2) the numbers of H. avenae were inversely related to the level of fungal parasitism (Kerry, Crump and Mullen, 1982b), (3) the reduction in the number of nematophagus fungi induced by the application of formalin resulted in an increase in nematode multiplication (Kerry, Crump and Mullen, 1980, 1982a, b; Crump and Kerry, 1981) and this effect was only observed in soils where V. chlamydosporium and N. gynophila were present and not where they were absent (Kerry, Crump and Mullen, 1980), and (4) watering only increased fungal parasitism and reduced the numbers of H. avenae in untreated soil and not in soil treated with formalin, where there was no effect on nematode numbers (Kerry, Crump and Mullen, 1980). These studies are the first example of density dependency in the biological control of plant pathogens but since then other examples of density dependence in the biological control of soil borne pests and diseases have been reported (Jaffee, 1993). Although suppressive soils have provided an insight and significant levels of nematode control in some soils, the use of microbial agents to control plant parasitic nematodes has produced highly variable results and the successful use of microbial agents will require a deep knowledge of the interrelationships between agent, pest and plant and the environmental factors which influence them (Kerry, 1993). The purpose of this chapter will therefore be to focus on the use of the natural parasites of cyst nematodes in nematode biomanagement and review the work that has accumulated with respect to these organisms since the publication of Stirling’s book the Biological Control of Plant Parasitic Nematodes (1991), and to examine the research strategies available to understand the nematode/hyperparasite interactions at the population and molecular level as a way forward.