Virulence Management in Biocontrol Agents

Abstract

Although biological control is founded upon the virulence of natural enemies to the targeted pests, there has been little effort to understand how this might change, let alone to manage it. Frank Fenner and colleagues can be credited with being the first (and last!) to monitor changes in virulence of a biological control agent, namely the myxoma virus used to control rabbits in Australia (Fenner and Fantini 1999). This is despite a body of literature showing that the virulence of natural enemies can and does change in response to selective forces, either natural or artificial. These studies cover a wide taxonomic range of organisms, including fungal pathogens of plants (Burdon and Thrall 1999; Brasier et al. 1999; see also Chapter 31), microsporidian parasites of daphnids (Ebert 1994), pathogens and parasites of humans (Ewald 1994; Chapters 2 and 28), malarial parasites of rodents (Chapter 12), pathogens and parasites of social Hymenoptera (Schmid-Hempel 1998; Boot et al. 1999; Oldroyd 1999), nematode parasites of fig wasps and fruit flies (Herre 1993, 1995; Jaenike 1996, 1998), and hymenopteran parasitoids of aphids (Henter 1995; Henter and Via 1995). The results of this work have suggested that the course of virulence change can be predicted and possibly even manipulated. In biocontrol, a predator, parasitoid, or pathogen is used to control a pest population, a pest being defined as an animal, plant, or microorganism that is perceived to be damaging to some human activity. In this setting, the practical aim of virulence management would usually be to increase virulence. This contrasts with the management of virulence of pathogens or parasites of humans, domestic animals, or crop plants, in which low virulence is the aim. So how does one attempt to manage the virulence of a biocontrol agent? Theory predicts that increased virulence is selected for when a natural enemy is limited in its control over the resources contained within a victim. This occurs through competition with other natural enemies (including other strains of the same species) or through external sources of victim mortality (Van Baalen and Sabelis 1995a, 1995b; see Chapters 7, 9, 11, and 22). Both of these factors are potentially manipulable and in this chapter we present a framework in which this might be attempted. The field of biological control employs a range of natural enemies to control pest organisms. Workers using pathogens as biocontrol agents have traditionally recognized virulence (commonly termed “aggressiveness” in plant pathology, see, e.g., Jarosz and Davelos 1995) as an important attribute to assess, but we hope to