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Selecting biological control agents

Factors influencing host selection in the target range

Fundamental vs ecological host range

Fundamental host range of a natural enemy is defined as the sum of all species on which the agent can complete its life cycle, and the sole barrier to host use is whether the host stage is an acceptable substrate for development. This might more accurately be described as the taxonomic rather than the phylogenetic host range, because nomenclature does not always reflect phylogeny.

The ecological or realised host range is the subset of the fundamental host range that can be used by the natural enemy under natural conditions where there are additional, ecological barriers to host use. Not only must the host be suitable to support development of the immature control agent, but females must be able to locate a susceptible host and lay fertile eggs there. Some of the ecological and behavioural barriers that must be overcome to ensure this include host-acceptance cues and host-finding, niche-finding, habitat preference and life cycle synchrony. These factors all affect host use and should be considered in assessing the risk of non-target attack.

For biological control of weeds it is generally accepted that simplistic small arena tests estimate fundamental host range well but overestimate the ecological host range because host acceptance/rejection mechanisms are compromised (Cullen 1989). The behaviours leading to host acceptance or rejection in parasitoids are often sequential and more complex, and often are not independent of the substrate and prior experience of the agent. Small arena tests may therefore overestimate or underestimate the host range depending on the parasitoid/host system under study. Vinson (1998) summarised the mechanisms leading to successful parasitism as:


Cullen J.M. (1989). Current problems in host-specificity screening. Pp. 27-36 In: Proceedings of the VII International Symposium on Biological Control of Weeds, E.S. Delfosse (Ed.) CSIRO Publications, Melbourne.

Vinson S.B. (1998). The general host selection behavior of parasitoid Hymenoptera and a comparison of initial strategies utilized by larvaphagous and oophagous species. Biological Control 11: 79-96