Are all biological control agents equally capable of damaging their host plant? Intuitively we would answer no, because, for example, a stem borer would surely be more damaging than a leaf miner. However, if the stem borer only feeds in the pith it may do little damage, while the leaf miner may have a significant affect on photosynthesis and may even cause leaf abscission. Harley and Forno (1992) suggest that for a biological control agent to inflict "critical damage" it must either:

Briese (1993) suggests biological workers should give consideration to the biology and ecology of the target weed in order to discover aspects of its life cycle that either may be exploited by agents to have a significant effect on the plant, or may diminish the impact of the agent on the plant. Marohasy (1995), considering the reproductive biology of acacias, suggests insects which feed within flowers are unlikely to be effective against the weed Acacia nilotica because flowers are normally produced in excess with a very large percentage being subsequently lost through abscission. In contrast, flower-galling midges may be very damaging as galled flowers are not shed but develop into galls which act as energy sinks, consuming resources which would otherwise be available for growth and pod maturation (Marohasy 1995). Successful biological control of A. longifolia was achieved in South Africa using a wasp which galled flowers (Dennill 1988). The wasp sometimes committed A. longifolia to the production of 200% more galls per branch than the normal quota of pods and Dennill (1988) dubbed this phenomena 'forced commitment'. Harris' scoring system (Harris 1973) had suggested galling insects are poor potential biological control agents because they '...have evolved a homeostasis with their host that renders them incapable of inflicting serious damage to it'. It is now recognised that galls can be very damaging, acting as physiological sinks depriving other plant parts of energy and nutrients (Dennill 1988; Shorthouse 1990).

Although potential biological control agents may be shown to have an affect on individual plant performance, it may be an entirely different matter to demonstrate that this 'critical damage' can have an affect on plant population dynamics (Crawley 1989). If the growing season of a plant exceeds the feeding period of an insect or if the plant has food reserves which are inaccessible to the insect, the plant may be able to compensate for any damage. The seed bank in the soil might be so large that even a dramatic reduction in seed production through insect damage will not lead to a reduction in seedling recruitment (Crawley 1989).

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