"Strains" and the classical biological control of insect pests

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Abstract
The classical biological control technique of introducing two or more populations of the same species of beneficial agent to increase the genetic diversity of that species (and so increase the chances of achieving a successful project) is reviewed. From standard literature sources, all cases of multiple introductions of conspecific populations against insect targets were listed and the effect of subsequent introductions on the outcome of the project was recorded. Of 178 projects identified, involving 417 separate importations, only 11 (6.2%) were successful through a second or later importation of the same morphologically defined species of beneficial agent. Of these, five involved host-related "strains" that are likely to be cryptic species, so the success rate for the introduction of conspecific populations falls to 3.4%. The possibility that some (or even all) of the other six cases also involved cryptic species awaits investigation. Our analysis demonstrates that introducing two or more populations of the same species is less likely to result in enhanced success than if other species of natural enemies are sought for "normal" classical biological control (historical success rate 12–16%). In our discussion we focus on the genetic theory of species which underpins this area of applied biology and find that there is also no theoretical support for the continued introduction of strains.