Host Specificity Testing: Why Do We Do It and How We Can Do It Better

Host specificity testing is universally used in weed biological control to predict nontarget effects of potential agents. Despite this, there is some confusion regarding the role of host specificity testing in making such predictions. One possible role is as an assay of field host range. In this case, the ideal host specificity test will simulate conditions encountered in the field, and the result (the estimated field host range) will be judged according to how accurately it matches the realized field host range. An alternative approach is to separate the description of innate host specificity (which includes fundamental host range, the relative acceptability and suitability of hosts, the ability to learn, and time dependent effects) from the prediction of how it will be expressed in the post-release environment (in terms of field host range and relative attack). In this case, host specificity testing is used to describe properties of the insect, which are then used in combination with ecological information to predict where, when, and to what extent non-target attack would occur. I argue that the latter approach is more powerful because non-target effects under any particular environmental conditions are predicted, rather than being estimated by attempting to experimentally simulate the release environment. Here I discuss this more basic approach to host specificity testing in some detail in relation to the meaning of the terms host specificity and host range, and I point out the implications of this approach for the way that we conduct host specificity testing. My approach to host testing can be divided into three steps: (1) identification of aspects of life history that need to be host-specific if the insect is to be safe for release; (2) description of the fundamental host range of the organism; and (3) if non-target species are included within the fundamental host-range, prediction of whether they will be attacked under field conditions and the frequency and severity of such attacks.