Host Range Testing of Insects for Biological Weed Control: How Can It Be Better Interpreted?

Abstract

I nonindigenous species cause major environmental damage in many different ecosystems worldwide. In a recent assessment, the costs to the US economy inflicted by invasive species were estimated at $137 billion annually (Pimentel et al. 2000). The US government therefore has expanded its efforts to combat exotic pests, as well as to accelerate research on habitat restoration and biologically based, integrated pest management tactics. Classical biological control (that is, the deliberate release of natural enemies to control exotic pests) is an important tool in integrated pest management, and it is usually considered to be economically sound and environmentally safe. However, safety of classical biological control has recently become a matter of debate concerning the risks posed by the introduced enemies to nontarget organisms and to biodiversity in general. This shift in the assessment of biological control arose mainly from recent findings of nontarget effects caused by biocontrol agents introduced through classical biological control programs (Simberloff and Stiling 1996, Louda et al. 1997, Strong 1997). The importance of the topic emerged in subsequent publications discussing such topics as benefits of biological control (Gurr and Wratten 2000), ecological relevance of nontarget attack (Follet and Duan 2000, Wajnberg et al. 2001), evolutionary stability (Jervis 1997), and the use of modeling in biological control (Thomas and Willis 1998). One of the most critical points of contention is whether and to what extent host-specificity tests of potential biocontrol agents are useful in assessing potential risks toward nontarget organisms (McEvoy 1996). Although prerelease studies in general, and host range testing in particular, only recently have begun to be part of biological control programs against insect pests, they have a long-standing tradition in the classical biological control of weeds. The report of host range expansion of the weevil Rhinocyllus conicus Froel. (Figure 1; Louda et al. 1997), introduced in North America to control exotic thistles, has therefore puzzled many environmentalists and theoretical ecologists and has contributed to a general discomfort about the effectiveness of prerelease studies. In this paper, I discuss the potential and limits of host range testing in predicting ecological and evolutionary changes in the host range of biocontrol agents after their release in a target area. Particular emphasis is placed on the usefulness of detailed behavioral and genetic studies in improving the interpretation of standard host range tests.