Introduction

Biological control of arthropod pests using arthropod natural enemies has been practised for centuries (Flint and van den Bosch, 1981), and although it has been widely appreciated that these natural enemies can have adverse effects on the environment, it has been largely assumed that they are either absolutely small or small relative to the benefits of biological control. For example, Samways (1988) suggested that there are no quantified cases where the introduction of an arthropod agent has been shown to have harmed a specific conservation programme or has been conclusively damaging to native fauna. This suggestion has proved difficult to test under natural conditions. For example, many natural enemies have been released to control forest Lepidoptera pests. Some of these enemies kill non-target, nonpest insects, and it has been suggested that such unintended mortality might destabilize these nonpest populations thus causing them to become sporadic pests. Recently, Pimentel et al. (1984) reviewed the evidence that biological control agents can cause adverse environmental effects. Their review clearly documented that biological control can entail environmental risk and that categorical dismissal of this concern is unwarranted. More recently, Howarth (1991) argued that the introduction of biological control agents into Hawaii and New Zealand is one of the major causes of extinctions of the native, insular, endemic arthropod faunae associated with these two islands. In most of those cases, vertebrate natural enemies were implicated, but Howarth (1991) developed a reasonable argument that arthropod natural enemies could also affect the native faunae.