The Strength of Biological Control in the Battle against Invasive Pests: a Reply

Louda and Stiling (2004 [this issue]) state that biological control is not a simple matter of community re-assemblage because deliberate introductions of exotic natural enemies for suppression of exotic pest populations can have reticulate impacts that are difficult, if not impossible, to predict a priori. In an insightful retrospective study, Hawkins et al. (1999) analyzed 68 life-table studies of native insects and introduced insect pests to determine whether biological control is analogous to naturally occurring control (i.e., the action of native natural enemies on native hosts). Hawkins et al. (1999) show that successful biological control programs result in less reticulate trophic relationships than those seen in natural food webs of native insects. The most successful biological control programs are those that do not have “natural” food-web structures because biological control food webs consist of short, linear food chains that are devoid of complex reticulate trophic interactions. This result occurs because biological control systems often consist of exotic species that share few ecological or evolutionary links with native biota. Furthermore, control is enhanced in simplified habitats characteristic of agroecosystems and, arguably, native systems invaded by exotic plants because both often consist of vast monotypic stands of exotic vegetation. Host-specific natural enemies that cause population declines of the target pest are themselves subject to density-dependent population regulation as the biological control agent’s food source is depleted, and they are unable to adequately exploit other hosts in the environment to maintain high population densities because they lack significant trophic linkages to other hosts. The more specialized the natural enemy the less likely it is to infiltrate native communities and attack nontarget native species (Hennemen & Memmott 2001). Generalist natural enemies that have low levels of host and