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

The Strength of Biological Control in the Battle against Invasive Pests: a Reply MARK S. HODDLE Department of Entomology, University of California–Riverside, Riverside, CA 92521, U.S.A., email mark.hoddle@ucr.edu Nontarget Impacts of Biological Control 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 ene- mies 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 oc- curring control (i.e., the action of native natural enemies on native hosts). Hawkins et al. (1999) show that suc- cessful 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 ecologi- cal 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 mono- typic stands of exotic vegetation. Host-specific natural en- emies 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 habitat fidelity and exploit a wide range of hosts in a vari- ety of habitats are more likely to cause unwanted collat- eral damage to nontarget species either directly through direct attack or indirectly through apparent competition because of reticulate food-web linkages. Two plant-feeding insects (Rhinocyllus conicus [inten- tionally released; Gassmann & Louda 2001] and Larinus planus [an accidental arrival in the United States that was eliminated as a potential biological control agent because of broad host breadth on Carduinae thistles in its home range; Louda & O’Brien 2002]) mentioned by Louda and Stiling as having “unexpected levels of nontarget feeding” were anticipated from host-specificity tests because both weevils were known to feed and reproduce on a variety of thistle species in their home and introduced range. As Louda and Stiling point out, both insects are “thistle specialists,” but thistles are a speciose group with repre- sentatives in genera that occur in Europe and North Amer- ica, and these weevils, due to their broad dietary breadth within the thistle group, have greater numbers of signif- icant food-web linkages than desirable, thereby making significant nontarget impacts more likely. The real issue here concerns legislation (i.e., redistribution of L. planus and R. conicus) and changing social values (i.e., what con- stituted acceptable damage to nontarget species, and the value of native flora and fauna in the 1960s versus 2003), as opposed to an inherent flaw in the theoretical prin- ciples underlying concepts of host specificity and host range assessment as they pertain to biological control. The moth Cactoblastis cactorum, a native of Argentina and “poster child” of biological control (Stiling 2002) has invaded the continental United States from the Caribbean and is attacking native cactii. Louda and Stiling consider C. cactorum a “specialist” on the cactus genus Opuntia, a group with approximately 200 species in the new world but no representatives in Australia, making C. cactorum a specialist on this continent when it was released in Paper submitted April 15, 2003; revised manuscript accepted May 23, 2003. Conservation Biology, Pages 61–64 Volume 18, No. 1, February 2003