Exclusion of Natural Enemies as a Tool in Managing Rare Plant Species

In their criticism of our paper, Lesica and Atthowe (2000) seriously misrepresent it. First, they dismissed our main point, which likely reflects their assumption that coevolved insect herbivores are not likely to affect contemporary plant dynamics. Second, they chose to ignore the caution with which we presented the implications of our experimental results. The main point of our paper was that natural enemies such as native insect herbivores can limit the survival, growth, and reproduction of threatened plant species such as Pitcher's thistle (Cirsium pitcheri; Stanforth et al. 1997; Bevill 1998; Bevill et al. 1999). In spite of our evidence of a significant negative effect of coevolved native insects on key demographic parameters, Lesica and Atthowe (2000) suggest that ". . target herbivores of Pitcher's thistle are native and have probably been associated with it for hundreds of generations. They are not likely to be the primary cause of the plant's endangerment." This reasoning is incorrect for two reasons. First, there is already strong evidence that chronic herbivory by coevolved insect herbivores limits both population density and lifetime fitness of the ecologically similar (Louda 1994), closely related, and likely progenitor species (Johnson & Iltis 1963) Platte thistle (Cirsium canescens; Louda & Potvin 1995). Second, our stated aim was not to determine the cause of endangerment, but rather to examine the role of natural enemies in remnant local populations or in restorations of extirpated populations. We conclude that a bias against the herbivory hypothesis and a focus on a different issue obscured Lesica and Atthowe's assessment of our research. In their effort to challenge the irresponsible use of broad-spectrum chemicals in natural habitat management, they departed from what we actually said. They agreed with our main conclusion in essence by saying that "Insecticide use may be an appropriate option for endangered species management. . ." (Lesica & Atthowe 2000). Yet they harshly criticize us for not developing arguments against massive, widespread, long-term application of broad-spectrum insecticides in endangered species management. Perhaps we should have, but we did not think the argument was necessary for this audience, especially because we neither used, recommended, nor condoned such applications. Next, Lesica and Atthowe suggest that we showed a "single-minded concern for specific endangered species," leading to a "disregard [of] the long-term well being of other organisms in the system." Although we focused on insect herbivory in limiting local populations of a listed species within its protected habitat, we did consider all of the interacting species observed (Bevill 1998). Furthermore, current U.S. law mandates a special concern for listed species. But studies such as ours on important interactions within critical habitat areas lead to protection for that ecosystem as well as to improved management for the listed species. By their reference to Silent Spring, they insinuate that we favor irresponsible use of broad-spectrum insecticides and imply that we killed bees, poisoned birds, and endangered endemic specialist insects. They further imply that our recommendations will lead to destruction of top-down predator-parasitoid control, evolution of pesticide resistance, and endangerment of endemic fauna. A careful reading of our paper should make the error of these accusations obvious. We applied insecticide cautiously in a rigorous, controlled, small-scale experiment. Insecticide was applied by hand, leaf-by-leaf and plant-byplant. The insecticide was an over-the-counter preparation that reduced (not eliminated) phytophagous insects. We isolated plants. We stopped spraying individual plants when flower bud development began, before bees and birds would be involved. No rare endemic insects occurred. We repeatedly and purposefully emphasized our consideration of short term, small scale, and local exclusion in our use of insecticide. We did not advocate any particular exclusion technique. The choice, including hand-picking, netting, cages, or insecticide, depends on the situation. Our point was that it may be important, at times, to manage natural enemies locally, for a short