Issue link: https://read.dmtmag.com/i/73811
Bio Control - Cont'd From Page 10 through host-range testing. Candidate agents are presented with a series of native species more and more distantly related to the invader to determine if the agents will feed on, and more importantly, complete their life cycle on any of those native species. These tests are relatively straightforward to perform. The hard part is evaluating the results. Have the right species been tested? How much feeding on a native species is "significant?" How will a change of context from the lab to the wild—and from the point of introduction to a different ecological context—affect the behavior of the biocontrol agent? The wrong interpretation could result in ecological disaster. The scientists responsible for the thistle bio-control effort justified the weevil's release based on host specificity tests whose basic principles were largely the same as those in use today. These tests indicated that, although the weevil could complete its life cycle on native and introduced thistles in three genera, it had a strong preference for and developed more rapidly and to a larger size on the non-native thistles. On the basis of these results, the researchers argued that the weevil would have no significant impacts on natives. The 35 years since its release have proven the researchers wrong. The weevil attacks and significantly reduces seed production of the native Platte thistle (Cirsium canescens) as well as other native thistles. In hindsight, this weevil should never have been introduced, for it is too much of a generalist to be considered safe. The question, then, is whether these impacts could have been foreseen— and therefore avoided. In other words, was it inherent unpredicability of species introductions or unsound evaluation of test results that led researchers to release what has turned out to be such a damaging invader? In a recent article in Biological Conservation, University of Nebraska researchers Amy Arnett and Svata Louda reported on an experiment designed to answer just that question (1). They hypothesized that the unexpected impacts of R. conicus on native thistles were the result of a post-release expansion in host range; that is, that the weevil had adapted over time to reproduce with greater success on North American thistles than was the case before its introduction to North America. If this proved true, the future for biocontrol would be bleak—no predictions about impacts could be made with any certainty. What they found was exactly the opposite. When the researchers subjected 28th-generation naturalized weevils collected from a stand of Platte thistles in western Nebraska to exactly the same tests that were used in the original 1968 evaluation, they found that host specificity had not changed at all. R. conicus still showed a strong preference for the non-native thistles it was introduced to control. This meant that the weevil was feeding heavily on Platte thistle and other native thistles simply because in western Nebraska the natives were the only hosts available; the introduced musk thistle is not present in that part of the country. In short, the reduction in fitness that the weevils suffered as a result of this host switch was not enough to reduce its population to nondamaging levels. Arnett and Louda's research indicates that even a "strong preference" for a target species may not be sufficient to guard against collateral damage. It also shows the importance of evaluating host specificity data with an eye to worst-case scenarios. If a biocontrol agent is able to reproduce on something other than its target, one must assume that somewhere, at some point, it will. Even if the rate of reproduction is reduced, the impacts of that biocontrol agent on native species can be considerable. The key is to evaluate how impacts might vary across the full range of ecological contexts a biocontrol agent is likely to reach—not just the context in which it is to be introduced. Notice how much has changed in the last 35 years. The researchers who made the decision to release the weevil discounted what they knew to be the worst-case scenario; the nontarget impacts on native thistle species were unimportant by the standards of their time. Today's norms are much more attuned to broader ecological consequences, and researchers have begun to take a more precautionary approach. The data remain largely the same. All that has changed is the scientific and cultural consensus on how much collateral damage is acceptable and under what circumstances it makes sense to proceed even when nontarget impacts are likely. In a perfect world, biocontrol would only be used for those invaders whose natural enemies attacked absolutely nothing but the target invader. But sometimes you have to bend the rules. In cases where chemical, mechanical, or cultural management tools are useless and impacts are significant, conservationists must weigh the impacts of the biocontrol agent on nontarget species against the consequences of doing nothing. Such is the case of the imported red fire ant (Solenopsis invicta), an invader in the southeastern United States that inflicts painful bites and causes significant crop damage. It is also the single most important factor in the decline of native fire ants (as well as many other ant species), outcompeting them wherever the invader and the native come into contact. To make matters worse, available poisons do not discriminate between native and imported ants and therefore have no value as a tool for native species conservation. So in the face of serious and unmanageable ecological impacts, protecting native ant species will happen via biocontrol or not at all. Under such circumstances, the current consensus in the conservation community appears to favor biocontrol, so long as the benefits to native species outweigh the costs. This is the standard that researchers held themselves to as they considered whether to introduce a natural enemy of the imported red fire ant called Pseudacteon curvatus, a species of decapitating fly native to South America. Decapitating flies are parasites that reproduce by attacking fire ants and laying their eggs inside the ants' head cavity. Most are tightly co-evolved with a specific species of ant, making them ideal candidates for use as biocontrol agents. Although the fly shows a strong preference for the imported fire ant, host specificity tests indicated that if it were given no other option, the fly would also attack several native fire ant species in the same genus as that of the imported ant (2). Tests also showed that parasitized native ants were adequate hosts for larval development and that at least some fly eggs laid in native ants survived to adulthood. By the standard of "no collateral damage," it would appear that this particular species of decapitating fly was not an acceptable candidate for introduction. However, several factors suggested that the net impacts on native ant species would not be as uniformly negative as the raw numbers implied. As in the case of the thistles, the flies showed a strong preference for their non-native target, attacking native fire ants only 6 percent to 35 percent as often as they attacked the imported fire ant. Whereas these results by themselves would not be sufficient to justify release (after all, the fly did attack and kill native fire ants), the rest of the evidence tipped the scales in favor of introduction. First, the frequency with which the fly attacked native fire ant species was too low to create a self-perpetuating fly population. As such, the fly would not be able to survive in locations where all it had available were native hosts. (Recall that ability to survive in the absence of the target non-native species was the key contributing factor to the R. conicus weevil's impact on native thistles.) Second, native fire ant species already are subject to parasitism by native decapitating flies in the same Continued On Page 30