Brush Management: Past, Present, Future

11. The Future of Biological Management of Weeds on Rangelands

11 The Future of Biological Management of Weeds on Rangelands An Entomologist’s Viewpoint The flood of exotic plants being introduced into North America may be slowing due to increased education and legislation providing resources to monitor and reduce the intentional and accidental introduction of unwanted plants. However, we can expect a massive increase in invasive weed problems on rangelands as the plants introduced over the past 100 years become naturalized and begin to spread (McFadyen 1998). New exotic weeds will continue to be introduced into the United States and ultimately into southwestern rangelands. Some exotic weed problems will be unique to southwestern rangelands and others will simply spread from severely infested sites in northern and western states. In the northern half of the western United States, exotic weeds cause considerable economic damage while native weeds are generally considered to be more important than exotic weeds on most southwestern rangelands (DeLoach 1995). Weed management on southwestern rangelands during the last couple of decades has relied primarily on the use of herbicides. Although herbicides are the best choice for many weed management programs, there are increasingly more limitations placed on their use (Lym 1998). Many of the exotic weeds that we are faced with managing today are difficult to control with conventional herbicides. Some are tolerant of repeated herbicide applications , many occur in areas where the use of chemicals is unsafe or unwelcome , and others, by today’s standards, may not warrant the economic expense involved in controlling them with herbicides. Integrating biological control into the future management of many rangeland weeds will become a necessity for land managers who desire more stable, long-term, selfsustaining control. Biological control of rangeland weeds is dependent on numerous uncertain factors, including management goals for individual properties, economics , and politics. Most of these are very dynamic and will play important roles in future weed management decisions. Recognizing this fact, the future of weed management, especially using biological control agents, is as unpredictable as the factors that govern it. The future of biological control on rangelands will continue to emphaDavid C. Thompson and Kevin T. Gardner 143 size the introduction of the natural enemies of weeds from their country of origin, but there will be more research on augmenting or conserving the natural enemies already in place than in the past. Biological control has experienced many successes that completely eliminated the need for other management alternatives (Julien and Griffiths 1999; see Chapter 9, this book). However, the majority of weed biological control programs have established insects and pathogens that negatively influence the fitness and, thus, the competitive abilities of target plants without completely controlling the weed. These many successes are often overlooked, although the agents may have played a very important role in the integrated management of the weed. In this paper, we discuss the future of biological control as a management tool for terrestrial weeds in the southwestern United States, and some of the issues that affect this field of research. Weed Management When considering a weed management program, it is important to understand that no single control option will likely provide a cure-all for any given weed problem over its entire range. A variety of weed management options may be necessary where landowner goals vary. For instance, herbicides are very area specific. Herbicides allow a manager to manage a weed on a target area without influencing the neighbor’s property. Conversely, biological control agents do not respect property boundaries and if released on one property will eventually spread to neighboring properties where the target weed is present. The value of a plant, whether positive or negative, often varies between neighboring properties and must be considered when a weed management plan is prescribed. The impact of a weed on the rangeland it occupies, whether economic, ecological, or aesthetic is difficult to assess, especially when more than one land manager is involved. Weed management decisions are influenced, and often dictated, by ranch economics. Rangelands typically produce small economic yields per unit area compared to other land uses and when coupled with budget constraints , such as increased operational costs and lower market prices, weed management with what once was a profitable method may now be cost prohibitive . Such is the case with herbicidal control of many of our native weed species. Currently, the cost of herbicide treatment exceeds the revenue generated by the treatment on many rangelands (Bangsund et al. 1996). Future weed management strategies obviously will...