Biological Control Options for Invasive Weeds in Nevada
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Special Publication 14-03 Biological Control Options for Invasive Weeds in Nevada Joy Paterson, IPM Educator, University of Nevada Cooperative Extension Jay Davison, Area Specialist, University of Nevada Cooperative Extension Jeff Knight, Entomologist, Nevada Department of Agriculture This publication provides a basis for evaluating the use of biological control agents of invasive weeds in Nevada. It includes a list of weeds with the currently available biological control agents that have shown promise in weed management programs. It also includes references for those interested in using livestock as a weed management agent. examples include: squarrose knapweed, native Introduction to southeastern Europe and Central Asia; yellow starthistle, native to southern Eurasia Noxious weed encroachment is a and the Mediterranean Basin; and saltcedar, serious threat to the economic well-being and native to central and southwestern Asia. In ecological balance of Nevada lands. A weed is each of these examples, the local insects and normally declared as “noxious” by individual disease complexes associated with the plants’ States. The state then decides the level of native community limit the plants’ density and control or management required for each spread in the native range. However, in a new noxious weed by the owner of the property ecosystem the weed is no longer challenged where it is found. In Nevada, this is codified by these biological control agents and soon under Chapter 555 of the Nevada Revised dominates large areas. Statutes. A noxious designation is necessary There are for invasive, hard-to- several tools being control weeds that once used to fight the established, come to spread of noxious dominate the infested weeds in the Great lands, resulting in lost Basin and Nevada. profits and greatly Prevention is increased costs always the associated with preferred method of management of these control, but some weeds. These exotic, unintentional invasive, weeds are spread of seeds or plants that have plant parts from demonstrated the ability waterways, to outcompete the vehicles and animal complex of native plants movements is occurring on lands that inevitable. they invade. This results Chemical control in extensive has proved monocultures of the effective on some introduced weed species species but may that gradually displace not be economically the more diverse plant or ecologically and animal communities. appropriate in every These native plant and instance. Cultural animal associations are practices such as not easily restored once repeated tillage lost to noxious weed Yellow Starthistle may be used with invasions. some annual weeds but perennials are often All of Nevada’s most troublesome spread or benefit by this disturbance. Biological invasive plant species were introduced from control using insects or diseases specific to a Europe or Asia. They are widespread in particular weed species is a tool that has been various areas of the western United States utilized with some success in controlling the because their native range had a similar spread and impact of noxious weeds. The best environment, and the naturally occurring pests approach is to utilize an integrated pest of these plants were not introduced into the management (IPM) model whereby multiple U.S. at the same time as the weed. Some control activities are used in a way that significantly reduces a noxious weed Biological control strategies should be population at the lowest economic and considered when an invasive weed population environmental cost. has become economically or ecologically too large to be managed using traditional control Understanding Biological Control methods. Large populations of a weed species Biological control is the intentional are normally required for an introduced introduction of a biological agent such as an biological agent to reproduce and survive. The insect that incrementally stresses the target weed species is never eradicated because the weed species in order to help suppress weed bio-control agent requires a remnant plant populations without population for reproduction affecting other desirable and survival. If eradication plants growing in that of the target weed species location. When a noxious is the goal, biological weed is stressed over control agents are not time, it becomes less usually an appropriate tool. competitive with native A biological based weed plants. In a successful management program biological control when applied appropriately program, the weedy is, however, the best long- species is not eradicated term option where invasive but the economic and weeds have occupied ecological impact of that extensive areas and are weed species is sharply well established, and reduced. After the conventional control successful establishment techniques are impractical of a biological control due to economic or agent such as an insect, environmental limitations a weed population will on the site. While there is typically experience a some disagreement dramatic decline in regarding the right numbers. This is combination(s) of agents followed by a decline in for attack on a targeted the insect populations plant, agents that attack due to the reduction in several different parts of a the targeted weed plant simultaneously numbers (food source). Canada Thistle generally increase the This allows the weed effectiveness of the population to increase again, which is then biological control program. As with any weed followed by an increase in the insect management program, the use of a biological population. This cycle continues over several control agent must be evaluated for costs and years with a gradual, long-term reduction in the potential effectiveness at each location in targeted weed species. It is not a short-term which it will be applied. method of weed management, but when There are several ways that weeds can successful, provides a low-cost, noxious weed be managed using biological control methods. management option for landowners dealing Traditional biological control has focused on with large-scale infestations. Biological control introducing an organism, usually an insect, to is not a first-line treatment for small patches or feed on a plant. However, in the past 10 years newly infested areas, because biological biological control has grown to include any control agents usually take many years to management practice that uses a living become established and significantly suppress organism to suppress a weed population. a weed population. Livestock, disease-causing pathogens, mites and insects are all now being used as part of seeds have all passed the digestive system biological control strategies to suppress weeds. before being moved to new grazing lands. Any While the focus of this publication is on insects, weeds that grow in the holding areas should be which are the traditional biological control killed to prevent additional spread. agents, it briefly discusses a new approach These are just a few of the using livestock that has been effective in the considerations necessary to implement a United States and is growing in popularity. successful biological control program using Biological Control With Livestock Grazing livestock. Resources with comprehensive guidelines for using livestock to control weeds Some weeds can be reduced by can be found in an Extension bulletin, 06-05, selectively and intensively grazing the target “Livestock Grazing Guidelines for Controlling weed with the proper species of livestock. Noxious Weeds in the Western United States,” When using livestock, several items must be available from University of Nevada considered before a management plan can be Cooperative Extension at successfully implemented. As with any biological control agent, the use of livestock to http://www.unce.unr.edu/publications/files/ag/2 manage weeds requires a long-term 006/eb0605.pdf and in the book “Targeted commitment to be successful. Selecting the Grazing: A Natural Approach to Vegetation proper species of livestock, training it to eat the Management and Landscape Enhancement” target weed, and applying the grazing (Wilson, et. al. 2006). The book is available treatment at the appropriate from the University of Idaho and the American time are the most critical parts Sheep Industry Association. of a successful management Traditional Biological Control program. Although livestock are Using Insects able to eat weeds that are not In the United States, the normally considered forage, procedure for determining they may not eat the weeds acceptable control agents for without proper training or release is quite rigorous and exposure to the target weed. expensive, once a target weed The timing or when the grazing species has been determined. treatment is applied is also Potential insects that may be critical to success. Proper effective biological control agents timing will increase the effects are identified and collected from the of grazing, resulting in home ranges of the target weed. maximum damage to the The insects or pathogens undergo targeted weed with minimal extensive testing in the insect’s damage to desirable plants on Urophora affinis a native range to determine their the site. Grazing levels or knapweed seed head gall effectiveness at controlling the intensity must be controlled for fly on knapweed target species and to ascertain their the same reason. In addition, potential for harm to any other steps should be taken to prevent viable weed related or native plant species. These seeds passing through the grazer’s digestive “starvation” tests demonstrate that the potential system or those trapped in the coats of the biological control agent will starve to death livestock used to graze the infested areas from rather than feed on plant species other than spreading to lands not infested by the target the target. Any agents that fail these tests are weeds when the livestock are moved. Animals eliminated before continued trials occur in the should be isolated in holding pens for enough United States. Figure 1 shows the process that time (five to seven days) to ensure that viable must Figure 1. Required testing stages before an insect can be introduced as a biological control agent in the United States.