Yellow Starthistle Information

 Mech control
   -hand
   -tillage
   -mowing

 Cultural control
   -grazing
   -burning
   -re-vegetation

 Biocontrol
   -insects
   -biocontrol table
   -plant pathogens

 Chem control
  - risks
    --spray
    --water
    --toxicology
    --herbicide resist
    --effects
   -herbicides
    --preemergence
    --postemergence
    --late season
    --pre- & post-
    --imazapic
    --clopyralid
    --picloram

 Integrated app

 Summary

Management

Chemical control (continued)

Toxicology

When used improperly, herbicides can pose a health risk. This can be minimized with proper safety techniques. Applicators should follow label directions and wear appropriate safely apparel. This is particularly true during mixing, when the applicator is exposed to the highest concentration of the herbicide.

Although animals can also be at some risk from herbicide exposure, most herbicides registered for use in non-crop areas, particularly natural ecosystems, are relatively non-toxic to wildlife. To prevent injury to wildlife, care should be taken to apply these compounds at labeled rates.

The trend in herbicide toxicity of the past 25 years has been toward registration of less toxic compounds. From 1970 to 1994, the percentage of herbicides with an LD₅₀ value (lethal dose in mg herbicide/kg fresh animal weight which kills 50% of male rats) of between 1 and 500 mg/kg decreased from 15 to 7%, while herbicides in the least toxic category (>5000 mg/kg) increased from 18 to 42%. In addition, the average LD₅₀ of herbicides registered in the United States during this time period increased from 3031 to 3806 mg/kg (DiTomaso 1997).

Herbicide Resistance

Selection for herbicide-resistant weed biotypes is greatly accelerated with the continuous use of the same herbicide, or several herbicides with a common mode of action. The first case of herbicide resistance in yellow starthistle was detected in 1989 in Dayton, Washington (Gibbs et al. 1995, Sterling et al. 1991). This selection for resistant starthistle occurred through the continuous use of picloram (Callihan and Schirman 1991). In this case, the level of picloram resistance was between 3- and 35-fold greater than a susceptible population, depending on the site of application and growth conditions (Fuerst et al. 1994, 1996). This population was also cross-resistant to clopyralid, dicamba and fluroxypyr, which have a similar mode of action as picloram (Valenzuela-Valenzuela et al. 1997), but not to triclopyr or 2,4-D, which also have the same mode of action (Fuerst et al. 1994). Although this resistant biotype has been studied by several researchers (Fuerst et al. 1996, Prather et al. 1991, Sabba et al. 1998), the specific mechanism has yet to be elucidated.

The development of picloram-resistant starthistle indicates the potential for development of resistance to clopyralid if the herbicide is used year after year. Integrated approaches for the control of invasive weeds can greatly reduce the incidence of herbicide resistant biotypes.

Effects of Herbicides on Plant Diversity

Continuous broadcast use of one herbicide or a combination will often select for plant species demonstrating greatest tolerance. In the absence of a healthy plant community composed of desirable species, one noxious weed may be replaced by another equally undesirable species insensitive to the herbicide treatment. When broadleaf selective herbicides are used, noxious annual grasses such as medusahead (Taeniatherum caput-medusae), cheatgrass or downy brome (Bromus tectorum), or barbed goatgrass (Aegilops triuncialis) may become dominant. Population shifts through repeated use of a single herbicide may also reduce plant diversity and cause nutrient changes that decrease the total vigor of the range (DiTomaso 1997). For example, legume species are important components of rangelands, pastures, and wildlands and are nearly as sensitive to clopyralid as yellow starthistle. Repeated clopyralid use over multiple years may have a long-term detrimental effect on legume populations. Thus, herbicide use in rangelands should be part of an integrated weed management system.

Interestingly, in a study conducted by Northam and Callihan (1989), they showed that the number of plant species per square meter in a yellow starthistle infested area increased from 11 (untreated) to 12 following clopyralid treatment. In contrast, more non-selective postemergence herbicides, including 2,4-D and dicamba, decreased the number of species per square meter to less than 9. This experiment, however, measured species changes after only a single year of treatment. Multiple years of herbicide application may have a more negative impact on plant diversity.

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