Biology Reference
In-Depth Information
target plants (Rinella et al. 2008). A single or infrequent application of herbicides could
actually increase the subsequent densities of the target plant by reducing plant competi-
tion. Prescribed summer fire applied on an annual basis can reduce densities of spotted
knapweed (Emery and Gross 2005; MacDonald et al. 2007), but like the use of chemi-
cals, the cost and nontarget effects limit its use. Given the problem with chemical and
mechanical tools, the biological and cultural tools would therefore encompass the only
viable activities for control of a regionally abundant species such as spotted knapweed.
However, some evidence supports the hypotheses that spotted knapweed is a superior
competitor in North America via allelopathy, mycorrhizae, or other soil microbial feed-
back mechanisms, and if this is the case then enhancing plant competition may not be
effective. As mentioned previously, the remaining biological tools have begun to show
success in some areas. However, if spotted knapweed is not negatively impacted by
herbivores or can benefit from herbivory in some situations, then the current suite of
biological control agents may only exacerbate the overall problems caused by spotted
knapweed (Ortega et al. 2004). Until now, our current science has provided managers
with a confusing, mixed message about controls for spotted knapweed.
We argue that the studies that offer the most useful insights into sustainable
management are those that provide a long-term assessment of knapweed densities,
and are conducted within a whole system perspective. While individual experi-
ments on plant-soil, plant-plant, or plant-herbivore interactions may be perfectly
valid given the context of the experiment, interpretations and conclusions may
change when the full suite of direct and indirect interaction effects of the activity is
imposed. Management decisions should be based on those long-term studies that
measure changes in spotted knapweed abundance within the affected communities.
Considering that this plant grows in a variety of habitats and climates across much
of North America, experiments that assess control tools under a variety of resource
conditions are also the most useful for application to management plans.
11.4.1 ClassicalBiologicalControl
As previously discussed, biological control agents were first released on spotted
and diffuse knapweed in 1970. By 2000, 13 species of insects had been released in
an attempt to reduce the abundance of these plants (Story and Piper 2001). A
number of these had established on knapweed; however, no significant reductions
in the abundance of knapweed had been documented prior to 2006. Given the previ-
ously described concern about hantavirus and the influence of knapweed on human
health (Pearson and Callaway 2006), the benefits of identifying sustainable man-
agement controls of knapweed include not only the need to reduce the target plant's
effects on forage loss and biodiversity, but also to reduce problems associated with
nontarget effects caused by this biological control insect food-web alteration.
Biological control agent effects on spotted knapweed have been increasingly studied
in the last two decades. In a pot study, Steinger and Müller-Schärer (1992) demon-
strated that the growth response of spotted knapweed to root herbivory was mediated
