Biology Reference
In-Depth Information
other plant species (Blair et al. 2005, 2006). A similar finding for diffuse knapweed
suggests that this species also lacks sufficient quantities of its putative reported
allelopathic chemical to be capable of reducing plant competition as well (Norton
et al. 2008). Significant reductions in the densities of spotted knapweed (Corn et al.
2006; Story et al. 2006) or large reductions in seed production by this species
(Seastedt et al. 2007) suggested that classical biological control by insect herbiv-
ores may in fact be effective. Additional studies suggest that insect herbivory
(Jacobs et al. 2006) and competition from native plant species (Pokorny et al. 2005,
Rinella et al. 2007) have significant negative effects on the densities of this species.
These studies suggest that disturbance events, which lower the productivity of an
invaded plant community, may be essential to successful spotted knapweed inva-
sion. Collectively, these results appear to refute much of the evidence presented in
previous paragraphs suggesting novel allopathic mechanisms, superior resource
use, or the ability of biological control herbivory to improve plant fitness under
most field conditions.
11.4
Management of Spotted Knapweed in Natural Areas
Spotted knapweed appears to flourish in many human-generated habitats such as
roadsides and areas of soil disturbance, especially where competition from other
plants is absent. Because the weed is adapted to such sites, control in these areas
without modifying land use may be difficult, and can perhaps be accomplished only
with repeated control activities. Such habitats could function as source habitats to
disperse seeds into other, less-disturbed plant communities used for grazing or
conservation purposes. If, as we suggest later, sustainable control of the weed is
possible within relatively
nondisturbed
areas, then the compelling reasons to con-
trol the weed in disturbed sites are reduced. In fact, persistence of the weed may be
essential in maintaining refugia for biological control agents that provide sustaina-
ble control of this species in more intact vegetated areas.
Classical weed management usually attempts to categorize effects into what
tools
work to control a weed species. Using this approach, choices for grasslands
and rangelands include
cultural
(enhancing plant competition or preventing
weed introduction),
biological
(classical biological control, adding herbivores or
pathogens),
mechanical
(tillage, mowing, or fire), and
chemical
(herbicides).
Readers are referred to Sheley et al. (1999) for an overview of all management
techniques, including those herbicides used to kill the plant. However, as we
note later, routine use of herbicides and particularly broadcast applications of
herbicides appear insufficient to reduce the long-term dominance characteristics
of this species.
While herbicides can provide temporary reductions in densities, the treatment rarely
if ever provides long-term benefits, and the weed is too widespread for this to be a viable
management option over a large area. In addition, herbicide treatments have been
shown to reduce native plant diversity while failing to reduce long-term density of the
