Biology Reference
In-Depth Information
by soil resource abundance. Since then, experimental studies using actual herbivores
or clipping manipulations suggested that
Centaurea
was capable of equal or overcom-
pensation to tissue loss by herbivory. For a species capable of producing allelopathic
chemicals, compensation to low levels of root herbivory resulted in a chemical release
or changes in root exudates that suppressed competing vegetation (Callaway et al.
1999; Thelen et al. 2005). At a minimum, these experiments suggest that the relative
fitness of
Centaurea
is increased when the fitness of competing vegetation is reduced.
However, the ability of plants to compensate for herbivory is mediated through relative
plant growth rate (RGR) responses (Hilbert et al. 1981). When a plant's RGR is con-
strained by resource availability, caused either by direct resource limitations or indirect
limitations generated by biotic effects (competition or additional herbivory), overcom-
pensation cannot occur. Under reduced resource conditions, the removal of tissue can-
not stimulate increased photosynthesis or materials allocation to damaged areas.
Further, the particular focal resource limiting plant growth, and information on how
herbivory affects a plant's ability to use that resource, may be necessary for under-
standing these compensation responses under different conditions (Wise and
Abrahamson 2007). A study by Steinger and Müller-Schärer (1992) demonstrated that
spotted knapweed can compensate for root herbivory by
Agapeta zoegana
(L.)
(Lepidoptera: Cochylidae) moth larvae, but not for herbivory by
Cyphocleonus ach-
ates
(Coleoptera: Curculionidae) weevil larvae, and that the ability to compensate
increased under higher plant resource conditions. A second study confirmed this com-
pensatory response; Newingham et al. (2007) found increased nitrogen allocation to
aboveground tissues after root damage from the
Agapeta
moth larvae. These two stud-
ies test compensation responses by spotted knapweed to herbivory; however, experi-
ments that demonstrate the possible degrees of compensation to both belowground and
aboveground herbivory under variable resource conditions have not been conducted.
A large body of evidence obtained by researchers and managers indicates that the
closely related species, diffuse knapweed, is controlled by insect herbivory across
much (and perhaps all) of its introduced range (Seastedt et al. 2003, 2005, 2007;
Coombs et al. 2004; Myers 2004; Smith 2004). Reports from Montana (Jacobs et al.
2006; Story et al. 2006; Corn et al. 2006) suggest that a similar response may be
occurring for spotted knapweed in the same region where allelopathy was reported as
being an important mechanism for the dominance of this species. There, the declines
are attributed to large numbers of two species of gall flies (
Urophora
spp), and the
root-feeding weevil,
Cyphocleonus achates
, that were released at those sites in 1988.
While reductions in densities of knapweed were being demonstrated in Montana, land
managers and researchers working in Minnesota and Colorado reported a similar
result using a suite of all available herbivore insects. Among those were
Urophora
spp,
Larinus minutus
Gyllenhal (Coleoptera: Curculionidae), and
Cyphocleonus ach-
ates
(Cortilet and Northrop 2006; Michels et al. 2007) (Fig. 11.1).
A very reasonable question to ask is, if the suite of biological control agents that
currently exist in the USA have all been approved and released since 1991, why
have not we seen more evidence for a negative effect on spotted knapweed? Given
the response we have observed of diffuse knapweed to insect herbivores, we sug-
gest that successful biological control of spotted knapweed has been similarly slow
