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Figure 3. Seed production from the insect release site (shaded bars) compared with sites experiencing
little or no herbivory by the seed head weevil,
Larinus minutus (
hatched bars). Weevil densities at the
release site were estimated to be below 0.1 weevil per seed head in 1999, but were above 0.5 per seed
head in subsequent years. Values are means and standard errors of a minimum of 180 seed heads sam-
pled from 30 plants at each site for each of the six years.
for substantial mortality of seeds, seedlings, and rosettes. We do not know the
extent to which that mortality results from plant competition
versus
direct mor-
tality from herbivory and pathogens, but we believe this mortality is likely
important in the sustained low densities of the knapweed. Interestingly, 'wee-
vil rain' (number of adult weevils emerging from seed heads) has been as high
as 2,000 weevils per m
2
. Unless an effective predator or parasite for this spe-
cies appears, knapweed experiencing these densities of herbivores is doomed
to either an early death or very low reproduction.
Harris [30] suggested that the objective of knapweed control should be to
achieve less than 5% cover by the weed on rangelands. This has been accom-
plished in Colorado, and similar results are underway in other regions (e.g.,
[25, 26] and unpublished results). Myers and Bazely [31] make the strong case
that this decline is likely due to the combination of effects that the insects have
on multiple stages of the knapweed lifecycle.
The reduction in knapweed densities in Colorado is attributed largely to
activities of the lesser knapweed flower weevil,
Larinus minutus
. The other
insects present in this study have not been able to control the weed [31], or, as
in the case of
Cyphocleonus achates
, were not particularly abundant during
intervals of knapweed decline (Seastedt, unpublished results). However, these
observations do not exclude the possibility that the addition of the other spe-
cies collectively have more impact on the rate and extent of knapweed decline
than
L. minutus
operating alone. Seedling mortality appears to be a significant
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