Agriculture Reference
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Accordingly, tests at multiple sites and under multiple climatic regimes are war-
ranted to establish the generality of top-down controls on diffuse knapweed.
Here, we use results from studies of insect herbivory on diffuse knapweed
populations in the Colorado Front Range to make inferences about factors con-
trolling invasiveness of diffuse knapweed and its relatives in North America.
We show that top-down controls negate the characteristics that made knap-
weed successful as an invader in Colorado. We also hypothesize that these con-
trols, as represented by the current list of biological control insect species
released in North America, may not be uniformly effective for all of the inva-
sive
Centaurea
species.
Monitoring effects of herbivores on knapweed populations and seed
production
A monitoring site dominated by diffuse knapweed (25-30% of plant cover)
was established in Boulder County, Colorado, USA in 1997. In that year small
numbers of
Sphenoptera jugoslavica
Obenb., (Coleoptera: Buprestidae),
Cyphocleonus achates
Fahraeus (Coleoptera: Curculionidae), both root feed-
ers of rosettes, and
Larinus minutus
Gyllenhal (Coleoptera: Curculionidae),
the lesser knapweed flower weevil, were released at the site. In addition, there
were existing populations of
Urophora quadrifasciata
Meigen, (the knapweed
seed head fly, Diptera: Tephritidae). A second species of fly,
Urophora affinis
Frauenfeld, the banded gall fly, invaded the site from unknown sources by
1999.
Knapweed abundance and reproduction was obtained by counting rosettes,
flowering stems, seed heads per plant, and seeds per seed head. The abundance
of
Larinus minutus
, which reproduces by placing eggs in flowers of the knap-
weed, was also obtained during the inventory of seed heads. Details on data
collection procedures at the Boulder site are reported in Seastedt et al. [24].
In addition to quantifying seed production and weevil abundance at the
above site, additional areas containing large densities of knapweed were sam-
pled for seeds and weevils. Sites included three mountain meadow sites and
eight additional grasslands at distances up to 100 km from the original study
site. Insects were released at these sites in the late 1990s or insects from other
sites eventually colonized the areas. Data on seed and weevil densities in seed
heads reported here were collected during the 2001-2004 interval. With one
exception, counts were based on inspection of 180 seed heads from 30 differ-
ent plants at each site collected in the mid August to mid September interval.
At one site this analysis was limited to 108 seed heads from 18 plants.
The patterns observed for knapweed stem densities at our 1997 release site
show that about four years were required for insect populations to build up to
levels where knapweed densities were significantly reduced (Fig. 1). In con-
trast to the sharp decline in densities observed at this site, other sites not expe-
riencing these levels of insect herbivory continued to maintain high densities
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