Biology Reference
In-Depth Information
Fig. 11.1
Biological control fly,
Urophora affinis
, and the weevil
Larinus minutus
ovipositing
into spotted knapweed flower heads. On the right is
Cyphocleonus achates
on a diffuse knapweed
plant in a prairie near the Colorado foothills (USA) (photo courtesy of David G. Knochel)
to manifest itself due to (1) the failure to find the right combination of control
insects (Müller-Schärer and Schroeder 1993), (2) the tendency of insects to disperse
or require a substantial time period before increasing to high densities at individual
sites, (3) other management activities have interfered with the efficacy of the
insects, and (4) lag effects were generated by the availability of a large and persist-
ent seed bank established prior to the increase in herbivore release and activity.
At our research site, we released the insects discussed here against a population of
spotted knapweed and began monitoring indices of spotted knapweed abundance in
2002, a year when our site was impacted by a severe drought (Seastedt et al. 2007).
We have followed stem densities and seed production of this population through the
present (Table 11.1). Knapweed seed production declined since 2003 and we inter-
preted this to suggest that insects released in 2001 were initiating top-down con-
straints, similar to that documented for diffuse knapweed. Our highest seed production
numbers in 2004 and 2007 are still well below those reported by Schirman (1981),
where spotted knapweed grew in areas lacking top-down pressure from herbivory.
The damage by
Urophora spp
. and
Larinus minutus
consuming seed and above-
ground tissues from the majority of the plants at out site, as well as the effects of the
Cyphocleonus achates
weevil feeding on root tissue, suggest that this combination of
insects may provide success in biological control. The abundance of all released
insects has increased since 2001, and experiments are under way to determine if the
insects reach high enough densities to continue to push the spotted knapweed popula-
tion into steady decline. Blair et al. (2008) compared herbivore loads on both
C. dif-
fusa
and
C. stoebe
in North America and Europe, and found that
C. stoebe
has largely
escaped root herbivores but not seedhead feeders in the introduced range. The root
herbivores may thus simply require greater time and assistance to become widely
established or effective on spotted knapweed populations. In a study by Pokorny
et al. (2005), 2,000 spotted knapweed seeds per m
2
were added to areas of manipulated
