Understanding invasions: the rise and fall of diffuse knapweed (Centaurea diffusa) in North America - Invasive Plants: Ecological and Agricultural Aspects - page 113

Figure 2. Relationship between seed and weevil production in seed heads of diffuse knapweed. Each
point represents the mean value of seeds per seed head and weevils per seed head for 12 sites in the
Colorado Front Range area collected during the 2001-2004 interval. (n = 38; not all sites were sam-
pled in all years.)
abundance is moderate-to-low, seed production can be low, moderate or high
(Fig. 2). The difference in seed production likely relates to the amount of defo-
liation experienced by the flowering plants. Larinus minutus over-winters as
adults beneath the knapweed, and they tend to defoliate plants in the spring
following their emergence from the soil. If sufficient defoliation occurs, the
vigor of flowering is suppressed, and weevils apparently lay few eggs on
stressed plants. Such plants produce few seeds and contain few weevils, pro-
viding the low seed-low weevil results seen in Figure 2. The weevils that defo-
liate flowering plants disperse to healthy plants that do produce seeds in those
seed heads not fed upon by the weevil larvae. Those plants produce the mod-
erate seed-low weevil points seen in Figure 2.
Knapweed not subjected to significant weevil herbivory generally produced
4-8 seeds per seed head (Fig. 3). Once weevil populations become estab-
lished, seed production is greatly reduced, both by the direct consumption of
seeds and the reduction in plant vigor caused by the feeding activities of the
adults. While the summer of 2004 was spectacular in terms of rainfall and
plant growth, weevil damage on a seed head basis was about the same as that
seen in previous years (Fig. 3). Seed rain, the amount of seed produced per m 2 ,
was initially above 4,000 seeds per m 2 in 1997, declined to about 700 seeds per
m 2 in 2000 (the year before the large population decline shown in Fig. 1), and
was estimated below 200 seeds per m 2 in 2004 (Fig. 3). The failure for knap-
weed to maintain its flowering stem densities, given these seed inputs, argues