Influence of Seed Head–Attacking Biological Control Agents on Spotted Knapweed Reproductive Potential in Western Montana Over a 30-Year Period

Abstract

Five insect biological control agents that attack flower heads of spotted knapweed, Centaurea stoebe L. subsp. micranthos (Gugler) Hayek, became established in western Montana between 1973 and 1992. In a controlled field experiment in 2006, seed-head insects reduced spotted knapweed seed production per seed head by 84.4%. The seed production at two sites in western Montana where these biological control agents were well established was 91.6-93.8% lower in 2004-2005 than 1974-1975, whereas the number of seed heads per square meter was 70.7% lower, and the reproductive potential (seeds/m(2)) was 95.9-99.0% lower. The average seed bank in 2005 at four sites containing robust spotted knapweed populations was 281 seeds/m(2) compared with 19 seeds/m(2) at four sites where knapweed density has declined. Seed bank densities were much higher at sites in central Montana (4,218 seeds/m(2)), where the insects have been established for a shorter period. Urophora affinis Frauenfeld was the most abundant species at eight study sites, infesting 66.7% of the seed heads, followed by a 47.3% infestation by Larinus minutus Gyllenhal and L. obtusus Gyllenhal. From 1974 to 1985, Urophora spp. apparently reduced the number of seeds per seed head by 34.5-46.9%; the addition of Larinus spp. further reduced seed numbers 84.2-90.5% by 2005. Path analysis indicated that both Larinus spp. and U. affinis contributed significantly to reduction of seed production over the 30-yr period. Spotted knapweed density may not decrease significantly until the seed bank falls below a critical threshold.

Hand Pulling Following Mowing and Herbicide Treatments Increases Control of Spotted Knapweed (Centaurea stoebe)

“…; Story et al 2008). We observed a reduction in the knapweed seed bank to an average of 68 6 26 m 22 after 3 yr of hand pulling, approaching seed bank densities of 32 to 42 m 22 reported by Davis et al (1993) after 7 yr of suppression of seed production using annual herbicide treatments.…”

“…The reduced knapweed seed bank density observed in our study also was similar to the seed bank density of 52 6 17 m 22 observed after 6 yr of competition with native grasses and 3 yr of annual burning in a previous study in an adjacent area of our study site (MacDonald et al 2007). The mean seed bank density on hand-pulled plots was below the approximate threshold of 160 seeds m 22 where knapweed populations remain low when seed production has been consistently reduced by biological control (Story et al 2008), suggesting that hand pulling provides an effective means of reducing seed bank densities on infested sites. Without continued suppression of seed production, seed bank densities likely will return to the greater than 400 m 22 found on untreated areas in this study, emphasizing the need for multiple years of hand pulling to Hand-pulling means Mowing…”

“…In comparison, Skurski et al (2013) found that a single hand pulling reduced knapweed cover from 10% to less than 2% after 1 yr, but knapweed cover then began to increase in the absence of further treatment. Ultimately, all knapweed life stages including adults, juveniles, seedlings, and the soil seed bank must be controlled when restoring an infested site, because populations can rebound from these life stages even if all adults are initially eliminated by single herbicide or hand-pulling treatments (Duncan et al 2011;Sheley et al 1998;Story et al 2008).…”

Rate of dispersal of spotted knapweed biocontrol beetles (Larinus spp. Curculionidae) in Wisconsin

“…This feeding reduces total number of viable seeds produced, and in turn reduces the seed bank surrounding spotted knapweed populations (Story, 2008). Since each species have similar impacts on spotted knapweed (Wilson and Randall, 2005), and are difficult to distinguish, spatial observation data do not discriminat between the two species.…”

Biological control and precipitation effects on spotted knapweed (Centaurea stoebe): empirical and modeling results

“…This finding suggests that seed reduction and increased mortality of adult plants might not be sufficient to overcome growth from juvenile plants. A further complication when interpreting results is that drought has been suggested as a contributing factor to the observed declines in spotted knapweed populations (Corn et al 2007, Story et al 2008 or as an alternative explanation instead of reductions due to biological controls (Ortega et al 2012). Experimental manipulations have been conducted on many stages of the life cycle; however, integration of these findings to compare how individual effects impact long-term population dynamics has not been undertaken.…”