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.

Additive effects of aboveground and belowground herbivores on the dominance of spotted knapweed (Centaurea stoebe)

“…Our general findings of negative impacts of herbivores corroborate with the observation of Story et al (2008) that these insects can have strong negative effects on C. stoebe densities. The cage experiment represented the first study to examine the single and combined effects of above-and belowground herbivores on spotted knapweed, and further, to monitor these effects across a gradient in resource availability.…”

“…(Story et al 2008) but also the negative effects of L. minutus and C. achates feeding on above-and belowground tissues. We exercise caution in scaling up our relatively short-term results from individual plants to longterm trends at the level of a field population, but we believe that the evidence reported here and from recently published field studies substantially improve understanding of above-and belowground herbivory in concert, and provide a probable explanation for some of the documented C. stoebe population declines across North America.…”

“…Further, long time periods are necessary to see the negative effects of insect activities on plant densities (Seastedt et al 2007;Story et al 2008;Knochel and Seastedt 2009). However, we expect the combination of below-and aboveground herbivory and plant competition to provide the cumulative stress needed to diminish the dominance of the aggressive invasive species C. stoebe in North America.…”

“…(Coleoptera: Curculionidae), consumes these critical tissues during bolting, the plant may be more severely limited by an inability to produce adequate photosynthates. These effects would be in addition to the well-studied and large negative effects of L. minutus larvae on seed production (Story et al 2008;Seastedt 2009, 2010;. Likewise, Cyphocleonus achates Fahr.…”

“…These insects have controlled diffuse knapweed across much of its introduced range (Seastedt et al 2003(Seastedt et al , 2007Coombs et al 2004;Smith 2004;Myers et al 2009). However, reports of these species successfully reducing spotted knapweed populations (Corn et al 2006;Cortilet and Northrop 2006;Jacobs et al 2006;Story et al 2006Story et al , 2008Michels et al 2009) are debated (e.g., Pearson and Ortega 2009), and the release of some insect species has led to unintended negative consequences (Pearson and Fletcher 2008).…”