Reconciling contradictory findings of herbivore impacts on spotted knapweed (Centaurea stoebe) growth and reproduction

Abstract

Substantial controversy surrounds the efficacy of biological control insects to reduce densities of Centaurea stoebe, a widespread, aggressive invasive plant in North America. We developed a graphical model to conceptualize the conditions required to explain the current contradictory findings, and then employed a series of manipulations to evaluate C. stoebe responses to herbivores. We manipulated soil nitrogen and competition in a field population and measured attack rates of a foliage and seed feeder (Larinus minutus), two gall flies (Urophora spp.), and a root feeder (Cyphocleonus achates), as well as their effects on the growth and reproduction of C. stoebe. Nitrogen limitation and competing vegetation greatly reduced C. stoebe growth. L. minutus most intensively reduced seed production in low-nitrogen soils, and removal of neighboring vegetation increased Larinus numbers per flower head and the percentage of flowers attacked by 15% and 11%, respectively. Cyphocleonus reduced flower production and aboveground biomass over two years, regardless of resources or competition. Our results, in conjunction with other published studies, demonstrate that positive, neutral, and negative plant growth responses to herbivory can be generated. However, under realistic field conditions and in the presence of multiple herbivores, our work repudiates earlier studies that indicate insect herbivores increase C. stoebe dominance.

Integral projection model of insect herbivore effects onCirsium altissimumpopulations along productivity gradients

“…First, significant spatial variation in the amount of damage characterizes many insect herbivore-host plant interactions (Parker and Root 1981, Louda 1982, 1983, Louda et al 1987a, b, c, Louda and Rodman 1996, Rand 2002. Second, abiotic resource availability varies across a range of spatial scales in ecosystems and often mediates plants' abilities to tolerate tissue loss (Maschinski and Whitham 1989, Wise and Abrahamson 2005, Knochel and Seastedt 2010. Third, the extent to which plant population sizes are limited by the availability of seed can vary greatly in space (reviewed in Turnbull et al 2000, Maron andCrone 2006).…”

“…For example, variation in abiotic resource availability along productivity gradients can affect plant tissue quality for insect herbivores (Louda et al 1987a, Crutsinger et al 2008, and it can drive large differences in plant compensatory ability (Maschinski andWhitham 1989, Wise andAbrahamson 2005). In montane meadows of Colorado USA, Knochel and Seastedt (2010) showed that attack rate of the flowerhead-feeding weevil Larinus minutus on the invasive weed, Centaurea stoebe, was increased by clipping neighboring vegetation, thereby reducing resource competition, and that tolerance for L. minutus damage was least where soil nitrogen was experimentally reduced. Further, bare ground exposure is emerging as a powerful predictor for where and when plant populations will be seed-limited (Turnbull et al 2000, Maron andCrone 2006); and, bare ground exposure is often correlated with ecosystem productivity.…”

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

“…Biological control insects that have been released to control spotted knapweed include insects that target plant tissues such as the root weevil Cyphocleonus achates, and those which target seed production such as the weevil Larinus minutus and several flies (Urophora spp.) (Sheley et al 1998, Story et al 2008, Knochel and Seastedt 2010. One of the most effective combinations of biological control insects for this plant has been C. achates and L. minutus: collectively these insects increase spotted knapweed plant mortality and reduce seed production , Crowe and Bourchier 2006, Knochel et al 2010bA.…”

“…Values for precipitation are mean with SD in parentheses. Sources for estimates are: 1, Knochel et al (2010a); 2, A. P. Maines, unpublished manuscript; 3, Sheley et al (1998);4, Knochel et al (2010b); 5, Knochel and Seastedt (2010);6, NOAA (2012). F1 is calculated from values in the table.…”

“…This value was used under all scenarios, since biological control should not affect seed bank survival. The estimate for seeds flower À1 was obtained from yearly surveys of seed heads at the field site (Knochel and Seastedt 2010). For the density model, the number of seeds flower À1 was calculated from the average number of seeds when L. minutus and Urophora spp.…”