Evaluating biological control of yellow starthistle (Centaurea solstitialis) in California: A GIS based supply-demand demographic model

The biological control of yellow starthistle (Centaurea solstitialis) by four capitulum-feeding insects (weevils Bangasternus orientalis and Eustenopus villosus, and flies Urophora sirunaseva and Chaetorellia succinea), as affected by plant competition with annual grasses, is analyzed using a weather-driven, physiologically based, age-structured, simulation model. Seed density in the soil seed bank for yellow starthistle and the number of overwintering insects provide between-season links. Seed germination intensity and pattern is determined by the timing and quantity of autumn rains and temperatures, while season-length is mostly determined by soil-water balance. Output from the yellow starthistle systems model was integrated into a geographic information system to examine regional differences in abundance for all species across several ecological zones of California. A marginal analysis of the simulation results was performed to examine the efficacy of each capitulum-feeding insect, their within-capitulum competitive interactions, and the effect of competition from annual grasses on yellow starthistle populations. The model suggests that lack of complete control of yellow starthistle may be due to plant compensation via increased per-plant seed production at lower plant densities and the incomplete destruction of seed in attacked capitula. The model suggests herbivory that reduces the plant's ability to compensate and/or kills whole plants before seed maturity would most likely lead to control of yellow starthistle.