PARENT SESSION
Oral Session #66: Invasions: Plants. Mechanisms, patterns, controls. Presiding: R. Mack.
Thursday, August 9, 2001. 1:00 PM to 4:45 PM. Hall of Ideas E.

Assessing the likely impact of biological control of yellow starthistle: Density manipulation of seedlings.

Rieder, Julie¹, Evans, Edward¹, ¹

ABSTRACT- Biological control is conceptually based on the hypothesis that invaders outcompete natives because they have escaped population-regulating mechanisms imposed by their natural predators. Flowerhead predators introduced for biological control of yellow starthistle (Centaurea solstitialis) are expected to reduce seed production by 60 to 95% (and consequently reduce densities of germinating seedlings). However, even this extreme level of seed predation may not control yellow starthistle populations. To address this concern, we conducted a density manipulation experiment to investigate how reduced yellow starthistle seedling density affects seed production. In northcentral Utah, twelve 1m² plots were created; six randomly selected plots were thinned in early spring to one-third the original density of seedlings, and six were unthinned. The number and stage class of individuals were counted every two weeks from May to September. Plants began flowering in mid-June and continued into September, and ranged in height from 7 to 60cm. Flowerheads were harvested, dissected, and the number of seeds and seed predators per flowerhead recorded. A comparison of thinned and unthinned plots shows that thinning successfully reduced the number of individuals in a plot, and that this trend was observed late into the season. However, a compensatory response to thinning was observed; at the end of the growing season, the number of flowering plants in thinned plots (300/m²) was 50% (not 33%) that of flowering plants in unthinned plots (600/m²).

KEY WORDS: Centaurea solstitialis, knapweed, seedling density, biological control