ILLEGITIMATE FLOWER VISITORS OF YELLOW STAR THISTLE, CENTA UREA SOLSTITIALIS L. (ASTERACEAE)

The evolutionary ecology of plant defenses against herbivores is generally under- stood in a cost-benefit framework. We studied the ecological consequences of the spines of Cen- taurea solstitialis L. (yellow star thistle) which are typically thought of as defenses against mam- malian herbivores. By experimentally removing spines, which consist of dead tissue, we show that "nectar robbing" lepidopteran visitors are deterred by naturally occurring spines (a benefit). Unlike the lepidoptera, legitimate pollinators (bees and flies) were not more likely to visit spineless flower heads, although bees and flies did spend 20% more time per visit on spineless flower heads. One potential cost of having spines is the lower time per visit of the bees. The net result of spine removal from flower heads was a 22% reduction in the percentage of filled seeds of spineless flower heads compared to spined controls. Thus, spines of yellow star thistle may not only deter mammalian herbivory, but also deter lepidoptera which are illegitimate flower visitors. Although the spines may be energetically costly (not measured in this experiment), or costly because of other ecological trade-offs, it appears that these costs are outweighed by the benefits of spines as a plant defense.