Benefits and constraints on plant defense against herbivores: Spines influence the legitimate and illegitimate flower visitors of yellow star thistle, Centaurea solstitialis L-(asteraceae)

The evolutionary ecology of plant defenses against herbivores is generally understood ill a cost-benefit framework. We studied the ecological conseque nces of the spines of Ccn taurea solstitialis L. (yellow star thistle) whic h are typically thought of as defenses against mammalian herbivores. By exp erimentally removing spines, which consist of dead tissue, we show that "ne ctar robbing" lepidopteran all visitors are deterred by naturally occurring spines (a benefit). Unlike the lepidoptera, legitimate pollinators (bees a nd flies) were not more likely to visit spineless flower heads, although be es and flies did spend 20% more time per visit on spineless flower heads. O ne 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 t he percentage of Riled seeds of spineless flower heads compared to, spined controls. Thus, spines of yellow star thistle mall not only deter mammalian herbivory, but also deter lepidoptera which are illegitimate flower visito rs. Although the spines may be energetically costly (not measured in this e xperiment), or costly because of other ecological trade-offs, it appears th at these costs are outweighed by the benefits of spines as a plant defense.