The effect of petal size manipulation on pollinator/seed-predator mediatedfemale reproductive success of Hibiscus moscheutos

The effects of petal-size manipulations on the behavior of pollinators and pollen/seed predators, and on pollen removal and deposition, were studied i n Hibiscus moscheutos (Malvaceae) populations. The ultimate effects on the female reproductive success of flowers, such as fruit set, seed predation r ate, and final seed set were also measured. We applied three levels of peta l removal (100%, 50%, and 0% size reduction in radius) to flowers in natura l populations. Two pollinators (Bombus pennsylvanicus and Ptilothrix bombif ormis) ignored flowers without petals, suggesting that pollinators use peta ls as a visual cue to locate flowers. Consequently, 100% petal removal redu ced female reproductive success considerably, mainly through a higher rate of fruit abortion due to failure of pollen deposition on stigmas. No signif icant differences between the 50% petal removal treatment and uncut control were detected in any components of female success examined. The results, t herefore, suggest that differences in petal size have little influence on f emale reproductive success of Hibiscus flowers at our study site. Final see d set varied considerably depending on the larval densities of two coleopte ran seed predators (Althaeus hibisci and Conotrachelus fissunguis). A. hibi sci responded to petal size, and a higher density of adults was found in fl owers in which petal size had not been reduced. Because Althaeus feed on po llen as adults and no effect of petal size on seed predation was detected, the preference of Althaeus for larger flowers may represent a foraging stra tegy for adult beetles and may exert counteracting selection pressure on pe tal size through male reproductive success of flowers.