Inflorescence Damage by Insects and Fungi in Native Pili Grass ( Heteropogon contortus ) versus Alien Foundation Grass ( Pennisetum setaceum ) in Hawai'i

The success of introduced invaders has often been attributed to their release from natural enemies. We compared rates of seed and ovule destruction by insects and fungal pathogens in an alien invader, fountain grass (Pennisetum setaceum), and a declining native competitor, pili grass (Heteropogon contortus), to determine whether the invader experienced less damage from natural enemies. Inflorescences were sampled on O'ahu from three sites on three dates, and seeds and ovules were inspected for insect damage or pathogen infection. Total seed and ovule destruction was significantly lower in alien fountain grass at all times and sites, with the exception of one sample date on Ka'iwa Ridge when very little damage (<1%) was observed in either species. Total seed and ovule destruction ranged from 0.8 to 5% in fountain grass versus 0 to 61% in pili grass. Most seed and ovule loss in pili grass was caused by infection with the smut fungus Sporisorium caledonicum. Between 5 and 35% of pili grass inflorescences showed signs of smut infection. No fungal pathogens were noted on fountain grass. The low impact of natural enemies on seed production in alien fountain grass, relative to native pili grass, could confer a long-term reproductive advantage to the alien.