Effect of simulated and actual herbivory on alligator weed, Alternanthera philoxeroides, growth and reproduction

Simulated herbivory has often been used as a substitute for actual herbivory when assessing the effects of herbivores on plants. However, mechanical damage does not always produce the same response as herbivore feeding. Similarity of plant response should be determined before mechanical damage is used as a surrogate for actual herbivory. This study compared two types of mechanical leaf tissue removal with actual herbivory on alligator weed [Alternanthera philoxeroides (Martius) Grisebach (Amaranthaceae)], a significant wetland weed in Australia. We contrasted the effects of simulated herbivory with those of actual herbivory by the alligator weed flea beetle, Agasicles hygrophila Selman and Vogt (Chrysomelidae: Alticinae), on plant growth and reproduction. There was no significant difference in the response of plant biomass to the three herbivory treatments. However, stem length and the number of stem nodes were reduced more by beetle damage than by the two simulated treatments. This was likely caused by preferential feeding on young leaf tissue and/or feeding damage to stem tissue as leaf tissue quality decreased at greater defoliation levels. To more closely mimic alligator weed response to A. hygrophila adult feeding for variables other than biomass, simulated damage methods should consider damage to new growth and stem tissue.