Impact of invertebrates on three aquatic macrophytes: American pondweed, Illinois pondweed, and Mexican water lily

The objective of this study was to investigate the impact of invertebrates on three native macrophytes: American pondweed ( Potamogeton nodosus Poir.), Illinois pondweed ( P. illinoensis Morong), and Mexican water lily ( Nymphaea mexicana Zucc.). Biomass production of the three plant species was measured and compared under two conditions: one with an uncontrolled population of herbivorous invertebrates and one in which most herbivorous invertebrates were removed by an insecticide treatment. The insecticide effectively removed most plant-feeding insects, including those in orders Coleoptera, Diptera, Trichoptera, and Lepidoptera, but did not remove one invertebrate group likely to impact plants, Hemiptera (aphids). Differences in plant biomass due to feeding and nonconsumptive damage by remaining invertebrates were variable and dependent upon plant species. Nontreated samples of Mexican water lily exhibited high levels of insect damage (primarily herbivory), as well as case making and egg deposition, but biomass differences between treatments were not detected. The impacts of invertebrate herbivory and nonconsumptive damage were more pronounced in both pondweed species as nontreated biomass was significantly less than biomass of insecticide-treated pondweeds. Biomass of American and Illinois pondweed was reduced by 40 and 63%, respectively, due to invertebrate herbivory. Invertebrate herbivory, once thought to be insignificant to aquatic macrophytes, was shown to cause substantial biomass reductions in two of the three plant species studied.