Poor cold tolerance of the alligatorweed flea beetle, Agasicles hygrophila, has resulted in inadequate biological control of alligatorweed, Alternanthera philoxeroides, as the weed expands into more temperate climates. The alligatorweed thrips, Amynothrips andersoni, may be more cold tolerant than A. hygrophila, but they have received little attention in the literature. To investigate the value of the thrips as a biological control tool, we (1) surveyed alligatorweed populations to better understand range of A. andersoni in the USA, (2) evaluated the short-term impact of A. andersoni feeding on alligatorweed, and (3) compared cold tolerance of A. andersoni to A. hygrophila. Impact of A. andersoni was measured by infesting shoots of alligatorweed haplotypes Ap1 and Ap6 with low and high densities of adult thrips and allowing populations to increase for one generation before assessing thrips reproduction and host plant growth. Cold tolerance of the two agents was compared using critical thermal minimum (CTmin) and chill coma recovery time. Thrips were detected at 45% of the 20 locations surveyed, including two new state records. In the greenhouse, thrips feeding resulted in a significant shift in biomass from emergent growth to below the water surface, but total biomass was unaffected. The thrips were more cold-tolerant as measured by CTmin, which was 2 degrees C lower than that observed for A. hygrophila. Our results suggest that A. andersoni is better adapted for cooler climates, but their long term impact may depend on their ability to infest new emergent growth over multiple generations.
