Biological control of Hygrophila: results of native range surveys

Hygrophila, Hygrophila polysperma (Roxb.) T. Anderson (Acanthaceae) is an invasive aquatic plant in the Southeastern United States. It is listed as a Federal Noxious Weed and a Florida Exotic Pest Plant Council listed Category I invasive species. This weed is typically found in flowing fresh water channels and structured shorelines as a rooted submerged or emergent plant. The dense vegetative strands formed by hygrophila can occupy the entire water column, affecting navigation, irrigation and flood control activities. Recent experiences in south Florida indicated that practical solutions for long term control of this plant are not currently available. Classical biological control was thought to be an effective option to manage the spread of this invasive weed. As a part of our ongoing efforts for biological control of this aquatic weed, exploratory field surveys were conducted in India (n=42 sites) and Bangladesh (n=13 sites). Natural enemies were collected using several collection techniques, e.g. hand picking, Berlese funnel extraction, as well as sweep and clip vegetation sampling. A number of insects, including two caterpillars (Precis alamana L., Nymphalidae and an unidentified Noctuid moth, Lepidoptera) that defoliate emerged plants, an aquatic caterpillar (Parapoynx bilinealis Snellen, Lepidoptera: Crambidae) feeding on submerged hygrophila, and a leaf mining beetle (Trachys sp. Coleoptera: Buprestidae), were collected during these surveys. In addition, a very damaging aecial rust fungus also was collected. The biological control potential of the crambid caterpillar P. bilinealis and the buprestid leaf-miner Trachys sp. will be investigated in future.