Biological control of hygrophila: foreign exploration for candidate natural enemies.
Hygrophila, Hygrophila polysperma (Roxb.) T. Anders (Acanthaceae) is an invasive aquatic weed of lotic habitats in Florida, USA. This rooted submerged or emergent plant is typically found in flowing fresh water channels and structured shorelines. Hygrophila forms dense vegetative stands that occupy the entire water column, interfering with navigation, irrigation and flood control activities. It is listed as a Federal Noxious Weed and a Florida Exotic Pest Plant Council Category I invasive species. A visible increase in the number of water bodies invaded by hygrophila since 1990 suggested that current methods employed to control this weed are inadequate. A previous study confirmed that hygrophila is a good candidate for classical biological control. However, little information was available on natural enemies affecting hygrophila in its native range. Exploratory field surveys were conducted in a range of habitats in India and Bangladesh during 2008 and 2009. In total, 41 sites were surveyed, including 28 sites in the states of West Bengal and Assam, India and 13 sites in Mymensingh, Bangladesh. The geoposition and altitude of each survey site were recorded. Several collection techniques, e.g. hand picking, Berlese funnel extraction, as well as sweep and clip vegetation sampling, were used to collect natural enemies. 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, Crambidae, Lepidoptera) feeding an submerged hygrophila, and a leaf mining beetle (Trachys sp., Buprestidae, Coleoptera) were collected during these surveys. In addition, a very damaging aecial rust fungus was collected.