Suppression of the aquatic weed Hydrilla verticillata (L.f.) Royle (Hydrocharitaceae) by a leaf-cutting moth Parapoynx diminutalis Snellen (Lepidoptera: Crambidae) in Jozini Dam, South Africa

Abstract

A classical biological control programme against Hydrilla verticillata (L.f.) Royle (Hydrocharitaceae) was initiated in South Africa following the discovery of extensive mats in Jozini Dam in northern KwaZulu-Natal (KZN) in 2006. However, in mid-2008, high densities of a phytophagous insect, Parapoynx diminutalis Snellen (Lepidoptera: Crambidae) were discovered on the H. verticillata infestation, causing high levels of defoliation and dieback of the weed. In order to quantify the impact of the moth and to assess the long-term threat of H. verticillata to South African water bodies, a monitoring programme was initiated in 2013. Although P. diminutalis did not demonstrate a preference for H. verticillata over a native aquatic plant, Potamogeton schweinfurthii A. Bennett (Potamogetonaceae), moth populations followed a cyclical pattern of abundance, with rapid population increases coinciding with increases in the abundance of H. verticillata. High damage levels to H. verticillata were associated with high densities of immature P. diminutalis, which consistently led to population crashes of H. verticillata. Parapoynx diminutalis appears to have great potential to contribute to the management of H. verticillata in South Africa. However, the moth’s oligophagous habits will probably preclude its intentional introduction into water bodies with H. verticillata where it does not already occur.

Keywords: biological control, cyclical pattern of abundance, insect herbivory, plant-herbivore interactions