Short communications A quantitative post-release evaluation of biological control of water lettuce, Pistia stratiotes L. (Araceae) by the weevil Neohydronomus affinis Hustache (Coleoptera: Curculionidae) at Cape Recife Nature Reserve, Eastern Cape Province, South Africa

Water lettuce, Pistia stratiotes L. (Araceae) is recognized as being among the world’s worst aquatic weeds. In its adventive range, the plant forms extensive mats capable of blocking navigation channels, impeding water flow in irrigation and flood control canals, and disrupting hydropower generation (Holm et al. 1977). Dense mats of the weed prevent light penetration into the water column which negatively affects submerged aquatic plant communities, causing a lowering of the oxygen concentration and thereby reducing benthic invertebrate and fish populations (Neuenschwander et al. 2009). Biological control of water lettuce has been highly successful in most areas around the world through the introduction of the weevil, Neohydronomus affinis Hustache (Coleoptera: Curculionidae) (Neuenschwander et al. 2009). This agent has been successfully released in at least 10 countries: Australia, Benin, Botswana, Ghana, Papua New Guinea, South Africa, Senegal, United States of America, Zambia and Zimbabwe (Julien & Griffiths 1998). However, information on the impact of the agent on weed populations has largely been anecdotal and seldom quantified. Adult N. affinis are small (3 mm long) are bluish grey to brown (depending on the age) with dense scales on the elytra. The eggs are cream and spherical (0.3 mm × 0.4 mm). Females chew a hole in the water lettuce leaf (usually on the upper surface), deposit a single egg and close the hole with frass. The eggs hatch within about four days at temperatures above 24 °C (Center et al. 2002). The insect goes through three larval instars that take between 11 and 14 days during which time they mine the leaves, causing severe damage to the plants, and they then pupate in the leaves. Adult feeding causes small shot-holes in the younger leaves. Generation time, depending on temperature, is 4–6 weeks (Neuenschwander et al. 2009). Neohydronomus affinis was first introduced into South Africa in 1985 onto a water lettuce infestation on Nhlangaluwe Pan in the Kruger National Park (Cilliers 1987). Since then the weevil has been released on, or has spread to most water lettuce infestations in the country (Coetzee et al. 2011). The control of water lettuce has been highly successful in South Africa (Cilliers 1991) as it has in most parts of the world (Neuenschwander et al. 2009), and in many areas the weed is no longer considered problematic. Although biological control is considered the most sustainable method for reducing populations of this weed to below an economic and/or ecological threshold, in small, eutrophic water bodies in temperate areas, biological control has been less successful (Hill 2003). In these systems, although insect populations are high, the plants are able to tolerate herbivory through rapid leaf turnover facilitated by high levels of nitrates and phosphates (Ripley et al. 2006). Biological control of floating aquatic weeds is traditionally less successful where the plants are growing in conditions of high nutrients in that the weed populations are not reduced below the acceptable level of control (Room & Thomas 1985; Coetzee & Hill 2012), or as was the case with water lettuce control on Sunset Dam in the Kruger National Park, the time taken to achieve control is longer (five to seven years) than in oligothrophic systems (one to two years) (Cilliers 1991). Water lettuce was first noticed on the wastewater treatment settlement pond at the Cape Recife Nature Reserve, Port Elizabeth, Eastern Cape Province (34°01’11.9”S 25°41’18.7”E) in March 2002, and within two months it had completely *To whom correspondence should be addressed. E-mail: m.p.hill@ru.ac. za