Status of aquatic weeds associated with biological control agents in the southern Mozambique rivers

Status of aquatic weeds associated with biological control agents in the southern Mozambique rivers
Silvia F.Langa1 ,2 and Martin P.Hill2
1. Departamento de Ciencias Biologicas, Universidade Eduardo Mondlane, C.Postal 257, Maputo, Mozambique;
2. Department of Zoology and Entomology, Rhodes University, P.O. Box 6140, Grahamstown,v South Africa

The release of biological control agents has reduced the problems caused by many invasive weeds throughout Africa. In Mozambique, the first biological control agents released on the aquatic weeds were Neochetina eichhorniae and Neochetina bruchi to control water hyacinth. Other biological control agents used on aquatic weeds included Stenopelmus rufinasus on A. filiculoides, Neohydronomus affinis on P. stratiotes, and Cyrtobagous salviniae on S. molesta, but nothing is known of their establishment and impact. The aim of this study was to identify the biological control agents associated with water hyacinth, water lettuce, salvinia and red water fern and to evaluate the impact of the biological control agents on the weeds in southern Mozambique rivers. At Maputo, Umbeluzi, Incomati, Limpopo, Inharrime, Govuro and Save, samples were taken once each in the dry season, and in the wet season to measure the damage caused by the biological control agents. It was observed that two arthropods fed on water-hyacinth plants, namely Neochetina eichhorniae and N. bruchi, and they varied from site to site. The number of weevils per plant showed varied in different rivers but those numbers did not show significant seasonal differences. The evidence of damage caused by the weevil N. eichhorniae on the water hyacinth leaves was remarkable only in the Umbeluzi and Incomati rivers, as shown by the relatively higher number of scars on leaf per total number of weevils found in each plant during the two sampling occasions. The number of scars per most recently opened leaf in the Maputo and Limpopo rivers was very low compared to that from the Umbeluzi water hyacinth communities. No Neochetina feeding scars on the leaf most recently opened water hyacinth plants were found in the Govuro and Inharrime rivers. Consequently, water hyacinth in the Maputo, Govuro, Inharrime and Limpopo rivers were healthy, since no weevils were found on the plants, even though there were some old scars on the leaves on plants in the Maputo and Limpopo. Water hyacinth appeared to be in poor health in the Umbeluzi and Incomati rivers; the plants had brownish leaves and, in some places, water hyacinth mats were mixed with grasses. Surveys also showed that the weevil Neohydronomus affinis was found in the studied rivers but at a very low density, too low to effectively control Pistia stratiotes. It was very difficult to find Stenopelnus rufinasus on the Azolla plant and no Cyrtobagous salviniae was found on S. molesta along the studied rivers. This study serves as a baseline of biological control of aquatic weeds in southern Mozambique rivers and should be added to over time. For biological control methods to be effective, it is recommended that the number of biological control agents in the weeds be increased and monitored further.