ESTABLISHMENT OF NEOCHETINA SPP.: THEIR PATTERN OF LOCAL DISPERSAL AND AGE STRUCTURE AT THE RELEASE SITE

Abstract

Study on the distribution pattern and age structure ofNeochetina spp. (Coleoptera: Curculionidae) at the release site was conducted to know a) the distribution pattern of the weevil, b) its establishment status, c) its survival rate in the field, and d) relationship between the attack of the weevil and the fungus, Altemaria eichhorniae Nag Rag & Ponappa (Hyphomycetes), in causing damage to water hyacinth. This study was conducted at Situ Bagendit lake, Garut, West Java. A release and recapture method was employed to study the mode and rate of dispersal of the weevils under field conditions. Regular sample collection at two-month interval was done to evaluate the pattern of distribution and to assess the age structure under field condition. Another two months regular observation was done to assess damage severity due to adult weevils and the fungus, A. eichhorniae, on water hyacinth. Results showed, that the weevils seem to disperse actively to all directions following the presence of water hyacinth. The data also showed that the dispersal rate of the weevils was about a few meters a week. Under field conditions at Situ Bagendit lake, the weevils were about evenly distributed throughout water hyacinth mass. The density of the weevils fluctuated from time to time, but the trend slightly increased. The survival rate of the weevils at Situ Bagendit lake was estimated not more than 5%. It was suspected that various limiting factors such as various predators have caused the low population increase under field conditions. The population increase through time confirmed that the weevils have established at Situ Bagendit lake. Field data showed that there were no interaction between the damage severity of the weed caused by both weevils and the fungus.