EFFECT OF PACLOBUTRAZOL AND WATER-HYACINTH WEEVIL (NEOCHETINA-EICHHORNIAE) ON PLANT-GROWTH AND LEAF DYNAMICS OF WATER-HYACINTH (EICHHORNIA-CRASSIPES)

Waterhyacinths grown outdoors were treated with combinations of 0 or 2 .2 kg ai ha-1 of the plant growth retardant paclobutrazol and 0, 25, o r 250 waterhyacinth weevils per culture tank under conditions of low ( 25% coverage) or high (100% coverage) plant densities. The low plant d ensity treatments simulated a regrowth or colonization situation. Weev il feeding activity, canopy height, plant volume, leaf production, and leaf mortality were monitored biweekly for 22 weeks or until all plan ts had died. In high plant densities, weevil infestations increased le af mortality rates while decreasing production rates, causing eventual plant death. At low plant densities, waterhyacinth plants partially c ompensated for weevil feeding by increased leaf production rates, thus enabling them to survive until the end of the study. Paclobutrazol re duced canopy height for about 3 months but had little effect on weevil feeding intensities. Combinations of weevils and paclobutrazol gave c omplete control of waterhyacinths regardless of plant densities. The c ombined effects were synergistic, with accelerated leaf mortality rate s exceeding production rates leading to early plant death. This study demonstrated the potential for increasing effectiveness of waterhyacin th weevils using integrated strategies. Nomenclature: Paclobutrazol, l )-4,4-dimethyl-2-(1,2,4-triazol-1-yl)pentan-3-ol; waterhyacinth weevil s, Neochetina eichhorniae Warner (Coleoptera: Curculionidae); waterhya cinth, Eichhornia crassipes (Mart.) Solms #3 EICCR.