The efficacy of grass carp Ctenopharyngodon idella (Cyprinidae) and weevils Neochetina
spp. (Curculionidae) to control the aquatic weed, water hyacinth, is investigated in a square net cage
(happas) setting at a farm in Cuddalore District, South India. This novel combination of insects and fish is
found to be superior to individual treatments for controlling the weed growth within 110 d. The biomass
of the weed, number of plants, percentage of flowered plants and chlorophyll contents were studied. The
weed biomass is reduced from 5 kg (day 1) to 0.33 kg (day 110) when exposed to grass carp and weevils.
The number of plants is reduced to 0.75 in grass carp and weevil exposed happas, while it is 741.5 in the
control. The mean number of leaves per plant is also reduced. In addition, the chlorophyll a and b are
significantly reduced in happas exposed to the combination of fish and insects when compared to the
other treatments. Based on the results of this study, we consider the combined use of grass carp and
weevils to be more efficient and sustainable for managing water hyacinths than the use of these organisms
individually.