Effect of some insecticides on the growth of hydrilla (Hydrilla verticillata (L.f.) Royle

Carbaryl, cypermethrin and mevinphos were applied at the rates of 0, 5, 10, 20 and 40 ppm on hydrilla (Hydrilla verticillata (L.f.) Royle) at Department of Entomology, Kasetsart University. All insecticides had the effects on its growth. The plant was rotted and died within 3 days at the rate of 40 ppm of cypermethrin and mevinphos. The water after applied with three insecticides at every rates were turbiness on the 1st day and after the 2nd day, the solution began to sattle. On the 5th week, at the rate of 20 ppm of all insecticides, hydrilla had a thin stem; leaves at 1-2 nodes at the end of fragment were fallen; the end of fragment was a brown color. On the 8th week, carbaryl at the rate of 40 ppm, the length, the node number, the wet and dry weight were 18.77 cm, 36.20 nodes, 97.50 g and 0.015 g, respectively. Cypermethrin at the rate of 20 ppm, the length, the node number, the wet and dry weight were 20.37 cm, 34.60 nodes, 118.00 g and 0.021 g, respectively. Carbaryl and cypermethrin at the rate of 0 ppm, the length, the node number, the wet and dry weight were 21.48 cm, 44.05 nodes, 135.00 g and 0.042 g, respectively. Mevinphos applied at the rate of 20 ppm, on the 8th week, the length, the node number, the wet and dry weight were 21.20 cm, 35.83 nodes, 117.50 g and 0.022 g, respectively. At the rate of 0 ppm of mevinphos application, the length, the node number, the wet and dry weight were 23.77 cm, 40.86 nodes, 175.00 g and 0.078 g, respectively. At the 8th week, pH of water at the rates of 40 and 0 ppm of carbaryl, the rates of 20 and 0 ppm of cypermethrin, the rates of 20 and 0 ppm of mevinphos were 8.40 and 9.20, 8.05 and 9.20, 8.28 and 9.23, respectively. Carbaryl, cypermethrin and mevinphos had the effects on the morphology changed of hydrilla, namely, the stem were pale green color, thin and broken easily; the leaves were fallen; the end of fragments had a brown color.