Toxicity to the Eri Silkworn of Some Insecticides Used in the Control of Castor-oil Plant Pests.

Abstract

Castor oil plants are commonly attacked by some important pests such as the castor semi-looper, leafhoppers, and scale insects. Cabaryl, cypermethrin, dimethoate, fenvalerate, malathion, methomyl, monocrotophos, oxydemeton-methyl, and trichlorfon are among insecticides recommended for the effective control of the pests. However, these insecticides are also toxic to the eri-silkworm larvae when treated leaves were used to feed them. Toxicity tests were made here in order to find the mean for safety uses of these insecticides in silkworm rearing areas. The above named insecticides were tested against five larval instars of the eri-silkworms using the single dose sandwich-method of feeding. LD50s of the insecticides as determined in micrograms of dosage per one gram of the larval body weight were compared. The second larval instar was found to be the most susceptible stage to methomyl, oxdemeton-methyl, and trichlorfon while the third larval instar was to cabaryl, dimethoate, fenvalerate, malation, and monocrotophos. Only cypermethrin was highest in its toxic to the fourth larval stage. LD50 values of most tested insecticides were distinctly high in the first, fourth and fifth larval stages. Leaves of castor-oil plants in the field which were treated with cabaryl, cypermethrin, dimethoate, methomyl, and oxydemeton-methyl were fed to the second instar larvae of the eri-silkworm at 3, 5, 7, 10, 15, and 21 days after the insecticidal applications. Safety time intervals of above insecticides after the treatment of the leaves to the harvest for the larval feeding, as determined from LT5 values at 24 hours were not less than 18, 36, 6, 16, and 3 days respectively. Monocrotophos and trichlorfon showed less toxic to the silkworm than cabaryl, and malathion was less than dimethoate. Time intervals recommended for monocrotophos, trichlorfon and malathion were 18, 18, and 6 days respectively. Cypermethrin and fenvalerate which are insecticides of the pyrethroid group were highly toxic to the eri silkworm. They are not recommended to use in this case.