Effects of sublethal doses of insecticides and host plant age on Cnaphalocrocis medinalis (Guenee)
1990
Wantana Telvapuchom
Larvae of the rice leaffolder, Cnaphalocrocis medinalis (Guenee), were collected from the IRRI experimental farms and farmer's fields in Laguna, Philippines, survived better on young rice plant age than on older ones. These differences were greater than could be accounted for by variation in feeding rates, lower larval and pupal mortality, higher pupation rates, heaviest pupae, and higher fecundity. Pupal and adult period were, however, not affected by plant age. Pupal weights of larvae being fed on older plants were significantly reduced and appeared to be linearly related. Larvae feeding on older plants had higher mortality. At the same time feeding rate per larval period was also reduced about 2.5 times. Toxicity of cypermethrin, diazinon, and BPMC on the fourth-instar larvae of C. medinalis were obtained using topical application. Cypermethrin was the most toxic, followed by diazinon and BPMC, respectively. The LC50 values of cypermethrin, diazinon and BPMC were found to be 40.69, 259.83, and 9896.63 ppm, respectively, while the LD50 values were computed to be 0.02, 0.13 and 4.95 micro g/larva, respectively. Sublethal doses of cypermethrin on C. medinalis resulted in substantial reduction of larval period, survival, and pupation, but did not affect pupal and adult period and pupal weights. Larval period was reduced from 21.9 to 17.5 days, and pupation rate from 42.2 to 19.8 percent. Sublethal doses of cypermethrin significantly increased larval mortality from 50.5 to 79.0 percent. Diazinon and BPMC, generally, were not affected to C. medinalis development, but higher dose showed tend to stimulated the fecundity. The effect of the treatments on total eggs laid per female was not significant. Acetone was found to significantly reduce larval period and fecundity.
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