Bioefficacy of certain chemical insecticides against rice yellow stem borer (Scirpophaga incertulas Wlk.)

Rice is the major staple food in Jharkhand in India and is threatened by various biotic and abiotic stresses. Among the insect pests that imperil the rice cultivation in Jharkhand, the yellow stem borer (Scirpophaga incertulas Wlk.) is one of the most notorious ones. The present investigation was conducted in Kharif 2018 to study the bioefficacy of certain chemical insecticidal treatments against yellow stem borers in rice measured in terms of mean values of dead heart (% DH) and white ear (%WE). Ten insecticidal treatments, namely, T1 - Flubendiamide 240 SC + Thiacloprid 240 SC (combination product) @ 200 ml/ha, T2 - Flubendiamide 240 SC + Thiacloprid 240 SC (combination product) @ 250 ml/ha, T3 - Imidacloprid 17.8 SL @150 ml/ha, T4 - Flubendiamide 480 SC @ 50 ml/ha, T5 - Rynaxypyr 20 SC @150 ml/ha, T6 - Fipronil 80 WG @ 65 g/ha, T7 - Dinotefuran 20 SG @ 200 g/ha, T8 - Fipronil 0.3 GR followed by Chlorpyriphos 20 EC @2000 ml/ha, T9 - Chlorpyriphos 20 EC@ 2000 ml/ha and T10 - untreated control  were taken for the experiment. The results showed that two of the insecticidal treatments,namely T2- flubendiamide 240 SC + thiacloprid 240 SC @ 250 ml/ha (2.07% DH and 2.30% WE)  and T4 - flubendiamide 480 SC @ 50 ml/ha (1.89% DH and 2.49% WE), were at par with each other. They performed better than all the other treatments, suggesting that using these two newer insecticidal treatments could provide all-around protection for the rice crop against rice yellow stem borer in terms of both dead heart and white ear.

Susceptibility baselines for the invasive mealybugs Phenacoccus manihoti and Paracoccus marginatus (Hemiptera: Pseudococcidae) in cassava ecosystem against selected neonicotinoid insecticides

In recent years, an invasive cassava mealybug Phenacoccus manihoti has been threatening cassava cultivation alongside another invasive papaya mealybug Paracoccus marginatus which invaded the country more than a decade ago. In order to evaluate their responses against the commonly used neonicotinoid insecticides: thiamethoxam 25 WG and imidacloprid 17.8 SL,  acute toxicity experiments to determine the susceptibility baselines in populations of two invasive mealybugs in the cassava agro-ecosystem, namely, cassava mealybug P. manihoti and papaya mealybug P. marginatus were performed upto 15 generations. A systemic uptake method was used for the bioassay. The LC50 values of thiamethoxam for F1 generation were 3.298 ppm whereas it was 1.066 ppm for F15 in cassava mealybug. The LC50 values of F1 generation were 2.014 ppm and that of F15 generation was 1.384 ppm when tested with imidacloprid. In the case of papaya mealybug, the LC50 values ranged from 6.138 ppm (F1) to 2.503 ppm (F15) for thiamethoxam and 7.457 ppm (F1) to 3.231 ppm (F15) for imidacloprid. All the susceptibility indices calculated were less than threefold. The rate of resistance development was negative in all cases showing that none of the tested populations harboured any resistance without insecticidal selection pressure. Tentative discriminating doses were fixed for both chemicals with the help of LC95 values obtained from the bioassay experiments, namely five ppm for both thiamethoxam and imidacloprid in the case of cassava mealybug and 10 ppm and 15 ppm, respectively, for thiamethoxam and imidacloprid in the case of papaya mealybug.