Lethal and sublethal effects of flonicamid (50 WG) and spirotetramat (240 SC) on Bemisia tabaci (Homoptera: Aleyrodidae): an age-stage two sex life table study

Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) is a sporadic pest of cotton and several other agricultural crops. The expansion of insecticide resistance has compelled the assessment of chemically novel insecticides. Therefore, the lethal effects of systemic insecticides, flonicamid 50 WG (pyridincarboxamide) and spirotetramat 240 SC (ketoenole) were assessed against different life stages (egg, nymph, pupae and adult) of B. tabaci in the laboratory. The sublethal effects of these insecticides in relation to life table parameters of B. tabaci were also investigated. Spirotetramat was most effective against all nymphal instars and pupae but was relatively less toxic against adult stage of B. tabaci compared to flonicamid. Whereas, flonicamid was highly effective against adult, less effective against nymph and least effective against pupae in comparison to the efficacy of spirotetramat. Flonicamid and spirotetramat possessed weak ovicidal potential at all different tested concentrations. LC₅₀ values (72.96, 76.14 and 85.51 ppm) revealed two-fold toxicity of spirotetramat as compared to LC₅₀ values (123.40, 133.43 and 178.56 ppm) of flonicamid against 1st, 2nd and 3rd instar-nymph, respectively. Flonicamid was 54 folds more toxic against B. tabaci adults with LC₅₀ value of 3.99 ppm than that of spirotetramat (218.36 ppm). With sublethal concentration of flonicamid and spirotetramat, the intrinsic rate of increase of population per day (r = 0.080 and 0.093, respectively) and estimated finite rate of increase in population (λ = 1.083 and 1.097, respectively) were considerably decreased than control (r = 0.157 and λ = 1.170). However, the values of life table parameters (r and λ) were significantly smaller after exposure of flonicamid than spirotetramat. The net reproductive rate (R₀₌ offspring/ female) and mean generation time (T = days) of female B. tabaci were significantly reduced (7.085 and 21.03, respectively) with spirotetramat than those of flonicamid (40.88 and 46.11, respectively). Flonicamid extended the developmental duration of immature stages and the emergence of female and male (sxj: 34 and 39 days, respectively) compared to Spirotetramat. Spirotetramat significantly decreased the rate of age stage survival (lx), age-stage specific fecundity (fxj), age-specific fecundity (mx) and age-specific maternity (lxmx) compared to flonicamid and control treatment. The male and female treated with spirotetramat survived (life expectancy: exj) longer (47 and 37 days, respectively) than flonicamid (26 and 12 days, respectively). The least vxj reproductive curve for females was observed with spirotetramat (11.83) at 16th day compared with control (24.46) at 16th day and the highest with flonicamid (66.50) at 35th day of age. This study is the first stride towards establishing a scientific base for the efficient use of tested insecticides for pest control.