Effects of sub-lethal concentrations of chlorpyrifos and isoprocarb on experimental population of Rhopalosiphum padi

Rhopalosiphum padi (Linnaeus) is a devastating pest insect of wheat around the globe. Chlorpyrifos and isoprocarb have been widely used to control R. padi in the field. Sub-lethal concentrates of chlorpyrifos and isoprocarb could affect the physiology and behavior of R. padi. To evaluate the effects of sub-lethal concentrates of chlorpyrifos and isoprocarb on experimental population of R. padi, the leaf dipping method was used to test acute toxicity of chlorpyrifos and isoprocarb to adult R. padi. In a laboratory experiment, the life-table was used to analyze the effects of two sub-lethal concentrates of chlorpyrifos sublethal (C-LC₂₀, C-LC₃₀) and also two sub-lethal concentrates of isoprocarb sublethal (I-LC₂₀ and I-LC₃₀) on the development and reproduction of R. padi F₀ and F₁ generations. The results showed that compared with the control, average longevity and average fecundity of R. padi F₀ generation significantly decreased after treated with the four sub-lethal concentrates. Average longevity of R. padi F₀ generation decreased respectively by 14.03%, 36.04%, 12.01% and 26.86%, while average fecundity of R. padi F₀ generation decreased respectively by 20.55%, 42.27%, 17.13% and 26.00% after treated with C-LC₂₀, C-LC₃₀, I-LC₂₀ and I-LC₃₀. Also compared with the control, C-LC₂₀, C-LC₃₀, I-LC₂₀ and I-LC₃₀ treatments significantly increased average development period of the larval stage of R. padi F₁ generation by 1.51 d, 1.92 d, 0.9 d and 1.19 d, respectively. However, average longevity of R. padi F₁ generation under the four treatments decreased respectively by 21.62%, 33.68%, 15.51% and 23.14%. Also average fecundity decreased respectively by 21.81%, 37.4%, 14.51% and 29.29%. Compared with the control, sub-lethal concentrates of chlorpyrifos and isoprocarb decreased survival rate, intrinsic rate of increase (r_m), net reproductive rate (R₀), finite rate of increase (λ) and gross reproduction rate (GRR) of R. padi F₁ generation. It also increased the development duration, the mean generation time (T) and the population doubling time (D_t) of R. padi F₁ generation. The current results suggested that the sub-lethal concentrates of chlorpyrifos and isoprocarb decreased population development rate and fecundity rate of R. padi.