Toxicity and sublethal effects of Pymetrozine insecticide on population growth parameters and detoxifying enzymes activity of Schizaphis graminum Rondani (Hemiptera: Aphididae)

The Greenbug, Schizaphis graminum Rondani (Hemiptera: Aphididae), is one of the most important pests in grain, especially wheat, and is an important cause for transferring a number of pathogenic viruses in plants. The purpose of this research was to evaluate the lethal and sublethal effects of Pymetrozine insecticide on life history parameters and α-esterase and β-esterase enzymes activity of S. graminum. Detoxifying enzymes play a vital role in the detoxification of chemical compounds in many living organisms. These enzymes increase the resistance of insects to chemical pesticides by the catalysis of these compounds in their body.

The S. graminum was reared on the wheat host in a growth chamber set at 25 ± 2 °C, 60 ± 5 % RH, plus 16: 8 (L: D) hour photoperiods. Daily observations and records were made for the development period and survival rate. The crude data were analyzed based on age-stage and two-sex life table analysis. TWO SEX-MSChart was used to evaluate the raw data based on the age-stage and two-sex life table. In addition, alpha-naphthyl (α-NA) and beta-naphthyl (β-NA) acetates were employed as substrates for the α-esterase and β-esterase activity, respectively. Esterase enzymes of tested treatments were measured using a microplate reader.

Exposing adults to bioassays indicated that the lethal concentration (LC50) of insecticide treatment in S. graminum adults was 84.68 mg a.i./L. The present study demonstrated that Pymetrozine insecticide showed more acute toxicity on adult stages of S. graminum. Moreover, exposure of adults to sublethal concentrations (LC30) negatively affected the development and reproductive characteristics and demographic factors of the Greenbug. In specific, results for sublethal experiments showed a significant increase in the development time of the pest on the insecticide treatment compared with the control. On the contrary, the lowest fecundity in pests was observed on Pymetrozine insecticide. In addition, the tested insecticide significantly reduced the population growth parameters of S. graminum, which can be identified using detoxifying enzymes as biochemical markers of the pest. Estimation values for the intrinsic rate of increase (r) in the control and insecticide treatment were 0.394 and 0.280 per day, respectively. Furthermore, the activity of alpha and beta esterase was not significantly different between control and insecticide treatment.

The results revealed that Pymetrozine insecticide exerts high lethal and sublethal effects on S. graminum, and thus, it is recommended to be applied in an integrated pest management program (IPM). Semifield and field studies are needed to obtain more applicable results using insecticides.