Effect of Paclobutrazol Application on Enhancing the Efficacy of Nitenpyram against the Brown Planthopper, Nilaparvata lugens

Abstract

The brown planthopper (BPH), Nilaparvata lugens, is one of the most destructive rice pests in Asia. It has already developed a high level of resistance to many commonly used insecticides including nitenpyram (NIT), which is a main synthetic insecticide that is used to control BPH with a much shorter persistence compared to other neonicotinoid insecticides. Recently, we found that an exogenous supplement of paclobutrazol (PZ) could significantly enhance the efficacy of NIT against BPH, and the molecular mechanism underlying this synergistic effect was explored. The results showed that the addition of a range of 150-300 mg/L PZ increased the toxicity of NIT against BPH with the highest mortalities of 78.0-87.0% on the 16th day after treatments, and PZ could also significantly prolong the persistence of the NIT efficacies. Further investigation suggested that PZ directly increased the content of flavonoids and H₂O₂ in rice and increased the activity of polyphenol oxidase, which might be involved in the constitutive defense of rice in advance. Additionally, there was an interaction between PZ and BPH infestation, indicating that PZ might activate the host defense responses. Therefore, PZ increased the efficacy of NIT against the brown planthoppers by enhancing the constitutive and inducible defense responses of rice. Our study showed for the first time that PZ could contribute to improving the control effects of insecticides via inducing the defense responses in rice plants against BPH, which provided an important theoretical basis for developing novel pest management strategies in the field.

Keywords: Nilaparvata lugens; hydrogen peroxide; nitenpyram; paclobutrazol; synergetic effect; total phenolic.

Figure 1

Toxicity of the chemical mixtures containing different PZ concentrations against BPH nymphs. The bioassay was conducted on 12th day after the host plants were dipped and the mortalities were recorded after 96 h. Data are presented as the means ± S.E. for five independent replicates. The bars with different small letters indicate significant differences among different treatments (p < 0.05, Tukey’s multiple range test).

Figure 2

Toxicity comparison of NIT and its mixture containing PZ against BPH nymphs within 4 weeks after chemical treatments. The bioassays were conducted at 0, 6, 12, 18 and 24 d after rice plants were dipped, and the mortalities were recorded at 24, 48 and 96 h after the beginning of bioassays. The dotted lines and arrows represent the beginning of the independent test bioassays. Data are presented as the mean ± S.E. for five independent replicates. Asterisks indicate significant differences between the treatments of NIT and NIT + PZ at the same time point (* p < 0.05; ** p < 0.01; *** p < 0.001; Student’s t-test).

Figure 3

The level of total phenolic, flavonoids and malonaldehyde (MDA) in different treatment plants. Mean levels (+SE, n = 5) of total phenolic (A), flavonoids (B) and MDA (C) in stem of rice plants at 96 h after infestation under different treatments for 12th day. The bars with different small letters indicate significant differences among different treatments (p < 0.05, Tukey’s multiple range test).

Figure 4

The level of H₂O₂ in different treatment rice plants. Mean levels (+SE, n = 5) of H₂O₂ in stem of rice plants at 6 (A) and 12 (B) h after infestation under different treatments at 12th day. The bars with different small letters indicate significant differences between different treatments (p < 0.05, Tukey’s multiple range test).

Figure 5

Mean levels (+SE, n = 5–6) of POD (A) and PPO (B) in stem of rice plants at 96 h after infestation under different treatments at 12th day. The bars with different small letters indicate significant differences between different treatments (p < 0.05, Tukey’s multiple range test).