Slow resistance evolution to neonicotinoids in field populations of wheat aphids revealed by insecticide resistance monitoring in China

Thiamethoxam-Induced Intergenerational Sublethal Effects on the Life History and Feeding Behavior of Rhopalosiphum padi

Thiamethoxam, a second-generation neonicotinoid insecticide is widely used for controlling sap-sucking insect pests including Rhopalosiphum padi. The current study aimed to investigate the life-history parameters and feeding behavior of R. padi following treatments with sublethal concentrations of thiamethoxam. The lethal concentration 50 (LC50) value of thiamethoxam against adult R. padi was 11.458 mg L-1 after 48 h exposure. The sublethal concentrations of thiamethoxam (LC5 and LC10) significantly decreased the adult longevity, fecundity, and reproductive days in the directly exposed aphids (F0 generation). In the progeny R. padi (F1), the developmental durations and total prereproductive period (TPRP) were decreased while the adult longevity, fecundity, and reproductive days (RPd) were increased at both thiamethoxam concentrations. The demographic parameters including the net reproductive rate (R0), intrinsic rate of increase (r), and finite rate of increase (λ) were prolonged only at the LC5 of thiamethoxam. The EPG results indicated that the sublethal concentrations of thiamethoxam increases the total duration of non-probing (Np) while reducing the total duration of E2 in directly exposed aphids (F0). Interestingly, the E2 were significantly increased in the progeny generation (F1) descending from previously exposed parental aphids (F0). Overall, this study showed that thiamethoxam exhibited high toxicity against directly exposed individuals (F0), while inducing intergenerational hormetic effects on the progeny generation (F1) of R. padi. These findings provided crucial details about thiamethoxam-induced hormetic effects that might be useful in managing resurgences of this key pest.

Emulsifiable oil-formulated Beauveria bassiana competes with imidacloprid for seasonal control of cereal aphids in Zhejiang, China

BACKGROUND

Alternatives to neonicotinoids against cereal aphids are needed to mitigate aphid resistance and non-target effects. The emulsifiable oil formulations of two Beauveria bassiana strains, namely Bb registered as a mycoinsecticide and TBb overexpressing an endogenous virulence factor, were tested for seasonal control of cereal aphids at the elongating (April 7) to milk ripening (May 12) stages of winter wheat crop in Yuhang, Zhejiang. Each of three field trials consisted of blank control and the treatments (three randomized 100-m2 plots per capita) of each fungal strain sprayed biweekly at rates of 1.0 × 1013 and 1.5 × 1013 conidia ha-1 and 10% imidacloprid WP sprayed biweekly at a label rate.

RESULTS

Tiller infestation percentage and aphid density in the 5-week field trials after the first spray were reduced to 18.7-22.4% and 9.1-12.4 aphids per tiller in the fungal treatments, and 12.8-25.3% and 2.8-20.9 aphids per tiller in the chemical treatment, contrasting with 49.2-60.3% and 37.1-108.5 aphids per tiller in the control. Percent control efficacies (±SD) computed with weekly aphid densities over the period averaged 84.0 ± 1.6 and 85.3 ± 1.8 versus 78.0 ± 4.0 and 79.9 ± 3.2 in the high-rate versus low-rate treatments of Bb and TBb, respectively, and 84.5 ± 7.8 in the chemical treatment. Imidacloprid showed faster kill action but more variable efficacy than the fungal treatments throughout the trials.

CONCLUSION

Either Bb or TBb formulation competes with imidacloprid in reducing percent infestation and aphid density. The overall efficacy was significantly higher in the treatments of TBb than of Bb. © 2024 Society of Chemical Industry.

Evaluation of Transgenerational Effects of Sublethal Imidacloprid and Diversity of Symbiotic Bacteria on Acyrthosiphon gossypii

Symbiotic bacteria and hormesis in aphids are the driving forces for pesticide resistance. However, the mechanism remains unclear. In this study, the effects of imidacloprid on the population growth parameters and symbiotic bacterial communities of three successive generations of Acyrthosiphon gossypii were investigated. The bioassay results showed that imidacloprid had high toxicity to A. gossypii with an LC₅₀ of 1.46 mg·L^-1. The fecundity and longevity of the G0 generation of A. gossypii decreased when exposed to the LC₁₅ of imidacloprid. The net reproductive rate (R₀), intrinsic rate of increase (r_m), finite rate of increase (λ), and total reproductive rate (GRR) of G1 and G2 offspring were significantly increased, but those of the control and G3 offspring were not. In addition, sequencing data showed that the symbiotic bacteria of A. gossypii mainly belonged to Proteobacteria, with a relative abundance of 98.68%. The dominant genera of the symbiotic bacterial community were Buchnera and Arsenophonus. After treatment with the LC₁₅ of imidacloprid, the diversity and species number of bacterial communities of A. gossypii decreased for G1-G3 and the abundance of Candidatus-Hamiltonella decreased, but Buchnera increased. These results provide insight into the resistance mechanism of insecticides and the stress adaptation between symbiotic bacteria and aphids.

Resistance Mechanisms of Sitobion miscanthi (Hemiptera: Aphididae) to Malathion Revealed by Synergist Assay

A resistant strain (MRS) of Sitobion miscanthi was cultured by continuous selection with malathion for over 40 generations. The MRS exhibited 32.7-fold resistance to malathion compared to the susceptible strain (MSS) and 13.5-fold, 2.9-fold and 4.8-fold cross-resistance for omethoate, methomyl and beta-cypermethrin, respectively. However, no cross-resistance was found to imidacloprid in this resistant strain. The realized heritability for malathion resistance was 0.02. Inhibitors of esterase activity, both triphenyl phosphate (TPP) and S,S,S,-tributyl phosphorotrithioate (DEF) as synergists, exhibited significant synergism to malathion in the MRS strain, with 11.77-fold and 5.12-fold synergistic ratios, respectively, while piperonyl butoxide (PBO) and diethyl maleate (DEM) showed no significant synergism in the MRS strain. The biochemical assay indicated that carboxylesterase activity was higher in MRS than in MSS. These results suggest that the increase in esterase activity might play an important role in S. miscanthi resistance to malathion. Imidacloprid could be used as an alternative for malathion in the management of wheat aphid resistance.

The effects of acetamiprid multigeneration stress on metabolism and physiology of Aphis glycines Matsumura (Hemiptera: Aphididae)

Aphis glycines Matsumura (Hemiptera: Aphididae) is a major soybean pest that often poses a serious threat to soybean production. In this study, we checked the effects of acetamiprid on redox, energy metabolism, and hormone expression in A. glycines. The LC50 and LC30 of acetamiprid were used to treat the fourth instar nymphs in each generation from F0 to F4 to measure the activity of peroxidase, pyruvate kinase, and trehalase using a microassays approach. The peroxidase activity was significantly higher than control when treated with the LC30 of acetamiprid in F2-F5 generations. The activity of pyruvate kinase was significantly higher, while trehalase activity was substantially lower than control in each generation. Besides, we monitored molting and juvenile hormone expression in soybean aphids using enzyme-linked immunosorbent assay. The juvenile hormone titer of third instar nymphs was significantly higher in the treatment group (F1, F2, F4, and F5), while no effects were noted in the F3 generation. Taken together, the activity of peroxidase and pyruvate kinase in soybean aphid first increased to the peak and then decreased, while the trehalase activity continuously decreased in all generations following exposure to acetamiprid. The juvenile hormone titer was significantly higher, while the molting hormone titer was significantly lower in LC50 -treated aphids than in control. Moreover, the LC30 of acetamiprid increased the molting hormone expression in soybean aphids. These findings indicated a baseline for the effective use of acetamiprid in controlling soybean aphids.