Detoxifying Enzyme

INTRODUCTION

  • The study focuses on four aphid species:

    • Sitobion avenae (Fabricius)

    • Rhopalosiphum padi (Linnaeus)

    • Schizaphis graminum (Rondani)

    • Metopolophium dirhodum (Walker)

  • These species are significant pests in wheat and other cereals worldwide.

  • Imidacloprid is highlighted as a commonly used insecticide for controlling these pests.

OBJECTIVES

  • Assess the susceptibilities of the four wheat aphid species to seven insecticides.

  • Determine the relationship between insecticide susceptibility and detoxifying enzyme activities:

    • Carboxylesterase (CarE)

    • Glutathione S-transferase (GSTs)

    • Cytochrome P450 monooxygenase (P450s)

METHODS

Insect Populations

  • Collected populations of aphids were reared under controlled conditions (20 ± 1◦C, 60 ± 10% relative humidity).

Insecticides and Synergists

  • Seven insecticides tested include:

    • Imidacloprid

    • Thiamethoxam

    • Beta-cypermethrin

    • Omethoate

    • Matrine

    • Rotenone

    • Avermectin

  • Three synergists evaluated:

    • Piperonyl butoxide (PBO)

    • Triphenyl phosphate (TPP)

    • Diethyl maleate (DEM)

Bioassay Procedure

  • Aphids were exposed to different concentrations of insecticides using a leaf-dipping method, followed by mortality assessment.

RESULTS

Susceptibility Assessments

  • The susceptibilities of the four aphid species varied significantly to the tested insecticides:

    • M. dirhodum showed the highest tolerance, particularly to imidacloprid.

    • S. avenae and R. padi were more susceptible compared to M. dirhodum.

  • Toxicity difference ratio (TDR) was calculated based on LC50 values.

Enzyme Activity Analysis

  • The activities of detoxifying enzymes were significantly higher in aphid species with greater tolerance to insecticides:

    • Carboxylesterase (CarE): M. dirhodum and R. padi exhibited higher levels compared to S. graminum and S. avenae.

    • Glutathione S-transferase (GSTs): M. dirhodum had the highest activity, indicating a strong detoxification capacity.

    • Cytochrome P450: M. dirhodum and S. graminum showed increased P450 activity post-imidacloprid treatment.

Synergism Studies

  • The combination of insecticides with synergists (e.g., PBO) significantly enhanced efficacy against resistant aphids:

    • PBO led to a 20-fold decrease in LC50 for M. dirhodum when combined with imidacloprid.

DISCUSSION

  • The differential susceptibilities among wheat aphids may be attributed to varying activities of detoxifying enzymes.

  • The research indicates that M. dirhodum could become the dominant aphid species in wheat crops treated with imidacloprid and thiamethoxam.

  • Effective management strategies of wheat aphids must consider the specific resistance mechanisms and variations in enzyme activities for each aphid species.

CONCLUSIONS

  • Overall, the study provides insights into the complex interactions of insecticides and the adaptive responses of wheat aphids, posing implications for pest management strategies in agriculture.