Impact of Direct Contact and Ingestion of Selected Insecticides on the Predator Harmonia axyridis of Citrus Psyllids

The Asian citrus psyllid, <i>Diaphorina citri</i>, serves as the primary vector for Huanglongbing (HLB) by transmitting <i>Candidatus</i> Liberibacter asiaticus. Lambda-cyhalothrin and thiamethoxam are commonly employed for the control of <i>D. citri</i>. The multicolored Asian lady beetle, or harlequin ladybird, <i>Harmonia axyridis</i>, is an important predator of <i>D. citri</i> in both greenhouse and field settings. The effectiveness of integrated pest management (IPM) relies on the judicious use of selective insecticides that minimize harm to natural enemies. However, the effects of these insecticides on <i>H. axyridis</i>' predation of <i>D. citri</i> remain thoroughly unexplored. In this study, we assessed the toxicity of lambda-cyhalothrin and thiamethoxam to <i>H. axyridis</i> and examined their impact on the functional response of this predator to <i>D. citri</i> using direct contact exposure methods. We also evaluated the indirect effects on predator voracity through ingestion exposure. Our results demonstrated that exposure to both insecticides at LC₅₀ concentrations significantly prolonged the developmental durations of <i>H. axyridis</i> larval stages. The type-II functional response model effectively described the prey consumption patterns of <i>H. axyridis</i>, revealing a significant reduction in predation capacity across all life stages, particularly among second instar larvae, which experienced reductions of 85.30% and 88.58% following lambda-cyhalothrin and thiamethoxam treatments, respectively. Furthermore, <i>H. axyridis</i>' predation significantly declined when feeding on <i>D. citri</i> contaminated at LC₅₀ concentrations. These findings indicate that lambda-cyhalothrin and thiamethoxam adversely affect the predation of <i>H. axyridis</i>, both via direct contact and ingestion. Evaluating the potential impacts of these insecticides on <i>H. axyridis</i> is critical for the development of effective IPM strategies targeting <i>D. citri</i>.