Hormetic effects of thiamethoxam on Schizaphis graminum: demographics and feeding behavior

In agroecosystems, insects contend with chemical insecticides often encountered at sublethal concentrations. Insects' exposure to these mild stresses may induce hormetic effects, which has consequences for managing insect pests. In this study, we used an electrical penetration graph (EPG) technique to investigate the feeding behavior and an age-stage, two-sex life table approach to estimate the sublethal effects of thiamethoxam on greenbug, Schizaphis graminum. The LC₅ and LC₁₀ of thiamethoxam significantly decreased longevity and fecundity of directly exposed adult aphids (F₀). However, the adult longevity, fecundity, and reproductive days (RP_d)-indicating the number of days in which the females produce offspring - in the progeny generation (F₁) exhibited significant increase when parental aphids (F₀) were treated with LC₅ of the active ingredient. Subsequently, key demographic parameters such as intrinsic rate of increase (r) and net reproductive rate (R₀) significantly increased at LC₅ treatment. EPG recordings showed that total durations of non-probing (Np), intercellular stylet pathway (C), and salivary secretion into the sieve element (E1) were significantly increased, while mean duration of probing (Pr) and total duration of phloem sap ingestion and concurrent salivation (E2) were decreased in F₀ adults exposed to LC₅ and LC₁₀. Interestingly, in the F₁ generation, total duration of Np was significantly decreased while total duration of E2 was increased in LC₅ treatment. Taken together, our results showed that an LC5 of thiamethoxam induces intergenerational hormetic effects on the demographic parameters and feeding behavior of F₁ individuals of S. graminum. These findings have important implications on chemical control against S. graminum and highlight the need for a deeper understanding of the ecological consequences of such exposures within pest management strategies across the agricultural landscapes.