Synergistic and Antagonistic Effects of Insecticides and Heavy Metals on the Bird Cherry–Oat Aphid, Rhopalosiphum padi (L.), Under Laboratory Conditions

Co-exposure to insecticides and heavy metals alters the way the chemicals affect the aphid. This interaction can induce synergism through enzymatic inhibition of the insect, increasing mortality, or antagonism through competition for cellular receptors. The aim of this study was to determine the synergistic or antagonistic effect of four insecticides and five heavy metals on the median lethal concentrations (LC50 and LC90) and their toxic efficiency between 24 and 48 h post-exposure to the bird cherry–oat aphid under laboratory conditions. The four pesticides were acetamiprid, imidacloprid, emamectin benzoate, and thiamethoxam. The five heavy metals were Cr, Ni, Pb, Cd, and Cu. The results revealed LC50 and slope values for all four tested insecticides against the aphid. After 24 and 48 h from treatment, acetamiprid was the most toxic compound according to LC50 values, followed by emamectin benzoate, thiamethoxam, while imidacloprid was the least toxic one. The results indicated that thiamethoxam was synergized by Cr, Cu, Cd, Ni, and Pb by 18.31, 5.36, 74.12, 4.58, and 225.46-fold, respectively, as compared with LC50 values of thiamethoxam alone. This finding indicated that Pb was the most effective heavy metal in synergizing thiamethoxam. The results indicated that the toxicity of imidacloprid increased by adding each of the tested heavy metals except for Cu as compared with LC50 of imidacloprid alone. Results showed that LC50 value of each insecticide decreased with Cd as compared with LC50 value of the insecticide alone at both 24 h and 48 h. Under the conditions of this study, it can be concluded that the Cd acted as a synergist for all tested insecticides.