By analysing our previous results concerning a toxic effect of Cd on Scotia segetum D c n. et S c h i f f., Pb on Mamestra brasicae L. and Cu on Scotia segetum and Leptinotarsa decemlineata S a y, we have searched for appropriate criteria to preindicate influence of above mentioned heavy metals on reduction of leaf-eating insect species populations in agroecosystems and natural ecosystems. The biological and ethological expression of intoxicated postembryonal developing stages was evaluated. As a result of Cd, Pb and Cu treatment, the abnormalities in reproductive process were observed, e.g. fecundity decrease and eggs sterility. The lowest limit to manifest the toxic effect of mentioned heavy metals, also confirmed by histological studies on intoxicated ovaries of adults, was determined as follows: 12.64 μg for Cd, 13.7 μg for Pb and 9 μg for Cu, calculated per pupa and imago, respectively. Analysing host plant leaves treated by Cd, Pb and Cu, the lowest limit to express toxicity of particular heavy metals towards the studied insect species were determined as follows: 9 μg of Cd, 11 μg of Pb and 10 μg of Cu, calculated per 1 cm2 of a leaf area.