In triatomines, the measurement of fluctuating asymmetry (FA) might help us infer the (non-lethal) effects of insecticides on morphological characters and how these changes can impact activities of utmost importance in vector management. Insects that survive chemical control present morphological alterations that impact their final development. Our work measures for the first time the changes in FA in nymphs and their comparison with the occurrence of these changes in adults before and after a chemical control. We determined the occurrence and quantified the FA of the head as an indicator of stress during development in adults and fifth instar nymphs of Triatoma infestans - one of the main vectors of Trypanosoma cruzi in the southern cone of South America -, to estimate the effect of exposure to insecticide in the different stages. We measured the FA of the heads of peridomestic populations of the Rioja Llanos (Argentina), pre-treatment with insecticide (15 years without chemical treatment) and post-treatment (3–4 months after the application). Our results show lower FA values in nymphs compared with adults. Furthermore, shape FA was higher post-treatment, although with some exceptions depending on sex. According to what was observed, we conclude that the post-treatment population would have differences over the pre-treatment population (FA increases), at least for fifth instar nymphs and adult males. The results are discussed considering the stage of nymphal development where the insecticide was applied, the possible repellence caused by the insecticide, the influence of insects from neighboring habitats, seasonality and the compensatory mechanisms of development, among others. Working with field insects allows us to understand how populations react to an insecticide application, but without leaving aside characteristics of T. infestans populations under natural conditions.
