It is widely accepted that sensitivity towards pesticides varies significantly between species. Much less is known about the potential differences in pesticide sensitivity and its biological mechanism throughout the lifecycle of a single species. In the present study we used three life-stages (larvae, pupae and adult) of the holometabolous insect Tenebrio molitor to investigate: i) Life-stage specific differences in sensitivity towards the pyrethroid insecticide α-cypermethrin after topical exposure, and ii) whether these differences can be explained by the degree of uptake and/or excretion. Finally, we investigated if an efficient excretion coincided with higher activities of the detoxifying enzymes cytochrome P450 (P450), esterases (EST) and glutathione-S-transferease (GST). We found that mobility of adults of T. molitor was more affected by α-cypermethrin treatment than larvae and pupae. Mortality was relatively low for all life stages and did not vary significantly with dose within the duration of the experiment, which indicated that death was (at least partly) due to starvation (indirect effect of paralysis) rather that direct effects of the insecticide. Insecticide treatment during the pupal stage further impaired normal development from pupa to adult. Toxicokinetic measurements showed that cuticle penetration of α-cypermethrin differed significantly between life-stages. Approximately 50% of the applied insecticide had penetrated the adult cuticle after 1 h, whereas a maximum of 30% and 16% had penetrated the waxier cuticle of larvae and pupae. Further, the pupal stage lacked the ability to excrete compounds, and hence internal insecticide concentrations in pupae increased or stagnated until emergence of the adult. Finally, quantification of detoxification enzymes showed a markedly higher activity of P450 in adults and larvae compared to pupae. These findings suggest that assessing toxicity and/or risk of pesticides collectively for a species may not be adequate without taking into account the potential sensitivity differences between life stages.