Tuta absoluta, one of the most damaging pests of tomato crops and other solanaceous plants, has been extensively managed by the use of synthetic pesticides. Unfortunately, the reliance on chemicals has negatively affected human health, biodiversity, and the environment. Therefore, the development of eco-friendly alternatives is highly encouraged to replace hazardous chemical pesticides. Inhibitory cystine knot (ICK) peptides have a leading future as they present a narrow spectrum activity, and remarkable stability, making them promising bio-insecticide modalities against pests of crops of economic importance. In this study, the impact of Pea albumin 1-b (PA1b), a plant ICK peptide extracted from seeds of peas (Pisum sativum, Fabaceae) was investigated through foliar application to be delivered to 2(nd) instar larvae of T. absoluta. However, due to the failure of delivery upon foliar application, microinjections of the peptide into 3(rd) instar larvae were performed to assess the insecticidal activity of PA1b when being into the body cavity of the larvae. Toxicity bioassays were conducted in Petri dishes in a developed experimental arena that allowed us to easily survey the status of the larva and the leaf for up to 18 days. Our study showed that PA1b has a significant toxic effect against T. absoluta when delivered through injection. Further studies must be conducted to optimize the administration of PA1b into the larvae of T. absoluta, investigate the binding sites of the peptide in this pest, and ascertain the insecticidal capacity of such a peptide at an agricultural-relevant scale.