<p>Caste determination in social insects is influenced by both innate and socioenvironmental factors. In termites with bifurcated caste pathways—where individuals develop along either the nymphal (alate-reproductive) or apterous (worker-soldier) line—such as <i>Reticulitermes speratus</i>, heritable factors present at the egg stage can bias caste differentiation, while subsequent socioenvironmental influences can further modify caste fate. As hemimetabolous insects, they retain the pluripotency to molt into other castes until they differentiate into their terminal caste. However, whether an individual is able to perceive a particular external stimulus or not is expected to depend on its developmental stage. Here, we demonstrate that in <i>Reticulitermes speratus</i>, the critical period during which individuals are influenced by foster parents occurs during the early first instar stage. A fostering experiment was conducted using parthenogenetically produced larvae whose caste fate (nymph or worker) changes depending on the foster parents, specifically whether they are reared by workers or colony founders. Foster parents altered the caste fate of larvae within 10&#xa0;days post-hatching but had no effect beyond this period. Even under the same foster parent conditions, larvae that hatched earlier were significantly more likely to differentiate into workers, suggesting that parthenogenetic eggs laid during the early foundation stage differ in caste predisposition from those laid later. This study identified the critical period for socioenvironmental influences on the caste fate of offspring, contributing to a deeper understanding of the complex interplay between innate factors and environmental influences in termite caste determination.</p>