<p>Leaf anatomical traits can play a pivotal role in mediating plant resistance to herbivory and shaping pest dynamics in agroecosystems. Understanding how leaf anatomical traits shape insect–plant interactions is essential to elucidate these relationships and to develop more sustainable pest management strategies. In this context, we investigated an underexplored leaf structural feature—the vascular bundle sheath extension (BSE). Using tomato (<i>Solanum lycopersicum</i> L.) isogenic lines harboring the <i>obscuravenosa</i> (<i>obv</i>) mutation, we assessed how the presence (+) or absence (−) of BSEs impacts interactions with the tomato pinworm, <i>Phthorimaea</i> (= <i>Tuta</i>) <i>absoluta</i> (Meyrick) (Lepidoptera: Gelechiidae), a major invasive pest of the tomato. The absence of BSEs did not influence oviposition preference or egg incubation time, but it significantly reduced larval leaf consumption and prolonged larval development, without affecting survival. Pupal development was also altered in BSE- plants, resulting in lower body mass and delayed adult emergence, while sex ratio remained unaffected. Phytohormonal profiling revealed differences between BSE+ and BSE- plants, particularly in jasmonate levels, suggesting that BSEs may influence plant defense signaling pathways. Together, these findings highlight the potential of BSEs as morphophysiological modulators of insect–plant interactions, emphasizing that internal leaf anatomical traits, alongside chemical defenses, contribute significantly to herbivory resistance.</p>