Many herbivorous insects are mono- or oligophagous, having evolved to select a limited range of host plants. They specifically identify host-plant leaves using their keen sense of taste. Plant secondary metabolites and sugars are thought to be key chemical cues that enable insects to identify host plants and evaluate their quality as food. However, the neuronal and behavioral mechanisms of host-plant recognition are poorly understood. Here, we report a two-factor host acceptance system in larvae of the silkworm Bombyx mori, a specialist on several mulberry species. The first step is controlled by a chemosensory organ, the maxillary palp (MP). During palpation at the leaf edge, the MP detects trace amounts of leaf-surface compounds, which enables host-plant recognition without biting. Chemosensory neurons in the MP are tuned with ultrahigh sensitivity (thresholds of attomolar to femtomolar) to chlorogenic acid (CGA), quercetin glycosides, and β-sitosterol (βsito). Only if these 3 compounds are detected does the larva make a test bite, which is evaluated in the second step. Low-sensitivity neurons in another chemosensory organ, the maxillary galea (MG), mainly detect sucrose in the leaf sap exuded by test biting, allowing larvae to accept the leaf and proceed to persistent biting (feeding). The two-factor host acceptance system reported here may commonly underlie stereotyped feeding behavior in many phytophagous insects and determine their feeding habits.

许多草食性昆虫是单食性或寡食性的，已经进化到选择有限范围的寄主植物。它们利用敏锐的味觉专门识别寄主植物的叶子。植物次生代谢物和糖被认为是关键的化学线索，使昆虫能够识别寄主植物并评估其作为食物的质量。然而，人们对宿主植物识别的神经元和行为机制知之甚少。在这里，我们报告了家蚕幼虫的双因素宿主接受系统，家蚕是几种桑树的专家。第一步由化学感应器官——上颌须（MP）控制。在触诊叶子边缘时，MP 会检测到痕量的叶子表面化合物，从而无需咬合即可识别宿主植物。 MP 中的化学感应神经元对绿原酸 (CGA)、槲皮素糖苷和 β-谷甾醇 (βsito) 具有超高灵敏度（阿托摩尔至飞摩尔阈值）。只有检测到这 3 种化合物，幼虫才会进行试咬，并在第二步中进行评估。另一个化学感受器官上颌帽状腱膜（MG）中的低敏感性神经元主要检测试咬时渗出的叶汁中的蔗糖，使幼虫接受叶子并进行持续的咬食（取食）。这里报道的双因素宿主接受系统可能通常是许多植食性昆虫定型摄食行为的基础，并决定它们的摄食习惯。