An unconventional three‐domain odorant‐binding protein decodes contrasting host pheromones in the parasitoid wasp Anastatus japonicus

Abstract Invasive insect pests threaten global food security, necessitating sustainable management strategies beyond chemical pesticides. Biological control agents such as egg parasitoids are promising alternatives, yet the molecular basis of their host‐seeking behavior is poorly understood. Odorant‐binding proteins (OBPs) are central to olfactory signal detection, yet the function of atypical three‐domain OBPs has remained elusive. Here, we identified a three‐domain OBP, designated AjapOBP20 , from the egg parasitoid Anastatus japonicus , a key natural enemy of the brown marmorated stink bug ( Halyomorpha halys ). Uniquely composed of two Classic and one Minus‐C OBP domains, AjapOBP20 was predominantly expressed in female antennae. In vitro binding assays demonstrated its broad binding affinity to both plant‐derived and host‐related volatiles. Critically, RNA interference–mediated knockdown of AjapOBP20 significantly impaired both the parasitoid's attraction to n ‐tridecane (a synergist of the host's aggregation pheromone) and its avoidance of ( E )‐2‐decenal (the host's alarm pheromone). Docking analyses further indicated that n ‐tridecane bound more stably across all three domains compared to ( E )‐2‐decenal. Our findings establish the three‐domain AjapOBP20 as a key decoder of opposing host chemical cues, representing the first functional evidence for a three‐domain OBP in insects. This work advances our understanding of olfactory evolution and highlights a promising target for behavior‐based enhancement of biological control.