Entomopathogenic nematodes (EPNs) are soil-dwelling, insect-parasitic nematodes used in biological control. Infective juveniles (IJs), a third-stage free-living stage, are responsible for foraging and infecting suitable insect hosts, exhibiting cruising, ambushing, or intermediate foraging strategies. Steinernema siamkayai, an ambush forager, is a valuable biological control agent in Thailand, yet little is known about its behavioral response to host cues and insect odors. This study examines the olfactory responses of S. siamkayai IJs to insect odorants, including CO