In particular, air currents with high humidity or CO2 contents are used. Can search for area host is likely to be located, then search for host. Chemical odors of plant are detected by parasite. Homes in on plant and ignores everything else at this point. Once it reaches host plant then begins to search for host.
Some parasites restrict search areas to certain parts of the plant.
Can look for other clues left by host. The frass has the odor of the insect. Several parasites cue in on this frass. The parasites oviposit where the frass is and do not even verify if the host is there. In other cases the frass elicits probing behavior once contact is made.