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I have always understood that the spines are sensory - being articulated and having nerve endings in the basal socket. But I can only guess they would need extra senses to find their hosts and I can't think how the bristles would help in this. Having vibration detectors would seem a rather low-tech and less accurate idea than, say, having larger and more sensitive antennae. I think most hymenopteran parasitoids rely mainly on their antennae to detect the presence of key detector chemicals in the environment. These can be the 'smell' of the host itself or the chemicals released from the host's foodplant when the plant has been eaten by the host.

Sorry, not a very comprehensive answer! :)