Will Harpy traps work on Erynia's ?

Their only use is to destroy harpy nests for the Harpy Contract.

If you need to slow down attacking harpies, erynias, or Celaenos, Yrden and Aard work a treat.

I'm not 100% sure, but I believe the harpy traps are only good to fullfill this harpy contract in Chapter II. I once deployed a harpy trap against... harpies, in an area without any nests. Not one of the beasts picked it up or was bothered by it. They really only worked for me, while I had to destroy the 7 nests for that quest.

Aye, like the others have already said, I don't think they even work outside that specific quest; and even if on that quest, only 'regular' harpies will ever pick them up.

I can attest to that personally. The first time I tried to complete that contract, I thought that nest out by that tower where you find the dream crystal counted as a harpy nest and deployed harpy traps and not a single one of them payed any attention to it. They only work in the gulley near the actual harpy nests the game requires you to destroy.

They work only in those areas where are nests. Elsewere they wont:) How could a harpy far from its nest bring back a trap?

Its sure that if you notice anywhere a nest, you can put then down almost anywhere in that area.