When should I set up the H-trap?

Pending on temperature the horse flies will come out of the ground from Early spring (European situation: from April onwards) The female horse flies will than immediately start looking for blood, this to fertilize the next batch of eggs. It is important that we have the H-trap in position before this starts happening. This way we will not only catch the horse flies, but also a good amount of potential horse flies, which will become a pest in the next year(s)