I'm no legal expert Karol, these are just my opinions but I would say it depends on what steps have been taken by the park owners and the professional to minimise the risk to its customers.
Placing the wasp traps in itself isnt the issue. I would say that If the park owners have made sure that there are sufficient warning notices informing its customers of the danger, the traps are placed at sites as far away from customer areas as possible, and the areas directly around the traps are cordoned off, then they have done as much as they reasonably can to minimise the chance of injury, and therefore have a defence of "due diligence"
If on the other hand, there are no warning notices, the traps in placed directly in areas of high visitor traffic, and visitors are allowed to walk within inches of the traps, then yes, the park are most definately liable.
As you can see, not black and white. In Scenario one, the park has done as much as it can, and the warning notices allow the visitor to make an informed decision on the potential risk.
Scenario 2 put the visitor in UNNECESSARY danger without thier knowledge and could be avoided or minimized with just a few simple precautions.
Its a fact of life that sometimes, **** happens, but if it happens because someone else has cut corners, and it costs you, either financially OR physically, then why shouldnt you be compensated?