I have used the swat but I didn’t think about the duck eating it. You need to give her a bath in Epsom salt water and let her soak that should kill any maggots that hatch then any left you’ll need to take a pair of tweezers and pick them out. Might be better since she’s a wild duck to see if you can find a rehabber to take her they have the know how and means to care for them.Is this a Mallard? CB is my phone typing what it wants!