There are effective alternatives to toxic chemicals that are safe around ducks:Well I'm in Georgia and its parasite central. To treat them we use nail polish. I use bleach it burns like heck but it makes the itch stop. As a preventative I use bug spray with DEET. I don't put anything on yard or chickens just us. Mostly clothing. On a visor, hat, to keep the gnats out of my eyes socks, shoes for the ants. I don't like the spray, but its better than the bugs.
Diatomacious earth - a fine powder you dust on the ground. It kills the larvae of worms and little bugs. It's also safe if it gets in the duck's food. Avoid breathing the dust in - wear a mask! It can irritate your lungs. For bug repellent I use a product called "Bug Off!" available at www.lsshaftsfarm.com. Ingredients are peppermint oil, tea tree/lemon oil, cedarwood oil, lavender, thyme and a carrier oil. All the ingredients repel bugs and this combo seems to work very well. Because it's oil based, I spray or rub it on my boots, belt, hat or anything leather so it lasts longer. I apply it really lightly to my hair - which makes it curl more

Hope this helps & give you some alternatives. Feel free to repost.