Hello, Erin! Thank you for your question to Ask Extension/Ask a Master Gardener, and thank you for the helpful photos. Zinnia are such a bright and beautiful garden plant, with the added benefit of being a great food source for our pollinators. Yours look like they are very healthy! Congratulations! Sorry to hear that something is munching on them!

There are not many pests that attack zinnia, and the most common are: aphids, spider mites (two spotted), leafminers and earwigs, as well as slugs and Japanese Beetles. It is hard to tell exactly which of these is the culprit unless you can catch them in the act! Since the damage on your plants looks like large, irregular holes that cross veins, my best guess is slugs or earwigs, both of which are active now. I would not rule out Japanese Beetles, but I would expect that you would see more “skeletonizing” of the leaves if they were present. Aphids would be fairly easy to see. Spider mite damage would produce smaller holes. Sorry to say, I cannot be certain from the photos. Here is a good article that can help you diagnose the damage. Meanwhile, look carefully under leaves and in your soil to see if you can locate the pest. If you discover who the bad boy is, please respond to this strand and I will follow up with additional information. I've given suggestions below that should help with pests.

https://apps.extension.umn.edu/garden/diagnose/plant/annualperennial/zinnia/leavesholesorchewed.html

Regarding controlling slugs: I recently suspected I had slugs eating my hosta plants. To verify, I put a shallow pie pan with a half inch of beer (despite protests from my husband for wasting his beer!) near the plant in the late evening. Early the next morning I found slugs in the pan. With confirmation of the culprit, I applied a slug pellets around the base of the plants and, using according to label directions, had no further problems. The pellets I used are organic and not harmful to pollinators.

Regarding control of other pests:

1. Many of the named pests can be “knocked off” the plant with a jet of water. Be sure to get the underside of the leaves and lower leaves where they hide. For zinnia, it is best to do this early in the day so the plant can dry in the sun, as damp zinnia are prone to powdery mildew.

2. Application of insecticidal soaps will kill many pests and deter them from returning. It is safe for pollinators, but it should be applied in the late evening when pollinators have stopped feeding. It is advised to NOT apply in bright sunlight, as this could result in sunscald on plant leaves. It needs to be reapplied 3 times a week and should be applied to the lower leaves and undersides, where pests hide. Here is a good article on insecticidal soaps: https://hgic.clemson.edu/factsheet/insecticidal-soaps-for-garden-pest-control/

3. Removing deadfall, weeds and leaves from around your plants will help reduce pest populations. Also, allowing for airflow between plants through spaced planting and trimming will likewise help reduce pests and pathogens.

4. Keeping your plants healthy through proper watering and fertilizing helps to keep them resistant to pests and pathogens.

And, finally, here are a couple of excellent articles from Ohio State University Extension’s Ohioline.

https://ohioline.osu.edu/factsheet/ENT-65

https://ohioline.osu.edu/tags/integrated-pest-management

I hope this information helps you solve your zinnia issues. If you have any further questions or if you would like to let me know if this worked for you just respond to this thread and I will receive your response.

Wishing you a beautiful garden and the best of luck!