Greetings, Kathryn, and thank you for your question about holes in your Indian Summer Rudbeckia plant.

I think you are on the right track in suspecting the damage is caused by slugs. The pattern in your photos is consistent with slug damage. They can travel a distance in search of food sources, and are not limited to damp environments, especially at night. They will feed on young Indian Summer Rudbeckia. The Missouri Botanical Garden fact sheet says: “Watch for slugs and snails on young plants.”

You suggested using a flashlight to check for slugs, which is an excellent idea. It will help you confirm that slugs are your culprit. Another option for “catching them in the act” is to place a dish of beer, yeast-sugar water, or other fermented liquid near the plants in question. The slugs are attracted to the beer/yeast/fermented liquid, fall in and drown. You might not destroy enough of them to control the issue, but it is an easy diagnostic tool. I’ve used it several times and it has worked every time. This article from the University of Minnesota Extension has more specific directions. To find them, you will need to expand the section called “Handpick or Trap Slugs”.

If you do find slugs, the same article has information on how to protect your garden, as does this article from The Ohio State University Extension’s Ohioline. As you can see, there are a variety of control methods including mechanical, cultural and chemical control. The choice is yours. If you choose chemical controls be sure to follow all package directions and use personal protective equipment when recommended.

I hope this information will help you stop the damage being done to your Rudbeckia. In preparing a response to your question I consulted research-based sites to verify that my response was accurate and current. Links to the best of those sites are embedded in the above response and/or listed in the resources below. If you need any clarification or further information, simply reply to this email and it will come directly to me. I would love to hear from you to find out if we have helped you to resolve your issue. And, if you have time, completing the survey at the end of this response will be very helpful to us.

Wishing you a slug-free garden!

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Resources:

Slugs and Their Management in Landscape from Ohio State Ohio Line https://ohioline.osu.edu/factsheet/hyg-2010#:~:text=Slugs%20are%20easier%20to%20detect,when%20slugs%20are%20most%20active.

Slugs in Home Gardens University of Minnesota Extension https://extension.umn.edu/yard-and-garden-insects/slugs

Rudbeckia hirta ‘Indian Summer’ Missouri Botanical Garden “Watch for slugs and snails on young plants.” https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=262421&isprofile=0&basic=Rudbeckia%20hirta

It is interesting that there were no trails. I would expect there to be.

If I find any more info, I will send it along.

Interesting! You would expect to see a slime trail with slugs.

I've continued to look for research- based sites to help you. Almost all say "slugs", but I found this from the University of Minnesota Extension that mentions hail and checkerspot butterfly caterpillar. 

I know there has been a lot of severe weather in Ohio recently, some with hail. Are you continuing to see new damage? If yes, this would rule out hail. 

I would be interested to know what happens if you put out beer or yeast+sugar-water. I've used a pie pan or shallow plate in the past and sunk it just a little into the soil. 

I'll lyk if I find anything else. 

Sorry! I neglected to include the link:

https://apps.extension.umn.edu/garden/diagnose/plant/annualperennial/rudbeckia/leavesholes.html