Hello,

In order to decide on any treatment for the cause of the holes in your shrub's leaves, it's important to find the cause. Butterfly bush or buddleia can be susceptible to a few pests and a few foliar diseases. 

If the cause is an insect, you may be able to see the insect at work by doing some investigating. You can do this by looking at the shrubs not only during the day but after dark when some insects are more active. All parts of the plant should be looked at, as well as the ground around it. I have listed an article below from the University of Kentucky which list four insects attracted to butterfly bush. Given the look of the damage and the time of year, two of the insects, four-lined bug and European earwig, might be possible suspects.  Now, that's not to say another opportunistic insect isn't nibbling on the shrub.

The damage might also be caused by a fungal or bacterial pathogen. Cool, wet spring weather encourages fungal and bacterial infections. One fungal-like disease which affects buddleia is downey mildew. It can cause necrosis of leaf tissue between the leaf veins and once the tissue dies it falls away to create holes. Evidence of downey mildew can be found underneath the leaf in the form of white tufts of fungal-like growth. This is not actually caused by a fungal pathogen but is caused by a fungal relative called oomycetes.

Butterfly bush can also be infected by nematodes. Nematodes are microscopic organisms which feed on leaf tissue. Their damage looks similar to downey mildew with dead leaf tissue falling out and leaving holes between leaf veins. 

The most accurate way to diagnosis a plant problem would be to send a sample of the infected shrub to a plant diagnostic clinic. The C. Wayne Ellett Plant and Pest Diagnostic Clinic (PPDC) in Wooster, Ohio, run by OSU, would be able to determine the cause of the problem. I have listed a link for their website. This website gives you all the information that you would need in order to send in a plant sample. 

Once you have a diagnosis, then you can take the steps necessary to remediate the problem. You may write back with your findings or the findings of the PPDC and then I can give you some options for improving the plant's health.

I have listed links below for articles which describe in more detail the pests and diseases that I've written about above. Please write back if you should have more questions about this information.

Thanks for asking Ask Extension.

Betsy B. - MGV




https://www.uky.edu/Ag/Entomology/treepestguide/butterflybush.html

https://content.ces.ncsu.edu/four-lined-plant-bug#:~:text=Entire%20leaves%20can%20turn%20brown,damage%20resembles%20fungal%20leaf%20spots.

https://plant-pest-advisory.rutgers.edu/downy-mildew-in-ornamental-crops/

https://ag.umass.edu/greenhouse-floriculture/photos/buddleia-foliar-nematode-aphelenchoides-species

https://ppdc.osu.edu/