Hi Helga,

Without seeing a specific insect, it can be a guessing game to know what has been there. 

Keep an eye out for four lined plant bug. The feeding pattern does look similar, and the timing is right for them. 

This MSUE resource discusses additional insect species that may be attracted to wild geranium. 

Under 'faunal associations' this Illinois Wildflower resource discusses additional insect species as well. 

Thank you for the information regarding the four-lined bug being the possible insect doing the damage.

Other than trying to find and detroy the bug, is there anything that deters them?  I'm not talking about insecticides which I avoid using.  

Once the bugs have feasted on the leaves, will the original plant recuperate?

You're welcome Helga. 

Generally there is no need to manage the bugs and the plants will recuperate. If you do get a severe infestation you may wish to remove plant material from the garden in the Fall to reduce any eggs that could be in the plant. 

There is no deterrents. 

Next Spring you can monitor the plants as they leaf out to look for larva. 

Manual removal of the larva, nymphs or adults along with removal of senesced plant material are your two best options. 

In addition to the University of Minnesota resource in the first email, you may also find this University of Florida resource helpful. It does mention parasitic wasps that will feed on the eggs as well as a jumping spider that is a predator. 

Congratulations on creating a wonderful habitat in your yard!