I shouldn't worry about it being one side of an arch, most birds do not mind where their bugs come from. If you have an arch, it's not much to tell people about, except "there's an arch".
Now, if one side was missing, THERE'S an interesting story.. "Oh, there used to be an arch there but some bugs made there home in it as it was probably a bit on the sick side. Anyway we had (then fill in with long list of birds) living on it for ages. Did you know..............etc etc". People will think how considerate you were, instead of someone saying "WHAT? LINDANE? That was banned years ago as it was so dangerous, what ever made you do that.......(and so it goes on).