If you are just putting them outside and monitoring them in the bush, and they are not eating it, then I wouldn't worry. I would not have something like that in my cage round the clock, where you could not see at all times if they might be chewing on it or eating it. Just putting them in it for some outside time would be ok I think like I said as long as you are watching them. I have a butterfly bush in my yard also, two of them actually. They get the little red flowers right? I see in your avatar you have a Veild, they are known to eat plants more so than panthers so just be careful.