Received what I am fairly sure is Gonocerus juniperi in a parcel from Germany. The insect is still alive, and I was hoping for advice on whether I should release it (in East Yorkshire UK) or if it is potentially invasive. There is not much information available online about this species. Thanks for any advice in advance.
Note: I at first thought it could be the invasive western conifer seed bug but it has a green underside and does not have any white zigzag markings.
I don’t know about this group of bugs, but I’m not convinced your identification is correct. I’d suggest keeping the bug in a container for now and tagging some of the top Hemiptera identifiers in Europe on your observation, and then asking those who add an identification what they think you should do. I’d also suggest trying to get a better photo. Even just cropping the photo you have taken with your phone will help to make more detail visible.
I don’t see much harm being done by releasing this one individual. Given the similar habitat in Germany to the UK It’ll likely either live out it’s lifespan before dying from old age or some sort of element related accident ( like drowning) or just become food for some sort of predator. If you’d found a small colony then It’d maybe be more problematic but I wouldn’t worry too much.
Personally, I’d put it in the freezer overnight - better safe than sorry. If it’s a gravid female it could lay eggs and become an invasive species. It’s not the most likely outcome from releasing it, but I’d say the small risk of a bad outcome outweighs the interests of the individual bug in a case like this. For context, I see zero other observations of the species in the UK on iNaturalist, so it doesn’t seem to disperse naturally from the continent.