One, or even 10 or 15, beetles do not necessarily indicate a problem that needs some control and it may not even mean that adult beetle was brought in on the coneflower. Those beetles may already be in your garden and that could well be where that one adult you saw came from.
Integrated Pest Management says to be sure you have a problem before taking step to control anything (this is also economical), properly identifying what is actually causing the problem, if there is one, Starting with the least toxic means of controlling that problem, if there is one, and then evaluating to see if what you did, if anything, controlled any problem.
If I only saw one potential problem beetle I would not get in a tizzy about that, 10 or 15 maybe.

I definitely know the beetle didn't come from my yard as she gave me the plant while we were at another location and I noticed it before I came home,I guess wasn't clear about that. I have a garden and other plantings that I had been trying to keep healthy and there are already plenty of infestations this year to worry about esp. when I'm trying to get a decent crop from my garden. Some things cannot be helped, I know. My question was not meant to sound crazy or like I was in a "tizzy" over seeing one bug! I was asking a legitmate question to find out if bringing a plant with a cucumber beetle to a garden with cucumbers would wreak havok even if I got rid of the bug I saw. Hence the question about larvae in the soil. You make it sound like I was ready to run willy nilly spraying my whole yard and everyones else's with toxic chemicals. that was not my intent and in fact have never used chemicals in my yard,yet! The reason I stuck the plant in the garage is because I did not want to plant possibly infected soil into my yard and create a problem I would then have to deal with. I would not want to create another problem in my garden especially, when it could have been prevented if I gotten some helpful advice from people more knowledgeable in cucumber beetles. Isn't that what this forum is for?

I agree with kimmsr that one cuke beetle isn't a worry. (And believe me, cuke beetles send me screaming....they've killed my cuke crops until this year, when I practiced defensive gardening...I consider them the WORST pest ever!)

You got rid of him, but I guess you're worried maybe there are eggs? What I would do, since you've already isolated the coneflower in the garage, is give it a nice spray of weak soap solution. I don't know for a fact that it kills eggs, but that's what I'd do.

Or...if you have some spinosad on hand, spray it with that, then wait a day and plant it.

I think that's probably over cautious, but given the havoc cuke beetles bring, I don't blame you for being cautious. I just don't know how big cuke beetle eggs might be. You could also just spray the whole plant with a hard water spray to knock off any eggs.

BTW, I would think that if there were any larvae in the soil, those would be hatched by now. What zone are you in? The further north you go, the less of a chance they get in the soil, at least as far as I've read. And the other thing I think I remember (I really researched these devils) is that the striped ones migrate from the south and don't overwinter.

I may have manufactured eggs in my head, thinking about squash bugs and how they lay eggs.

I still go back to what I said in the previous post: either give it a spray with soapy water (liquid dish soap and water....weak solution) or spinosad if you have it. I wouldn't order it just for this unless you want to have it on hand for next year.

I actually ordered some from Planet Natural and haven't used it. But I feel better having it in my arsenal just in case.

If you grow cukes and you've never had cuke beetles, give thanks! You're super lucky.

Thanks julianna, I am in Ohio. I checked the thing over carefully and couldn't find anything so I did plant it and so far no worries. As you said I wasn't worried about there being more as I got rid of the only one I saw. I was worried I would be introducing a potential problem if the soil was contaminated and that was what I was trying to avoid. thanks again.