There are a zillion varieties of begonias and seedlings can vary greatly so I doubt it's insect related.
I use BT as a general insecticide.

I bought 4 non-stop bright pink begonias at my garden center. They were healthy plants but were not blooming. 3 of them turned out to be the regular non-stops and one is white and has the same shape leaves and flowers but is much smaller tham the others.

I had been meaning to call them and ask about it but didn't and your post jogged my memory so I have a call into them right now. The horticulturalist is on a break right now. I will let you know what they say when they call

Hi Well I heard back from my garden center and they said they didn't know what was wrongand asked if I had done anything different with this one--nope treated the same as the rest.
They did say however that sometimes they do get runts and they discard them and if I wanted a replacement they would give me one. I guess because I bought these small and they weren't flowering they didn't have a chance to discard mine, My lesson--buy them when they are bloooming.

If you are really dissatisfied with yours I would call and see what can be done

Thanks for the answers. I did take a few of the leaves into my local research horticulturist site. They were going to have one of the scientists look at it to see what may be wrong.

I went back to the nursery where I bought the plant and talked with the owner. She gave me a credit voucher and gave me Osmocote (sp?) and said maybe try feeding with the fertilizer.

By the way what is BT?

If you are talking about BT for insect control it stands
for
Bacillus Thuringiensius--This is being bred into more and more ornamental plants. It is a natural bacterial disease for soft bodied insects like catapillars and tree leaf eating larva.

What it does is form little crystals that cut up the insects insides and eventually kills it