Can you mine solo with a block erupter?
Yes you can, however the chances of you hitting a block are incredibly low so its not worth the time and the effort. You will loose money to electricity charges, you won't loose a lot of money but still a small amount of money to electricity charges.
You can compare it to buying lottery tickets, your chances of winning are incredibly low. When it comes to mining with a block erupter your chances of hitting a block are far far far, lower than winning a lottery.
some math 3 block erupter = 1 gh
1000 gh = 1th
1000 th = 1ph
so 3,000,000 block erupters = 1 ph
if you mine for 4 days at 1ph your have a good chance of hitting a block
if you mine for 12,000,000 days or 32,876 years your chances would be the same if difficulty never changed.
the problem is difficulty changes every 2 weeks. This why people say the chances are terrible since difficulty usually rises.