Let me explain in more detail. Bitcoin is based on SHA256 algorithm. Block Erupters use ASICMiner chips designed to do one thing – calculate double SHA256 hash. This specialization allows them to be highly efficient at Bitcoin mining and consume minimal amount of electricity. They can also be used to mine alternative coins based on SHA256 algorithm, such as Terracoin or PPCoin.
The ASICMiner chips can not mine coins that use scrypt algorithm, such as Litecoin or Feathercoin. In fact, as of now, there are no ASICs in existence that can mine coins based on scrypt hash. The main reason for this is that such ASICs are considered to be more expensive and inefficient at scrypt mining in comparison with higher end GPUs. However this may change in the future if somebody figures out how to pair specialized scrypt-mining CPU with large amounts of fast memory (which is what scrypt mining relies on) in an inexpensive and energy efficient way. Until then GPU mining is the only practical way to mine Litecoins.
[Update: Jan 2014. Alfa Technology reported that it has been working on an scrypt ASIC and is currently accepting pre-orders. While I would like to caution anyone considering pre-ordering equipment (my Butterfly Labs miner arrived 8 months late, after all), a working scrypt ASIC would certainly have tremendous impact on Litecoin mining, similarly to how SHA256 ASICs transformed the Bitcoin mining.]