"Mariposa de la luz" is not a standard spanish phrase, but the language (any language) is short on words to define all kinds of insects, so you need to abide by the meaning, instead of the word-by-word translation.
A "lamp bug" is a bug that is attracted by the light of a lamp. This can only happen during night, since during day that light is not important under the Sun, so Steinbeck is talking about a nightly insect. It is most probably that he is talking about moths, the best known insect that is attracted by human lights on the night. In this case the translation would be "polilla", which is actually the term for nightly butterflies (and also for clothing moths) in spanish.
The translator would have used "polilla" directly if the author would have used "moth", but since Steinbeck taken a detour, so did he.