Which literary device is "brown beetles"?

The two words, “brown beetles,” show the use of alliteration.  Alliteration is the repetition of like consonant sounds most often used at the beginning of words. The use of alliteration can create a rhythm or mood to a passage or line of poetry, and it is usually used for effect or creative meaning. 

For example, this sentence uses alliteration through the repetition of the “r” sound: "The rainstorm relentlessly rattled the ranch style house." The use of alliteration in this instance gives the reader a visual description, a description of sound, and perhaps, an idea that the house isn’t in good repair or standing up well to the rainstorm. 

Alliteration’s “cousin”, assonance is the repetition of like vowel sounds.  Here’s an example: "Anna ate the apple all the way down to the core." The “a” sound is being repeated.

For fun, let's extend "brown beetles" to, "The brown beetles burrowed between the basement wall and the box elder bushes."