Dead bugs/insects in laptop
Just doing a quick bit of research, I ran across this post on Apple's support forums that sounded eerily similar to your situation.

What I gathered from the article is that


You should immediately ask for photographic evidence
You should document with Apple customer relations (1-800-676-2775 in the US)
The bugs themselves won't void the warranty, but the Apple tech can void the warranty on his own accord (if he believes they are a biohazard)


There are a couple of things you can try:


Take the MBA back, remove the back cover and using a can of compressed air, blow out any remnants of the bugs.  There are no tamper resistant stickers or tags that will indicate that you did this.  Bring it back to another Apple store, tell them what the first one told you and claim it's BS.  Depending on the year of the MBA (2012 and later) you might need a Pentalobe screwdriver.  Older than that, and a regular jeweler's Phillips will do just fine.
If that option isn't feasible, avoid the "Geniuses" at all costs.  I have never found them competent in solving problems.  There are some good Apple repair shops.  One that I have found is based in NYC and have done better work that the authorized guys (https://www.rossmanngroup.com/)  Disclaimer:  I am not affiliated with him, I just had really good results on may occasions.