Nice graphics, fairly good gameplay. Could have used a little music.
I think you made it go for too long. I got bored around wave 11. For a game jam, I think you'd want your game to show everything it has to offer faster than this.
Also, the little red bugs feel like they're a bit too fast. It makes them feel like cheap difficulty.
I feel like you're not making much use of the limitation here, though. That's a bit disappointing.