I absolutely love that the game was so easy to pick up! And the difficulty ramped up nicely! The graphics were fun, and the music was enjoyable! And the theme is there of course, it all works together nicely!
I actually think it would be nice to turn down the cooldown on the plants as sometimes when they went back to place it was just a bit too slow, as there was moments I couldn't do anything. To compensate, you could make more flies spawn or something and make them move faster! Maybe a powerup you can get which is like a golden fly and if you get it, you move faster for a period of time or something!