Fluid controls, fast-paced action. I love how you absolutely have to aim correctly or you will (a) burn your crops or (b) make the pests gigantic. The dual-use of your spray nozzle is a very good implementation of the jam focus.
Some audio and visual feedback when killing an aphid would be nice. It's just not as satisfying to watch them disappear. Also, the music got somewhat grating.
Also, some more variety would help keep things fresh. I played for about 4 waves and got some bigger aphids, but the cycle was mostly the same... not sure if the game changes after that.