This circuit is intended to drive-off the female mosquitoes with a super sonic sounder. Most of the time when a mosquito is pregnant, she needs more valuable food and will look for humans or other sources of nutrition. In the meantime, she dislikes keeping in close with the male mosquitoes and prefers to look for food alone. Thus, this circuit is intended to produce a sonic frequency range that drives-off the female mosquito.
The two transistors link as a multi-vibrator oscillator circuit while R1, R3, and C2 control the super sonic frequency. The piezo element is the sounder.