In this work, a capacitive sensor circuit implemented on a flexible indium gallium zinc oxide (IGZO) thin film transistor (TFT) technology is presented, tailored for insect detection. The circuit employs a charge-based capacitive measurement (CBCM) architecture to map the differential capacitance between a sensing and a reference capacitor into a dc voltage at its output. The sensor, designed for measuring capacitances in the 100 fF range, makes use of both traditional and pseudo-CMOS current mirrors, given the lack of complementary devices in the technology. We show a circuit with a total area (including the sensing structures) of 480 µm by 480 µm, and demonstrate its operation by measuring polymer traces over its sensing electrode, being able to detect differential capacitances from 2 fF. Connected to a 6 V supply, the sensor boasts a sensitivity of 18 mV fF⁻¹ with a dc power of 1.5 mW. Finally, the circuit is tested by placing a firebug over the sensing plates, showing a 200 mV voltage shift, sufficient for accurate insect detection.