Novel microwave devices capable of monitoring pest activity or moisture content in grain. A planar active microwave resonator with a regenerative element is placed in proximity to the grain to be tested, for example by immersion within in a bulk grain store to monitor moisture levels or pest activity therein, mounting within a sample container to monitor moisture levels or pest activity in a smaller volume of grain sampled from a grain store or shipment, or exposure to individual sample kernels for detecting internal pests therein.