Tracking Insects with Harmonic Radar: a Case Study

Harmonic radar technology can be used to track the dispersal of tagged insects. The tag consists of a wire antenna attached to a Schottky diode, which uses the original radar signal as an energy source, re-emitting a harmonic of the transmitted wavelength. Two forms of harmonic radar use this basic technology to study insect movement. The more sophisticated form consists of a ground-based scanning radar station that tracks the movement of a tagged insect on a circular radar display. A simpler, “off-the-shelf” form of harmonic radar is a commercially available, light-weight, handheld transmitter/receiver from RECCO Rescue Systems. We briefly review both of these forms and describe our experience monitoring the movement of carabid beetles in agricultural habitats with the handheld transmitter/receiver. We identified a commercial source of diodes compatible with the RECCO transmitter/receiver and tested several diode and wire combinations. We found that a tag built with a diode attached to a single section of 8-cm wire (monopole) was more appropriate for marking carabids. Tags built from flexible Teflon-coated wires were an improvement on tags built with stiff, aluminum wire, but beetle movement was still hindered. In corn and soybean fields, large carabids ( Scarites quadriceps Chaudoir and Harpalus pennsylvanicus , (DeG.) Coleoptera: Carabidae) could be recaptured even when they burrowed out of sight 3 to 9 cm below the soil surface. We discuss the trade-offs between tag detection and durability that occur when designing a tag for a given organism. Although the technique shows promise, producing a tag that does not hinder movement of the target insect in the field will require further development.