Tracking insects with harmonic radar: A case study

Abstract

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.