Harmonic Radar Tags for Insect Tracking: Lightweight, Low-cost, and Accessible

Using tracking devices to follow animal movements has long been a standard approach for studying the behavior and ecology of vertebrates. However, until relatively recently, these tracking methods have been unavailable to entomologists due to the prohibitively large size and mass of the tracking devices that must be attached. Technology supplied to insect tracking include radio telemetry, Bluetooth, RFID, and harmonic Radar.

Of these, radio telemetry is the most used method to track larger insects. It offers longer detection range and unique signals for each tracked insect, but requires expensive tags which have limited shelf and field life and are heavy, generally at least 150 milligrams. By contrast, harmonic radar tags are lighter by at least an order of magnitude, inexpensive, and have detection ranges which allow field tracking of many smaller insects.

Harmonic radar uses a transceiver which both emits a signal that energizes the tag carried by the insect and receives the frequency-doubled return signal sent by the tag. We will demonstrate how to fabricate two sizes of harmonic radar tags, each built with a Schottky diode and nitinol wire. The nitinol wire is a critical design component as this wire is flexible, thereby allowing a tagged insect relatively free movement, and returns easily to a straight orientation, allowing maximum signal return.

We will also be demonstrating the use of an off-the-shelf transceiver marketed for backcountry rescue. Together, inexpensive tags and off-the-shelf transceivers make this insect tracking technique highly accessible to entomologists looking to study insect behavior and ecology. We will touch on making tags, attaching tags to insects, tracking these tagged insects, and we will provide you with some representative tracking results.

Here are the materials you will need to make your tags, the two different wires and the different sizes of diodes, as well as a variety of conductive materials. In our experience, we have found the silver epoxy to be the strongest and most effective method, but others could be used as well. Here you can see the different thicknesses of nitinol wire used in small and large tags respectively.

Here are the two different sizes of diodes that we will be using for our tags. These are the tools that you'll need to assemble your tags. The 3D-printed jigs will be used for wire cutting and tag assembly.

Note the attached document for the 3D print designs. First, we will be showing the assembly of the large tags. A critical component of the tag assembly is having the correct length of antenna, 8.25 centimeters for the large tag.

The measuring jig is used to easily and reproducibly cut the wires to the desired length. First, we tape the wire to the jig, wrap the wire around the jig, maintaining light tension, and cut the wires in the channel as shown. Before carefully removing the wires from the cylinder.

To prepare the diodes for wire attachment, secure the diode packaging to the microscope. Pull back the cover and remove the diodes with double-sided tape, while the diodes are in the correct orientation with contacts facing up to facilitate antenna bonding. A fixed tape to the jig.

Place card stock parallel to the diodes on each side and smooth the edges to prevent the antenna from sticking to the tape. Align the wire next to the diode in preparation of attachment, and under a microscope, position the wires so that they are touching a single electrical contact. It is important that they do not touch both.

Apply conductive material to each contact, ensuring to cover both the contact point and the wire. Use care not to spread the conductive material between each contact. Reapply conductive material as needed to secure each wire and ensure electrical conductivity.

Next, we will show the fabrication of the small tag which uses nitinol wire lengths of 4.1 centimeters. Attach the wire to the jig with the tape, being careful not to overtension the wire. Wind the wire around the jig.

And cut the wire along the groove. Remember that each tag requires two segments of this wire. To prepare diodes for attachment, place double-sided tape on the jig.

Transfer the diodes to the tape, maintaining an appropriate space between diodes. Under the scope, ensure contacts are facing up to facilitate the alignment of wire and diode. Realign these as needed and make sure that they are pressed into the tape to prevent movement.

Place single-sided tape along diodes to elevate the wire at the level of the contact and to prevent the wire from sticking to the underlying tape. Smooth down the tape and place the wires in the general vicinity of the diode. Under the scope, place the wire on the contact points with care to prevent wire overlap.

Ideally, your alignment should look like this. Next, apply the conductive material to the contact and wire and ensure that the conductive material does not flow together and flood the diode plane. Here is what your tag should look like after the appropriate curing time designated by the instructions for the conductive material of your choice.

Here you can see the large and small tag. Next we will demonstrate how to attach a large tag to an insect using the Queensland longhorn beetle as an example. There are multiple materials that you will use to secure tags to insects.

We will be demonstrating the attachment using UV cured adhesives. Two other commonly used adhesives are cyanoacrylates or cool melt glues. First, secure the beetle to facilitate access to the attachment site.

Apply a drop of adhesive to the attachment site. Orient the tag and place the diode with the electrical contacts facing down onto the thorax. Once satisfied with the position of the tag, cure the glue using a UV light at a variety of different angles for a total of 5 to 10 seconds.

Next we will demonstrate small tag attachment using to Tephritid fruit flies. The smaller tags are intended for use with medium or small insects. We will demonstrate tag attachment with the melon fly and the Mediterranean fruit fly.

Make sure to anesthetize your flies at four degrees centigrade. Dispense a drop of glue and immerse the tag into the UV adhesive. Make sure to roll the diode in the glue to ensure full coverage.

Use care to prevent excess adhesive from attaching to the wire. Secure the insect between the thumb and forefinger to present the attachment site. Orient the tag and place it longitudinally on the dorsal side of the fly thorax.

Move the tag back and forth to spread the adhesive and ensure a secure connection. Again, when satisfied with the placement of the tag, cure the adhesive using a UV light. We will now show the tagging process again.

If this process is conducted in the field, you will need to exclude excess UV light to prevent the glue from hardening before you are satisfied with the position of the diode. Chilling insects in the field can be done with ice instead of placing them in a cold room. Again, when curing with UV light, cure from multiple angles to ensure a strong attachment.

Finally, we will demonstrate tag attachment with the Mediterranean fruit fly under the microscope. Place the tag on the dorsal surface of the thorax and cure it with UV light. Here we see examples of the tagged insect post attachment.

Note, the tag position prevents removal by the insect cleaning efforts and appears to facilitate balanced flight. With flies, we have found that a transverse attachment close to the center of mass appears to lead to flight inhibition, potentially unbalancing the fly. It's important to thoroughly test the flight ability of the insect post tag attachment.

Typically, this is done by testing in a flight tube. Here we show representative takeoffs demonstrating that the flies are flight-capable. Note that the flies move freely and seem generally unencumbered by the tag.

Next, we will demonstrate the large tag in operation. Note the sound the transceiver makes when aligned with the tag. And here you can see the beetle with the tag attached.

Next, we will demonstrate the range of the large tag. A strong signal can be detected at about 30 meters. The maximum distance is about 60 meters.

Small tags have a much shorter range. A strong signal can be detected at about 10 meters. Signal strength is also affected by the orientation of the transceiver to the tag.

When the tag and the transceiver are aligned, we get our maximum signal. However, rotate the transceiver by 90 degrees and now the tag and transceiver are misaligned and we get weak or no signal. Detection range and tag size are probably the two most important features of insect-tracking tags.

Here we show representative results for the maximum detection range of the large and small tags fabricated in this video. 10 tags of each size were tested in an open field. Misalignment of the tag and transceiver, along with vegetation interference, often reduces the detection range under field conditions.

In our experience, we assume a detection distance of approximately 10 meters for larger tags and five meters for smaller tags when working in tree or field crops. Finally, we would like to share the results from a representative tracking study with QLB. The aim of this study was to investigate the movement of Queensland longhorn beetle as little is known about where adult beetles spend time in the environment.

As the beetles are cryptic, it is quite difficult to locate them in the environment without a tracking technique such as harmonic radar. This study was conducted in a stand of kukui trees on the Big Island of Hawaii. Multiple beetles were tagged and released during the course of this study.

This is an example of the path taken by one beetle over the course of five days. Note that the beetle was tracked to multiple kinds of locations, including high in kukui trees and in dead leaves. This was an important finding of this study.

QLB can be highly cryptic and farmers had often asked about where the beetles were hiding around the trees. By tracking individual QLB, we were able to determine that beetles were often concealed in dry leaves, making them difficult to detect. In this protocol, we have demonstrated how to fabricate two sizes of harmonic radar tags.

In addition, we have shown how to attach these tags to insects using UV-cured adhesives and demonstrated signal detection range. This protocol has been refined to allow the streamlined fabrication of many low-cost HR tags with minimal specialized laboratory equipment. Additionally, the protocol should be accessible to researchers with experience manipulating objects under a dissecting scope.

We recommend beginning with larger tags to develop skills in manipulating the diodes and applying the conductive adhesive before attempting to make smaller tags. Thank you for watching, and we hope this video has been helpful. Please feel free to contact us if you have questions.