Detecting Coconut Rhinoceros Beetle Breeding Sites Using Harmonic Radar

Coconut rhinoceros beetle, a major pest of coconut and oil palms, is causing severe economic and environmental damage following recent invasions of several Pacific islands. Population suppression and eradication of this pest requires location and destruction of active and potential breeding sites where all life stages aggregate. Three search tactics for discovering breeding sites have been used with limited success: visual search by humans, search with assistance from detector dogs, and search by tracking CRB adults fitted with radio transmitters. Here we suggest a fourth search tactic: releasing CRB adults fitted with harmonic radar tags to locate breeding sites. Our idea is to find static end points for tags which accumulate at breeding sites, rather than active tracking of individual beetles. We plan to use commercially available hand-held harmonic radar devices. If we are successful, this technique may be useful for locating other insects which aggregate, such as hornets and other social insects.

sap to fuel their flight muscles. They typically bore through several fronds developing within the crownshaft. When these fronds emerge and expand several weeks later, large vshaped cuts become visible, a distinctive sign of CRB damage. Palms are killed only when the apical meristem (growing tip), located at the base of the crownshaft, is damaged by boring activity. However, mortality caused by CRB is rare unless CRB population densities are high and individual palms are attacked simultaneously by multiple adults. Adults reside in crowns of live palms only briefly, exiting bore holes within a few days to aggregate at breeding sites where they mate and lay eggs. Each CRB may feed up to 6 times during its adult lifetime (Vander Meer and Mclean 1975), boring a new hole each time. Gressitt (1953) estimated that 88% of a CRB population occurs in breeding site aggregations. The remaining 12% accounts for adults temporarily visiting live palm crowns to feed on sap. Breeding site aggregations occur in a wide variety of decaying plant material including dead standing coconut palms, fallen coconut logs, rotting coconut stumps, decaying wood of many tree species, piles of compost, sawdust, and manure. Small breeding sites are sometimes located in live coconut palm crowns where grubs feed on accumulated detritus (Moore et al. 2015).
Severe damage by CRB is often triggered by an abundant larval food supply in the form of massive amounts of decaying vegetation generated by typhoons, large-scale land clearing and wars. CRB damage can be totally avoided if all breeding sites are located and destroyed prior to first emergence of adults at about six months after sites are established.
Location and destruction of breeding sites, usually referred to as sanitation, is essential for CRB population suppression leading to eradication. Sanitation is likely to be suppress CRB populations much more effectively than control programs aimed primarily at killing only adults, such as mass trapping and insecticides applied to live palms.

Invasion history
CRB is endemic to the tropical Asia region (including South East Asia). The beetle was inadvertently introduced into the Pacific in 1909 when infested rubber tree plants were transported to Samoa from Sri Lanka (previously known as Ceylon). The pest rapidly multiplied in Samoa and subsequently spread to several nearby Polynesian islands. Separate invasions further distributed CRB through Palau, parts of Papua New Guinea, and other Pacific nations through disruptions and uncontrolled movements during World War II. The invasive phase of the beetle was brought under control by the discovery and distribution of a viral biocontrol agent, Oryctes rhinoceros nudivirus (OrNV). OrNV causes persistent population suppression on many of the CRB infested Pacific Islands where it was introduced.
Detection of CRB on Guam in 2007 heralded a second wave of Pacific island invasions by this pest (Marshall et al. 2017). Following a failed eradication attempt it was discovered that the Guam beetles are an OrNV resistant form which is being referred to as the CRB-G biotype . This problematic biotype has been detected on several previously uninfested Pacific islands including Guam (2007) (2015). CRB-G is damaging and killing coconut and oil palms on these islands and it is expected to spread further if high populations are not suppressed ( Jackson 2015).

Eradication programs
The recipe for eradicating coconut rhinoceros beetle from an island is simple: • find and destroy all active and potential breeding sites • prevent re-infestation by closing invasion pathways However, eradication of CRB from an island has proven extremely difficult once this pest has become established. There have been many CRB eradication attempts and some are currently in progress. But there has been only one success. This was accomplished on the tiny (36 km ) Niuatoputapu Island (also known as Keppel Island), which lies between Samoa and Tonga (Catley 1969). During a period spanning 1922 to 1930 all CRB breeding sites were located and destroyed.
We suggest that harmonic radar may be useful for efficient detection of CRB breeding sites, thus facilitating efficient sanitation and improved probability of successful eradication.

Methods for Detecting Coconut Rhinoceros Beetle Breeding Sites
Three methods have previously been used for detecting CRB breeding sites: unassisted search by humans, search with assistance from detector dogs, and search with assistance from CRB adults equipped with radio transmitters. Pros and cons of these methods plus a fourth method, search with assistance from CRB adults equipped with harmonic radar tags, are presented Table 1 Search by humans Unassisted visual search by humans is limited because many CRB breeding sites are cryptic with a high probability of being undetected.

Search assisted by beetles equipped with radio transmitters
After discontinuation of the Guam CRB detector dog program we began investigating the prospect of replacing dogs with CRB adults for olfactory detection of breeding sites.
Location of mammals and birds is commonly done by attaching radio transmitters to individuals. These individuals can then be tracked using a directional antenna attached to a radio receiver. Miniaturized transmitters are now small enough to be carried by large insects and these can be tracked using receivers and antennae identical to what is used for locating mammals and birds. Our idea was to track CRB adults equipped with miniature radio transmitters to see if they would lead us to breeding sites.
A feasibility trial performed on Guam showed that the method worked ). However, this method has not been used operationally because of financial and logistic limitations: • transmitters are expensive, about $200 each • transmitters require batteries which are not replaceable or rechargeable. These batteries are relatively heavy, have a shelf life of a few moths and an operational field life of a few weeks

Search by beetles equipped with harmonic radar tags
To continue investigating the prospect of replacing dogs with CRB adults for olfactory detection of breeding sites, we are now considering use of harmonic radar tags which are much cheaper, lighter and longer lasting than radio transmitters. Harmonic radar (HR) has been used for locating and tracking insects for more than a quarter of a century. Mascanzoni and Wallin (1986) used HR to track carabid beetles and Riley et al. (1996) used HR to track bees.
HR can be used to locate and track tagged insects. The key to HR is a tiny tag consisting of a wire antenna and diode attached to the insect being tracked. When the tag is illuminated by a beam of fixed-frequency radio waves from a HR transceiver, the tag radiates at integer multiples of that frequency (harmonic frequencies). The HR transceiver is designed to detect harmonic frequencies and to reject the original frequency. In this way, the HR transceiver detects the harmonic frequencies radiated by the tag and rejects backscatter (reflections of the original frequency from foliage, the ground and other objects). More comprehensive descriptions of insect location and tracking using HR are presented by Mascanzoni andWallin (1986) andO'Neal et al. (2004).

Objectives
Given the importance of finding and destroying breeding sites in order to suppress and eradicate coconut rhinoceros beetle populations and the inherent difficulty of locating cryptic breeding sites which are found in a wide range of habitats there is a pressing need to develop cheap yet efficient detection methods to find these sites. We hope that harmonic radar will allow efficient detection of cryptic aggregation sites where tags have accumulated. We are planning a field trial on Guam to assess the feasibility of this approach. We will essentially repeat the previous trial in which we tagged CRB adults with radio transmitters ), but this time we will use HR tags.

Materials and Methods
HR tagged CRB will be released at two sites, War in the Pacific National Historical Park in Asan (13.4659 N,144.7109 E) and the University of Guam Agricultural Research Station in Yigo (13.5324 N,144.8733 E). After a period of several days, location of the tags will be will be determined.
We will use a handheld harmonic radar device (RECCO AB, Lidingö, Sweden) designed for finding avalanche victims.
We will fabricate dipole harmonic radar tags by attaching antennae to Schottky diodes (RECCO AB, Lidingö, Sweden). Two 8 cm lengths of super-elastic nitinol wire (0.076 mm diameter, McMaster-Carr, Aurora, OH, USA) will be attached to the diode with UV-activated adhesive (Bondic, Niagara Falls, NY, USA) so that each wire touches one of the diode contacts while avoiding the opposite diode contact and the other wire. Electrical connections between the wires and the diode contacts will be secured using conductive silver paint (GC Electronics, Rockford, IL, USA).
As with our previous work with radio transmitters, CRB adults caught in pheromone traps will be fed banana slices in the laboratory and their flight ability will be tested prior to selection for the feasibility study. Tagged beetles will be released in the evening about one hour after sunset.
A thorough ground search of the release site neighborhood using RECCO harmonic radar devices will start several days after release. Search paths and location of detected tags will be recorded using GPS devices.

Discussion
Development of a relatively cheap and efficient method for locating CRB breeding sites using harmonic radar will facilitate population suppression and increase probability of eradication. This method may also be applied to other invasive species, especially those that aggregate, such as hornets and other social insects.
Searches may be highly automated by mounting a HR transceiver equipped with a data logger on an aerial drone. This drone will fly programmed search paths close to the ground with the HR beam pointing downwards. At the completion of a search, a map will be compiled by merging the HR data log with search path coordinates recorded by the drone.  Pros and cons for methods used to search for Oryctes rhinoceros breeding sites.