A subsidiary page of The Radar Entomology Web Site.

Further information on specific topics in Radar Entomology and Insect Migration. The notes reproduced here are edited extracts from correspondence sent to Alistair Drake by TREWS readers, and from Alistair's replies. For privacy and copyright reasons, correspondents are identified, and incoming correspondence is reproduced, only if the writer's specific permission has been obtained.

Copyright of material on this page remains with the original authors! Quotation for the purpose of study or research is acceptable.

Radar Entomology:-

 Can the electronics for an entomological radar be bought? What would be the cost?

Answer:- The radars used by "radar entomologists" are derived from commercially manufactured "marine radars" produced for the commercial shipping and sea-fishing industries. If you look in boating magazines you will find small-boat radars advertised for prices as low as ~US2000-3000. These are not really suitable, but the next range up - radars aimed at the large-fishing-boat or small-ship market - can be adapted to our purposes. Prices here are in the range $US10000-50000, but entomologists really only need the transceiver (the subunit that generates and receives the radio waves), for around $US10000. We then have to build our own power supply, control unit, and data-acquisition unit "in-house". The antenna also has to be built, because the marine ones are of the wrong type for entomological applications. Entomological radars of the scanning type have also employed the display unit (the "plan position indicator" or "PPI") from the original marine radar, but future units may simply acquire the radar echoes with a microcomputer and store them directly onto disk. [Answer 97Fe, revised 97Oc.]

 What is the range of an entomological radar?

Answer:- Most entomological radars detect individual large insects (moths, grasshoppers - both ~20 mm long) to ~2 km, and concentrations of these types to ~20 km. NRIRU's millimetric-wavelength radar for observing smaller insects has a similar range performance. [Answer 97Fe.]

 Can radar be used for tracking animals on the ground?

Answer:- Conventional entomological radars can only observe insects that are well clear of the ground. These radars also receive echo from any vegetation, land surface, building, etc. that they are pointed at, and generally such targets have bigger echoes than insects and any insect echoes are "lost in ground clutter". Because of this limitation, the main application of entomological radars so far has been the study of insect migration - because migrating insects often fly at heights of 100 m or more. In favourable (i.e. flat, with only low vegetation) locations, conventional scanning entomological radars can work down to heights of about 5 m. This has allowed some additional applications, notably the study of honeybee drone flight - which occurs in open spaces at heights of around 10 m.

The "harmonic radar" technique overcomes the problem of ground clutter, but only by requiring the insect to carry an electronic tag. This approach can be regarded as a variant of radio-tracking methods, but with the radar transmitter providing the power source for the tag - which can then be reduced in weight sufficiently for an insect to carry it. Two types of harmonic entomological radar have been developed - hand-held units that have been employed in studies of ground-living beetles (Mascanzoni & Wallin 1986, Wallin & Ekbom 1988, Hockmann et al. 1989, Wallin 1991) and local butterfly movements (Caldwell 1997), and a scanning unit employed so far in studies of honeybee and bumblebee foraging flights (Carreck 1996, Riley et al. 1996, Osborne et al. 1997).

For animals able to carry a heavier (self-powered) tag, radio-tracking is probably still the preferred technique. Radio-tracking methods have advanced considerably over the last decade or so, and tags light enough for small mammals and birds to carry have been developed.

Perhaps the most unlikely entomological application of radar is its use to detect underground insects! Some species of termites build their nests in the ground, and a special-purpose radar has been constructed to find these when they occur in river dykes - where they constitute potential weak points in times of flood (Xu et al. 1996).

[Answer 97Fe, extended 97Oc.]

 "Some "email history" of the first use of an "entomological radar".

Bill de Beer (Nelson Polytechnic, New Zealand) wrote (97Mr-Ap):-

"In the mid-'60s to early '70s, I worked at Associated Electrical Industries (AEI, later Marconi Radar and Defense) in Leicester, UK, and was approached by Glen Shaefer to provide technical input to his work. At this time he was just [founding] Loughborough University's Biophysics Research Unit. Up to that time, he had been using a "No. 3 Mk 8 10-cm Gun Laying Radar" [ex British Army], and it was his intention to try one of AEI's 3-cm "Escort 654" marine radars, these having greater flexibility in selection of range scale, transmitter pulse width, and pulse repetition frequency. The only characteristic not suitable was the aerial, with its vertical beamwidth of 24 deg. Following various investigations, ..., the radar was modified and fitted with a 1-m parabolic aerial, the complete system being mounted into a Landrover. This system was being to investigate behaviour of locusts in North Africa and Australia.

"I found an old black and white photograph (at left) [of this unit, taken shortly before departure.]

"My [contribution] was to modify and construct several modules to enable an individual signal to be gated and fed to a sample and hold circuit, the output of which was recorded on an FM tape recorder."

Comment:- The Landrover-mounted radar was operated very successfully in Niger (southern Sahara) in 1968 (Schaefer 1969; 1972; 1976; Roffey 1969). This really founded the line of research which has come to be known as "radar entomology" (and which the "TREWS" web pages are about). For Schaefer's Australian exercise (1972), and also for his work in Sudan (1971-1974), a "Mk 8" AA radar was used, but with an Escort transceiver (and 3-cm wavelength).

"Gated sample-and-hold" circuits still form an important component of entomological radars. ASoP has just built a 16-parallel unit for use with its Insect-Monitoring Radar. FM tape recording has given way to direct digitisation on a PC.

[Comment 97Mr, extended 97No.]

 "Some "email history" of radar entomology in China

Yajie Sun (Jilin Academy of Agricultural Sciences, Jilin province, China) wrote* (99Nov):-

"The JAAS entomological radar (photo) was built in 1980-1983 (Chen et al. 1992, Sun 1998), with several field tests to determine its capacity for detecting insects undertaken in 1982-1983. Its first use on a natural insect population was in June 1984, when the flight of meadow moths (Loxostege sticticalis) was observed in Ying county of Shanxi province. The first radar observations of oriental armyworm (Mythimna separata) moths were in July-August 1984, at Gongzhuling, Jilin province. In July 1992, a third species, the Chinese pine moth (Dendrolimus tabulaeformis) was observed for the first time, at Yanqing, Beijing city."

"The JAAS radar is a facility of the forecasting laboratory of the Academy's Institute of Plant Protection. Radar work was initiated by the late Prof. Chen Ruilu, director of the laboratory until 1991, who from the 1950s onwards made major contributions to the study of insect migration, especially of the oriental armyworm, in China."

*Edited by Alistair Drake.

Comment:- The JAAS radar continued in use through to 1993, and was returned to operation in 1999 and 2000. The laboratory is currently directed by Prof. Sun. The Chinese studies of oriental armyworm migration in the 1950s, to which Prof. Chen made a major contribution, were of key importance in advancing recognition of the insect migration phenomenon and understanding of how it occurs.

[Comment 2001Dec.]

 What radar systems and characteristics are being employed [in entomological radars] today? [Also from Bill de Beer.]

Answer:- ASoP's Insect-Monitoring Radars use transceivers from Racal (formerly Decca) marine radars, but otherwise have been constructed in-house. We have three transceivers, an old "65160" and its power supply from a Decca "RMB926" marine radar and two current-model "65625/DM" transceivers from a Racal "Bridgemaster" marine radar. All three are nominally 25-kW X-band units and have log. amplifiers. Another Decca/Racal item we use is the "echo box" (tuned resonant cavity) from their performance monitor accessory.

 Insect echo on weather radar in Alberta, Canada

Paul Joe (King Facility [now King Weather Radar Research Station], Toronto, Canada) wrote:-

"I think that I am seeing insects on one of [Environment Canada's] weather radars in [Alberta]. ... [Below] is an image of [the radar display during one] such event. ... I have been digging into the literature a bit and have satisfied myself that the [echoes] are [from] insects. Q: What type? Why do we see them this year in particular?



"The radar image is from the Carvel radar located about 40 km NW of Edmonton. It is a 0.5-degree PPI image taken on July 31 1997 at 21.00 GMT or 15.00 local time. The resolution of each pixel is 1 km × 1 km and the colours indicate intensity of the return echo. The scale and information should be visible on the right side of the image. The radar is a C-Band (5-cm wavelength) unit with a 1-degree beamwidth, peak power of 250 kW, and a sensitivity of about 0 dBZ at 75 km (meteorological terminology); pulse width is 0.5 us (75-m pulse length), pulse repetition frequency 1200 and 900 Hz (alternating every 0.5 degrees to unfold the radial velocity data). It is built by Enterprise Electronic Corporation in Alabama, USA. The signal processor is built by SIGMET Inc. in West Park, MA, U.S.A and we (the R&D group at King Facility) have built the display systems. We have a 10-minute cycle where we collect a volume scan of reflectivity information (no Doppler) in 5 minutes and then a three-elevation Doppler scan in 5 minutes. We archive the imagery (or I should say, the Edmonton weather office archives the images).

"I have gone back to about the 27th of July and have seen the insects till the 8th of Aug. ... They started at 12.00 h and lasted about 6-7 hours everyday.

"Since that day, I have looked at several days of images and have found similar signatures. By the speckled pattern, I have come to the conclusion that the echoes are from insects and not clear-air turbulence. They seem to match with Schaefer's (1976) report. I am building a web page with lots of images* to illustrate the event. It would be nice to have some collaborating insect information. The event was noticed by the weather forecasters who were very concerned that something was wrong with the radar - it is not an often observed feature obviously."

Answer :- From the location, time of year, time of day, and radar wavelength, I would guess the targets are grasshoppers. I know grasshopper numbers build up to near-plague proportions in some years in the Canadian Mid-West, but I'm less sure about how much these grasshoppers take to the air (but see below on N. Dakota obs.). (I'm assuming Carvel is in rangeland or a wheat-growing area. If it's in forest, it's more likely you're seeing moths of some type.)

Year-to-year variability of population size is quite common in insects, especially those of extensive "simple" (homogeneous, low species-diversity) habitats such as broadacre farmlands, some rangelands, and boreal forests. Usually seasonal weather variations are involved, though non-weather-related biological factors (e.g. numbers of parasites, disease levels) may also be important. Often the insects are pests and high numbers constitute an "outbreak year".

With regard to the images and animations on your web pages*, I don't see any reason to think these echoes are from anything other than day-flying insects. The diurnal pattern certainly fits. The streakiness of some of the images is most likely due to insects concentrated into the updraft sections of longitudinal convective vortices. I've seen similar streakiness, aligned downwind as I think yours are, in images on the scanning entomological radar I used for many years. The day to day variation in intensity is presumably due to changing temperatures - insect flight being suppressed after passage of a cold front, which I presume is what happened on 23 July. All very similar to my obs. in summer in inland Australia during periods when grasshoppers were numerous.

Some interesting radar observations on grasshopper flights have been made not so far away, in North Dakota, USA (Achtemeier 1991a, 1991b, 1991c, 1992).
*Links broken - sorry; paragraph under review and may be revised.]

[Answer 97Au.]

 Can "ballooning" spiders be observed with radar?

Answer:- There's nothing in TREWS about spiders. I don't know of anyone seeing radar echo and concluding the source was spiders. The main reason is that "ballooning" spiders are very small, so they don't produce big enough echoes to be detected with most of the sorts of radars that either "radar entomologists" or "radar meteorologists" use.

NRIRU in UK has developed a radar unit for detecting small insects ("planthoppers") that might be able to see "spiderlings" too. The very sensitive "FM/CW" radars that a few radar meteorologists use for looking at the lower atmosphere might also be able to pick them up. Another approach would be lidar, which atmospheric scientists use to track dust and aerosol particles (usually pollutants from smokestacks). Obviously, part of the problem is establishing that spiderlings, rather than some other organism or particulate, is the source of echo. Making observations when spiderlings are particularly numerous is one way of building up confidence about the identification.

[Answer 97Se.]

 Are all these devices used to detect insects really radars? [Posed by Alistair Drake.]

Answer:- The word "radar" originated as an acronym for Radio Detection and Ranging. What this name emphasizes is that radars are very good at doing two things: detecting the presence of a target and indicating how far away it is. Depending on how they are designed, radars can also be good at several other functions - most commonly, determining the direction the target is in. And some radars don't attempt to measure the distance of the target, because the information isn't needed - the obvious example is a police speed radar.

However, the term radar implies more than just use of radio technology for detection and ranging. It's use is restricted to systems that achieve this by a) transmitting a radio signal and then b) listening for an echo. There are a number of other radio technologies that are designed to determine the direction of a radio signal, but not its distance. When the radio signal is produced by a transmitter, such devices are known as direction finders rather than radars, and their use is referred to as radio tracking rather than radar entomology; radio tracking, even of insects, is outside the scope of TREWS. However, when the device transmits a radio signal and listens for an echo, the term radar is often used and TREWS will endeavour to include coverage of it.

A device of the last type, referred to in TREWS as a harmonic direction finder, is used to locate tagged insects. This type of device should not be confused with the harmonic radar, which tracks (rather than just locates) tagged insects and which does determine the range of the target. The way these two devices are used, and the type of information they provide, are in fact quite different. Confusion is especially likely when reading the bibliography, because while TREWS carefully uses the two terms as just described, in the titles of publications (which of course we have not changed), both types of device are referred to as "harmonic radars".

While much of the equipment described in TREWS fits the popular conception of a radar as having an antenna that points into the air and scans round (e.g. photo), providing an image on a screen that shows targets in all directions (e.g. photo), some units have very different designs. Profiling radars (e.g. photo) simply point directly upwards and observe insects (or, in the case of meteorological profilers, rain, snow, hail, and the air itself) as they pass overhead. Even more unfamiliar are radars that point into the ground or into buildings and trees to detect termites under the surface. An example of the latter, the TermaTrac, a lightweight, hand-held unit, is now available commercially. (Other devices for detecting insects inside objects such as fruit or logs rely on acoustic technology - they listen for the sounds the insects make as they move and feed - and are not included in TREWS.)

[Answer 2002Ja.]

Insect Migration:-

 Daytime or night-time migration?

Frederic Fabry (J.S. Marshall Radar Observatory of McGill University in Montreal, Canada observed:-

"I have noticed [through radar observations] that while birds migrate generally by night, insects seem to do so by day. Is it just to avoid the birds or are there more profound reasons?"

Answer:- Some insects migrate by day and some by night (and a few both). Observations with "entomological radars" have mostly concerned night flyers, which tend to be the larger species with "strong" flight - especially grasshoppers and moths. Conditions for downwind migration can be particularly favourable at night (as you observe for birds), and bird predators are also off the scene. (Though in southern US, bat predators appear to be active.) But daytime migration (often aphids) is also frequently important. Migration in "swarms" (grasshoppers, locusts) is also a daytime phenomenon. [Answer 97Mr.]

 What kind of air speed do migrating insects have? [Also from Frederic Fabry.]

Answer:- Small insects probably have effective airspeeds of zero - they just stay aloft and let the wind carry them along. Moths and grasshoppers typically add 4 m/s to the wind speed - which is often 2-4 times greater. They also don't always head downwind. [Answer 97Mr.]

 Ballooning spiderlings? A "great mass of spider web" found (no date) in Louisiana - but no spiderlings sighted.

Answer:- I recall hearing reports of very large amounts of "gossamer" (the threads on which the spiderlings suspend themselves in the air) draped over vegetation at times - and have seen minor instances myself. Presumably, on completing their migration and landing safely, the spiderlings would scurry away into less exposed environments - e.g. at the base of vegetation or in leaf litter or even in cracks in the soil - perhaps to emerge to feed only at night.

Here are some scientific papers on spider "ballooning", as this form of migration is (inaccurately) termed:-

 Coyle, FA 1983 Aerial dispersal by mygalomorph spiderlings (Araneae, Mygalomorphae). J. Arachnol. 11:283-286. AN-

 Greenstone, MH 1990 Meteorological determinants of spider ballooning: the roles of thermals vs. the vertical windspeed gradient in becoming airborne. Oecologia 84:164-168. AN01294

 Greenstone MH, Morgan CE, Hultsch A-L, Farrow RA, Dowse JE 1987 Ballooning spiders in Missouri, USA, and New South Wales, Australia: family and mass distributions. J. Arachnol. 15:163-173. AN00880

 Humphrey JAC 1987 Fluid mechanic constraints on spider ballooning. Oecologia 73:469-477. AN00847

 McManus ML 1988 Weather, behaviour and insect dispersal. Mem. Ent. Soc. Can. 146:71-94. [Useful source of references on spider ballooning and effects of weather factors.] AN00971

[Answer 97Se.]

Note: Spiders, of course, are not insects. They belong to a different group of invertebrates, the Arachnida, along with ticks and mites. However, from an ecological perspective, the distinction doesn't seem so important. For more on spiders, visit The Arachnological Hub or Queensland Museum Explorer - Spiders. [Note 97No.]

Back to: The Radar Entomology Web Site home page.

Last revised 2002Jan31, by Alistair Drake (a.drake@adfa.edu.au).