Nocturnal grasshopper migration in West Africa: transport and concentration by the wind, and the implications for air-to-air control

During radar observations of the migratory flight of Oedaleus senegalensis and of other grasshoppers in West Africa, we have observed that nocturnally flying insects are sometimes concentrated by mesoscale zones of wind convergence. The concentrations were typically 1.2-2.0 km wide, often more than 20 km long, and were similar to those observed elsewhere. The convergence zones appeared to be usually caused by atmospheric gravity currents. Some of these currents were cold air outflows from rain storms, and others were possibly of katabatic origin. Occasionally zones may also have been caused by bores and gravity waves set off by these currents. In this paper, we investigate the practicability of controlling populations of sahelian grasshoppers by the air-to-air spraying of insecticide onto such concentrations of insects. Using our data on concentration in convergence zones and a rudimentary model of zone distribution and behaviour, we estimate that less than 30% of the flying population of grasshoppers would be entrained in convergence zones, and that effective search for the concentrations might require the simultaneous use of at least two aircraft per 500 km square. These results imply that strategic control by air-to-air spraying is unlikely to be practicable. It is necessary to emphasize, however, that the evidence on which this deduction is based is fragmentary. A much more definitive conclusion could be expected from the results of further research with an aircraft equipped with a windfinding system and a radar able to measure and delineate insect concentrations.

Biological Factors Introductory remarks

The Chair for this opening Session should, of course, have been taken by Dr Rainey, but you will be aware that, unhappily, Reg is not well enough to attend. This is very sad as Reg is the instigator and inspirer of this meeting and has spent at least two years in preparing for it with all his accustomed attention to detail. We are grateful to Reg and his co-organizers, Dr K. A. Browning, Dr R. A. Cheke and Miss M. J. Haggis that, despite illness, the meeting is taking place as planned.

Detection of mesoscale synoptic features associated with dispersal of spruce budworm moths in eastern Canada

Radar studies in eastern Canada of spruce budworm moth distribution patterns and associated windfields frequently revealed mesoscale synoptic features induced by the strong thermal contrast between the heated land surface and the surrounding coastal waters that resulted in significant redistribution of the airborne moths. Experience gained during a four-year study in New Brunswick enabled meteorologists to identify the synoptic situations favouring the development of these mesoscale features. This paper examines the details of a particular case, on 15-16 July 1976, when insect detection teams were alerted in advance to the existence and location of a significant wind convergence zone.

Training: the missing link in pest control

Today when scientific development has so much to offer, the Desert Locust still poses as much threat to many parts of the world as it did many decades ago. When I was first exposed to aerial control of Desert Locust in the late 1960s, everything was being done to tackle the problem scientifically, with a collective and Regional approach. However, the recent past points towards a negation of this, because of a total disregard of the necessary manpower training. Unless a remedy is found soon the war will be won by the pests.

General discussion

K. A. Browning, F.R.S. ( Meteorological Office, Bracknell, U. K. ). I would like to recall Mr Boulahya’s advocacy of an operational Locust Control Watch and suggest that in parallel we need a properly resourced research programme integrating the physical and biological aspects of the problem. Today we heard about the advances in our observational capability from satellite, radar and aircraft and about our understanding of aspects of the weather that affect insect migration. We seem to have a physically attractive hypothesis, that convergent wind patterns in the boundary layer play an important role in concentrating insects; but the hypothesis is still of unproven generality. There are a few case studies and a lot of circumstantial evidence, but to find out whether meteorological convergence events are abundant enough to be really important we need a research programme with much more comprehensive coverage in space and time of both meteorology and insects. Assuming we are able to prove that the convergence events are important, we would also need more research into the best methods to detect them and to control the insects, perhaps by using a combination of satellite, radar, aircraft and mesoscale numerical models as part of an integrated system. The question is who should run such an integrated research experiment bringing migrant pest, remote sensing and meteorological communities together. I would suggest that a joint FAO/WMO Programme is required. D. Rijks ( World Meteorological Organization, Geneva, Switzerland ). Given the available knowledge about relationships between meteorology and the physiology and behaviour of at least some insects, and the possibilities to use this knowledge to monitor and combat outbreaks of migrant pests, am I right to think that there is, in this meeting, a consensus that this knowledge should be used now, to install a permanent preventive migrant pest watch, using among others the available meteorological infrastructure and reinforcing it, where necessary?

Satellite environmental monitoring for migrant pest forecasting by FAO: the ARTEMIS system

Since 1975, the Food and Agriculture Organization of the United Nations (FAO) has been pioneering the development of the use of satellite remote sensing techniques for improving the surveillance and forecasting capabilities of the centralized Desert Locust Reporting and Forecasting Service at FAO Headquarters and, indirectly, those of Regional Organizations and National Plant Protection Services. On the basis of findings from experimental activities on the use of Landsat and NOAA satellite AVHRR data for Desert Locust habitat detection and monitoring through vegetation assessment, and the use of Meteosat data for rainfall monitoring, FAO defined an operational system for satellite environmental monitoring in support of the FAO Desert Locust Plague Prevention Programme and the FAO Global Information and Early Warning System on Food and Agriculture. The system, African Real Time Environmental Monitoring using Imaging Satellites (ARTEMIS) is an advanced computer hardware and software configuration, equipped for direct acquisition of hourly Meteosat digital data and for automated thematic processing of Meteosat and NOAA AVHRR data for large area precipitation and vegetation condition assessment, being the key environmental factors for supporting Desert Locust population development. Since August 1988, the ARTEMIS system has generated a number of operational products documenting the occurrence of rainfall and vegetation development in the Desert Locust recession area on a 10-day and monthly basis at spatial resolutions varying from 7.6-1.1 km. These products are being used by the FAO Emergency Centre for Locust Operations (ECLO), along with synoptic weather and locust data, for the preparation of the bulletins containing the Desert Locust situation summaries and forecasts. For making ARTEMIS output products and other relevant data available in a timely manner at regional and national levels, a dedicated satellite communications system, Data and Information Available Now in Africa (DIANA), is currently being developed by the European Space Agency in cooperation with the FAO Remote Sensing Centre. The DIANA system will, by mid-1991, provide a capability for high speed (64 kb s -1 ) two-way transfer of facsimile images of documents and maps, character- coded text documents and digital images in raw or processed form from computers at FAO Headquarters to personal computer based terminals of recipients, initially in Africa, by using the commercial Intelsat satellites.

A Meteosat motion picture of a weather situation relevant to locust reports

The motion picture, of Meteosat VIS and IR half-hourly image sequences, covers the North African area with close-ups of northwest Africa from 28 to 31 March 1985. A cold front and horizontal vortex rolls, with their convergent upward atmospheric motions, visualized through dust being transported south from the Atlas mountains, may have transported locust swarms from the Algerian breeding area down to southeast Mauritania, where specimens were observed a few days after the event. This followed more than a year of no locust observations at all. Observation of dust in the atmosphere may give some insight into the transport and behaviour of locusts, both on the synoptic and mesoscale.

Progress in controlling the reinvasion of windborne vectors into the western area of the Onchocerciasis Control Programme in West Africa

Since vector control began in 1975, waves of Simulium sirbanum and S. damnosum s.str ., the principal vectors of severe blinding onchocerciasis in the West African savannas, have reinvaded treated rivers inside the original boundaries of the Onchocerciasis Control Programme in West Africa. Larviciding of potential source breeding sites has shown that these ‘savanna’ species are capable of travelling and carrying Onchocerca infection for at least 500 km northeastwards with the monsoon winds in the early rainy season. Vector control has, therefore, been extended progressively westwards. In 1984 the Programme embarked on a major western extension into Guinea, Sierra Leone, western Mali, Senegal and Guinea-Bissau. The transmission resulting from the reinvasion of northern Cote d’Ivoire and Burkina Faso has been reduced by over 95%, but eastern Mali has proved more difficult to protect because of sources in both Guinea and Sierra Leone. Rivers in Sierra Leone were treated for the first time in 1989 and biting and transmission rates in Sierra Leone and Guinea fell by over 90%. Because of treatment problems in some complex rapids and mountainous areas, flies still reinvaded Mali, though biting rates were approximately 70% lower than those recorded before anti-reinvasion treatments started. It was concluded that transmission in eastern Mali has now been reduced to the levels required to control onchocerciasis.

Concentration of flying insects by the wind

The role of horizontal wind convergence in concentrating flying insects is examined. Because of the inverse relation between intensity and persistence of convergence zones, a tenfold increase in volume density is unlikely to be exceeded in a single atmospheric disturbance, and then only if the insects are not taken through the convergence zone. Radar has shown that the commonest disturbances associated with insect concentrations are windshift lines: modifications of the broad-scale wind flow due to buoyancy and blocking effects. Even in the absence of concentration, a tenfold increase in area density can be brought about by vertical circulations at windshift lines. Insect concentrations are likely to be most frequent and persistent at night, and over and near mountains, where searches are most difficult.

The Desert Locust: an international challenge

A new plague of the Desert Locust Schistocerca gregaria started in 1986; it developed quickly in 1987 in the Sahelian countries and reached northwest Africa at the end of 1987. It expanded in 1988 in north Africa, the Sahel, the Sudan, the Near East, southwest Asia, and in October 1988, swarms crossed the Atlantic to the Caribbean. The plague declined dramatically in the past quarter of 1988, and by March 1989 the plague was over. A study of the latest known upsurges provides more support to the theory that a build-up of locusts arises from initially low-density populations rather than from the persistence of undetected swarming populations. The decline of the recent plague was probably attributable to the cumulative effect of control operations combined with natural environmental factors. Prevention of new plagues of the Desert Locust will be much more difficult as the restriction on the use of dieldrin poses major technical, logistic and financial problems.