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Desert locust plagues
Desert Locust plagues A. van Huis Some 3000 years ago the prophet Joel listed four great plagues afflicting his people: the locust, the cankerworm, the caterpillar and the palmer-worm. Despite great advances in both chemical and biological methods of control, the Desert Locust still periodically wreaks havoc over large areas. Apart from the intrinsic difficulty of the problem, various constraints have been imposed. For example, insecticides that have proved very effective in the past are no longer environmentally acceptable. For economic reasons, government action is usually delayed until infestation has already reached plague proportions.
The Desert Locust, Schistocerca
gregaria
(Forskti), has been known from ancient times as a plague of agriculture in the Middle East and north Africa. Such plagues evoke images of huge sky-covering swarms that suddenly invade countries. Such swarms can develop when rainfall is abundant over subsequent breeding areas. Under these favourable conditions locusts are not only capable of increasing rapidly in numbers, but also of changing from a solitarious to a gregarious phase (Figure 1). The phase transformation is triggered when densities pass a critical level. During this process the whole appearance, physiology and behaviour of the locust changes. In 1921, BP Uvarov [l], the father of acridology (locust and grasshopper science), first described this phase transformation in the Migratory Locust (Locusta migratoria migratorioides). Before then the solitarious form of the Migratory Locust was erroneously assumed to be another species, viz. Locusta danica. Phase transformation distinguishes locusts from grasshoppers. Other important locust species are the Red Locust (Nomadacris septemfasciata) in central and southern Africa and the Brown Locust (Locustana pardalina) in southern Africa. Very damaging locust species can also be found in the Far East, Central and South America, and in Australia. Because of the spectacular appearance of swarms, locusts catch people’s imagination. This article will describe how the most dangerous locust species, the Desert Locust, can develop into a plague and what can be done to halt that development.
square kilometre belt of arid and semi-arid land extending from the Atlantic Ocean to north-west India and containing more than 25 countries (Figure 2). About 25 mm of rain in a month is sufficient to provide moisture for reproduction, successful egg development, and enough vegetation to feed and shelter the larvae. The wingless larvae are usually called ‘hoppers’. As the number of locusts rises rapidly, density increases and gregarious behaviour develops. Hoppers start to group, forming marching bands. The adults, which acquire wings during the last moult, may then form swarms and these can undertake long migratory flights. The great mobility of the locusts takes them beyond the deserts into agricultural areas where they can cause rapid and locally devastating crop losses. The invasion area extends over about 29 million square kilo-
metres, putting more than 65 countries intermittently at risk (Figure 2). There have been eight major plagues since 1860, varying in length between three and 22 years (Figure 3). The last plague occurred in large parts of Africa from 1986 to 1989. Twenty-six million hectares in the Sahel region and north Africa were treated with pesticides during this period, at a cost of about US$315 million [2]. Desert Locusts eat approximately their own weight (2 g) in fresh vegetation each day. Swarms are likely on average to contain 50 million individuals per square kilometre [3], so that even a ‘small’ swarm measuring 10 km2 eats some 1000 tons of fresh vegetation daily during migration. Although there are claims of impressive crop losses, hard evidence is lacking and attack is normally very localized [4].
Distribution and importance During recession periods, when density is low, the solitarious form of the Desert Locust generally inhabits a broad 16 million A. van Huis, Ph.D. From 1974-79 worked for the FAO in Nicaragua on integrated pest management in food crops, and this formed the basis of his Ph.D. thesis for the Wageningen Agricultural University in The Netherlands, where he is now Associate Professor in Tropical Entomology, dealing with a number of crop protection projects in developing countries, in particular, Africa.
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Figure 1 Locusts in the gregarious phase: (top) a young immature female; (bottom) a sexually mature male (a female would also be yellow but slightly less bright). 0 1995 Elsevier Science Ltd 0160-9327/95/$09.50
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Biology Life cycle Locusts have three development stages: eggs, hoppers and adults (Figure 4). Both hoppers and adults can cause crop damage. Eggs are laid in the soil in pods containing 50-150 eggs. Rainfall is required not only to incubate the eggs, but also to produce sufficient green vegetation to sustain hopper development following hatching. The period of egg incubation varies from several weeks to three to four months, depending mainly on temperature, while hopper development takes one to two months. The last moult produces winged adults, called fledgelings which then develop into fully adult locusts within a few days. Maturation Sexually immature locusts usually migrate about two to three weeks after fledging but the trigger leading to flight is not known. Solitarious locusts migrate by night and swarm by day, both being displaced downwind between successive breeding areas. Immature adults may mature rapidly, laying eggs within two to three weeks of fledging when conditions remain favourable in the breeding area. New green foliage, normally appearing after rains, accelerates maturation. This has been attributed to high concentrations of the plant growth hormone gibberellin A3, in new leaves [5]. Maturation may also be triggered by essential oils released by buds and flowers of the plant Commiphora myrrhae [6]. This is of functional significance because budding and flowering occur a few
weeks before the onset of the rains. Adult locusts may remain sexually immature, feeding on senescent vegetation, if rain does not fall in the areas to which they migrate. Eventually such locusts will die if there is no rain. Migration The Desert Locust is able to migrate over long distances. The swarms are carried by the wind, which may be why in parts of Africa locusts are called ‘teeth of the wind’. In October 1988 swarms commenced a six-day transatlantic journey from west Africa to the Caribbean, a distance of about 5OOOkm. Many of these locusts drowned in the Atlantic. Fortunately, the survivors failed to
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adapt to the Caribbean island environments and died without reproducing [7]. Natural mortaliw factors Besides drowning and starving, locusts are subjected to other natural mortality factors. Natural enemies like parasitic flies and trogid and blister beetles are able to consume most of the eggs in an egg-pod [8]. Hoppers and adults are preyed on by many vertebrate predators such as small carnivores, insectivores, rodents, reptiles, resident and migrant birds, and even men. Invertebrate natural enemies include predators such as scorpions, ground beetles, sphecid wasps, and parasitic flies. Although parasitoids and
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Major plagues Figure 3
Major plagues of the Desert Locust. (Source: FAO.)
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Larvae
Eggs
Females deposit their egg-pods close to those of other females [lO-121, thus producing high densities following hatching. Egg laying in groups is not caused solely by localized favourable properties of the soil (such as sandiness and humidity), but is also enhanced by a volatile signal from the egg froth [9]. Besides chemical, visual and tactile responses are thought to be responsible for aggregated oviposition [ 131. How do plagues develop? J. Roffey and G.B. Popov [14] described three processes characteristic of the phase transformation: concentration, multiplication and gregarization (Figure 5) [ 151. Roffey [16] gave a detailed description of these phenomena. He was an eye witness of an early upsurge in 1967 in Tamesna, northern Niger.
Adults
The first phase of the transformation process was concentration. Adults migrated from distances probably up to 1OOOkm. They landed on green vegetation, days if not weeks after the rains which led to its development. These adults matured rapidly on the green vegetation. Concentration within the habitat occurred by locusts restricting themselves to the scarce vegetation. It also occurred through adults laying egg-pods in clusters of about five per m2. Even up to 70 egg-pods have been found in areas of only 0.1 m2.
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Fiaure 4 The life cycle of the Desert Locust: ovipositing female; egg-pod; five larval instars (srages); copulating gdults.
predators reduce the multiplication rate and may hasten population decline, they do not prevent outbreaks. Augmenting these natural enemies is not feasible because the vast numbers required can be produced only in living host insects. Infections with men&hid nematodes and fungi occasionally kill entire swarms. The most prominent fungus isolated from grasshoppers and locusts is Entomophaga ‘gtylli’, a frequent cause of natural epizootics that reduce host density. With other pathogenic organisms, this fungus probably regulates some grasshopper and locust species in humid conditions. Entomopox viruses have been found in grasshoppers in west Africa. Protozoic diseases occur but are of little significance in field populations [8]. Semiochemicals Laboratory research has tried to elucidate the mechanisms responsible for phase tmnsformation and it is believed that semiochemicals play an important role. These are
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compounds that mediate the exchange of information between organisms. When used for communication within a species, they are termed pheromones. Why are gregarious locusts, contrary to solitarious locusts, so synchronized in their behaviour? Scientists at the International Centre of Insect Physiology and Ecology (ICIPE) in Nairobi, Kenya, believe that two distinct pheromone systems are responsible, one for the adults and the other for the hoppers. The chemical components involved have been identified and synthetic blends of these have been used to study their effects. The pheromone blend produced by older gregarious males caused gregarious behaviour in young and older males and females, and accelerated maturation. Hopper pheromones induced aggregation of all hopper stages except the first and retarded maturation of both male and female fledgelings. So, it seems that the pheromone systems of adults and hoppers may be the cause of the synchronous develop ments in gregarious populations [9].
Multiplication concerns the increase of the population from one generation to another. Roffey [ 161 claimed that during the upsurge in the Tamesna three egg-pods were laid with a total of about 400 eggs, which is double the egg production of gregarious females. Egg and hopper mortality was estimated to have been about 92 per cent, which means that the multiplication rate of the population was 16 (one couple giving 32 offspring). Gregarization occurs when there is no further rain and hoppers collect in the remaining favourable vegetation (Figure 6). They encounter each other more frequently in these restricted sites. Avoidance is then replaced by mutual attraction. The degree of gregarization in the adult stage influences the stage of gregarization of the hatchlings in the next generation. From outbreaks to plagues It normally takes several years for a plague to develop. The first phase leading to a plague is called an outbreak, or an early upsurge, and the next phase an upsurge. At the beginning of an upsurge, hoppers are in small patches or groups, while the adults are still scattered. As the population increases, bands and swarms develop, becoming larger and more cohesive each time. The population increasingly occurs in gregarious units and consequently the infested area shrinks. The dimensions and locust numbers in recession and early upsurge swarms are small compared to those characteristic of plagues.
Characteristic plague swarms may reach tens, and sometimes hundreds, of square kilometres at densities ranging from 20 to 150 million locusts per square kilometre [3]. A plausible size of settled swarm invading a country during a plague would be about 1200 km* (this might be composed of about 20 medium-sized swarms) [3]. The size of hopper bands varies greatly, and depends on the larval stage (Figure 4). Bands of newly emerged hoppers (hatchlings) measure several square metres, containing 30,000 larvae per square metre when settled and 250-500 when marching. Bands of last stage larvae vary from one hectare to several square kilometres, containing 1000 hoppers per square metre when settled and 10-30 when marching [3]. Drought, control, and movement of populations to areas unsuitable for breeding and survival are cited as playing a role in ending plagues. However, their relative importance is often not clear, and, in particular, the impact of control efforts is a controversial issue.
SCATTERED LOCUSTS
LARGE-SCALE CONCENTRATION within convergent winds and by settling in green areas
SMALL-SCALE CONCENTRATION occurs when immigrants move to preferred habitats and to restricted laying sites
MULTIPLICATION follows good rains numbers increase and close contact leads to
GREGARIZATION groups form and fuse to become
HOPPER BANDS AND SWARMS \
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Gregarization processes (after Ashalland and Hemming [15]).
Monitoring and forecasting The information required to forecast locust development includes the weather (wind, ram and temperature), and habitat conditions conducive to supporting locust development and the presence of locusts in those habitats. The Food and Agriculture Organization of the United Nations (FAO) in Rome co-ordinates locust monitoring. Reports of the locust situation are received by telex, fax and post from locust-affected countries and regional locust organizations. Reliable information on the locusts’ stage of
Patches of hoppers may occur at the base of or within clumps of vegetation. (Reproduced by kind permission of K. Cressman, FAO.)
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development, population density and degree of gregarization should normally be provided regularly by ground survey teams. Data regarding egg-laying, hatching and fledging are calculated using a computer model developed at the FAO and based on the relationship between temperature and development rate [ 171. The main aim of weather monitoring is to detect when and where it has rained in the Desert Locust recession area. Both daily rainfall data from meteorological services and satellite imagery are utilized. Infrared images received from the Meteosat satellite are analysed for cold clouds that may produce significant rainfall. Cloud-top temperature is translated into rainfall maps using predefined relationships between cold cloud duration and actual observed rainfall. The ARTEMIS (Africa Real Time Environmental Monitoring using Imaging Satellites) system, used by the FAO Remote Sensing Centre in Rome, estimates the number of rainfall days during a lo-day period [18]. Habitat monitoring involves detecting the presence of suitable vegetation in the Desert Locust recession area. The FAO uses satellite data to detect the relative amount of green-leaf vegetation biomass. However, high spatial resolution is required to detect the sparse vegetation in the desert areas where the locust matures and breeds. The technique needs to be further developed if it is to be of reliable use. Apart from satellite imagery, the suitability of biotopes also determines Desert Locust population development. G.B. Popov et al. [19] have described the bio-ecological potential of different biotypes and delimited their area in north-west Africa, west of the River Nile. The study was based on ecological data from the Sahara, the experience of locust survey teams, and historical data on locust outbreaks. J.U. Hielkema [ 181claimed that only approximately 6 per cent of the 16 million square kilometre area inhabited during a recession can support development of Desert Locust populations in terms of soil/vegetation and geomorphologicah topographic conditions. The information on suitable Desert Locust habitats is being combined with the rainfall and vegetation data obtained by satellite imagery in a Geographical Information System (GIS) based at FAO headquarters. This should improve reliability when identifying potential Desert Locust breeding areas. This information can then be used by ground survey teams to select high priority areas for monitoring. In order to help estimate swarm movements a wind trajectory model has just been developed for the FAO by Meteoconsult in Wageningen, The Netherlands. The FAO prepares the monthly Desert Locust Bulletin summarizing the current situation and forecasting likely events for the next six weeks. This bulletin is sent by fax, e-mail and post to locust-affected countries and donors.
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Control strategies Three different strategies of control can be distinguished: preventive control (current strategy), upsurge suppression, and plague elimination. These strategies relate to different times of human intervention during the development from an outbreak (the very early stage) through an upsurge to a plague. Preventive control is the current strategy of locust-affected countries and the FAO. Surveillance and control of the Desert Locust consist of the following three steps: 1. Monitoring of ecological conditions in the potential breeding areas, and delimiting of possible breeding areas using satellite imagery, weather reports and information from nomads. 2. Ground surveillance of potential breeding areas for gregarization of locusts carried out by teams who know the area and who are in close contact with nomads. In the initial stage of an upsurge, the hoppers often occur in patches and adults in groups. 3. Survey teams or special teams may control small infestations immediately, or keep the infestation under observation. In practice, the early stages described above are inconspicuous. Consequently, they are often not detected in time, and gregarization occurs. Moreover, an outbreak may occur in an area where access is difficult for logistical or security reasons. Then the strategy to be followed must be upsurge suppression, or containment. Hopper bands or swarms become larger and increasingly cohesive, and more of the population occurs in gregarious units. These are traced and controlled wherever and whenever possible to prevent upsurges developing into widespread plagues. The strategy may fail, partly because there may be insufficient time available for organizing adequate control. The strategy of plague elimination is aimed at destroying all bands and swarms wherever and whenever they are found. In the past, most of the control has been directed against hopper bands and swarms. Crop protection is carried out as the gregarious locusts move towards agricultural areas. In certain countries, like the Sudan, monitoring and control is often limited only to the agricultural areas because the whole breeding area of almost one million square kilometres is simply too extensive [20], The feasibility of the control strategies Dieldrin dilemma The feasibility of all these strategies may
be questioned, now that the organochlorine insecticide dieldrin is no longer available. The insecticide has been withdrawn from the market because of its persistence and bio-accumulation, and, consequently, its potentially negative effects on the environ-
ment. Dieldrin had been used extensively and effectively in locust control since the 1950s. It is not excreted from or detoxified in the insect’s body when a sub-lethal dose is ingested, but accumulates until a lethal level is reached and the insect dies. Dieldrin was used in ‘barrier’ spraying for hopper control. Vast tracts of land could be sprayed in a short time from the air, using distances between tracks of as much as 5km, with limited amounts of dieldrin (10-20 grams active ingredient per hectare). Control by this technique was very effective and cheap [21]. It had the additional advantage that individual bands did not need to be found before spraying, although the area within which hopper bands were present had to be identified. Contact insecticides
Instead of dieldrin, contact insecticides now have to be used. For logistical reasons, bands and settled swarms are treated with concentrated ultra low volume (ULV) formulations, using wide overlapping swaths. This can be done only by treating blocks which are almost invariably much larger than a single band. The detection and demarcation of blocks with a high percentage of band infestation is very difficult. On the other hand, treating individual bands by vehicle or on foot is time-consuming and only a small proportion of those present could be treated with the limited resources likely to be available. The situation is probably even more difficult with the numerous small gregarious hopper infestations characteristic of the early stages of an upsurge. Bands and swarms become progressively larger and more cohesive during the development of an upsurge. When the 1968 plague developed, locust numbers during two generations increased from two billion to 30 billion locusts, while the area infested decreased from over 100,000 km2 to about 5000 km2 [22]. Therefore, some countries, such as India, claim that only the later stages of upsurges can be contained [23]. Swarm versus hopper spraying Swarm spraying has been argued to be more
efficient than hopper control [24,25]. These authors even claim it to be the only feasible method of achieving general population reduction. PM. Symmons [25] calculated that to prevent the formation of a mediumsized swarm covering 40 km*, an area of something like 1000 km2 might have to be sprayed at the hopper stage. These calculations are based on the assumption that in blocks for ULV treatment, no more than 2 per cent is covered by bands and that 1 km2 of band gives rise to 2 km2 of swarm. Despite this, most campaigns concentrate on hopper control. This is unavoidable in the early stages of an upsurge when swarms are small and perhaps transitory. In late upsurges, swarm control is theoretically attractive. However, swarms may escape control efforts because of their
mobility and the limited time available. Control of swarms requires timely action and excellent organizational and logistic capabilities. The impact of chemical control operations
Theoretical calculations may give some indications about the feasibility of plague prevention by chemical control. By assuming an initial population, different multiplication rates and several generations, the final population can be calculated. For example, let us assume that no control is carried out and that during five generations the multiplication rate is 10 (this is plausible taking into account the rate of 16 claimed during upsurges). The population would then increase from, let us say, 10 million locusts (2 km* swarm) to 106 million, which is a 10s times increase (Figure 7). By controlling 90 per cent of the locusts at each generation, the population would remain at the same level. If control were not possible in one of the five generations, then the control rate in the other four would need to be between 94 and 95 per cent. A multiplication rate higher than five requires more than 80 per cent control. These theoretical calculations show that with a multiplication rate of five, plague prevention is possible only when insecticides can kill more than 80 per cent of the population. This assumes that all the hopper bands are sprayed as well as the scattered hoppers during the early upsurge breedings and it has been argued that such a coverage is practically impossible. Chemical control at high multiplication rates of the Desert Locust would then only slow the development of a plague but not suppress it. Doubts are also cast on efficiency rates of pesticide treatments in locust control. The standard method for the treatment of hoppers is to spray heavily infested blocks using ULV techniques. However, the method is often poorly understood and may seldom be applied correctly (P.M. Symmons, pers. commun., 1994). A replacement for dieldrin?
Testing of the benzoylurea insecticides (diflubenzuron, teflubenzuron) is continuing. These compounds inhibit the synthesis of chitin necessary for the formation of a new cuticle when moulting to a next stage. Death occurs during moults several days to weeks after ingestion of a lethal dose on treated foliage. Because of its action, the compound is active only against the nymphal stages of the locusts. It has few side-effects, as it acts only against arthropods that possess a chitinous exoskeleton. Although their degree of persistence on vegetation is satisfactory, these insecticides do not accumulate in the insect. It is doubtful whether they can effectively and economically replace dieldrin in hopper control. Safer contact insecticides?
International projects also focus on a replacement for the synthetic chemical con-
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Figure 8 Locust infected with the pathogen Metarhyzium anisopliae. (Source: International Institute of Biological Control.) ’ -
tact pesticides in order to avoid their negative side-effects. Fungal pathogens show promise. The fungi Metarhyzium spp. and Beauveria bassiana have lipophilic spores that are easily mixed with oils. Therefore, they can be used by ULV application. The oil penetrates into the thin membranes of the joints’ articulating surfaces, carrying spores to the most vulnerable parts of the cuticle. The spores then germinate and penetrate the cuticle. The fungus develops inside the body, competing with the tissues for nutrients. In one to two weeks, depending on the spore dose and the temperature, the insect dies from physiological starvation (Figure 8). But these entomopathogens do not provide a replacement for dieldrin although they do reduce environmental impact.
Conclusion The Desert Locust has an amazing capability to respond to environmentally favourable conditions. A rapidly increasing population is characterized by a complete change in behaviour, form and colour. Within the uninhabited desert area of millions of square kilometres these gregarious locusts can migrate large distances, often several thousand kilometres. Locust-affected countries and the international donor community are normally much slower in their response to a rapidly developing locust plague. Most locust-affected countries are able to maintain only a very low capacity of survey and control. In plague situations they have to rely on donor assistance for vehicles, aircraft, pesticides, fuel, etc. Also, the lack of trained people is a severe handicap. The 123
high number of countries involved makes international co-ordination difficult. During recession periods national locust control capability almost inevitably declines. Regional locust organizations face similar problems as they depend on contributions from member states. The main constraints~ to preventing upsurges can be summarized as follows: Technical
constraints
1. Excessive resources are necessary effectively to monitor and control early upsurges. 2. Monitoring is not possible due to inaccessibility or insecurity. 3. Detection rates of gregarious and gregarizing locusts at the start of an upsurge. 4. Insecticides are no longer available that are persistent and accumulate in the insect body. 5. Immediate available resources or backup capacity to respond quickly to developing upsurges are lacking. Organizational
constraints
1. Monitoring and control operations are organized on a country basis, making international co-ordination complex. 2. Organizing campaigns which require donor assistance is difficult (delays often occur). 3. There is a lack of trained manpower. 4. Locust-affected countries and donors lack interest during recession periods. Chemical control is still the only way to control locusts. New pesticides such as the insect growth regulators have not yet proved to be an effective replacement for die&in. Fungal spore preparations may replace chemical contact pesticides as an environmentally safe alternative, but they will not solve the logistical and organizational problem of control. Although techniques are improving, the better to identify suitable breeding areas, gregarizing or gregarious locusts still have to be found and controlled in large and remote areas. What is required is a clear strategy in order effectively to carry out control in space (priority areas) and time (at what time during an upsurge, and hopper versus
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swarm control). Methods should be able to ensure that pesticides have a considerable impact on the prevention or containment of plagues and do not simply pollute the environment. Updated studies on the feasibility and impact of different control strategies are still lacking. Donors and locust-affected countries should press for urgently needed studies on the feasibility and impact of diffeent control strategies. However, as J. Hewitt (quoted in [26]) has already mentioned: ‘The real problem facing operations of any kind during years of recession is that governments, while responding to crises, never anticipate them.’ Acknowledgements The suggestions made by K. Cressman, C. Kooyman, L. McCullock, E Meerman and P. Symmons are gratefully acknowledged. References 111 Uvarov, BP Bull. Ent. Res. 12, 135-63, ,011 17LI.
121 Gruys, P Grasshopperand locust campaigns 1986-89 and FAO’srole: a review. Unpublished report FAO-AGPP, 1991. r31 Pedgley,D. Desert Locust Forecasting Manual, Vol. 1. Centre for Overseas Pest Research,London, 1981. 141 Krall, S. in New Trends of Locust Control (eds S. Krall and H. Wilps), pp. 7-22. DeutscheGesellschaftftir Tecbnische Zusammeuarbeit(GTZ), Eschbom, Germany, 1994. I51 Ellis, PE., Carlisle, D.B. and Osborne, D.J. Science 149,546-47, 1965. 161 Carlisle, D.B., Ellis, PE. and Betts, E.J. Znsect Physiol. 11, 1541-58, 1965.
171 Richardson, C.H. and Nemeth, D.J. Geo. J. 23, 349-57, 1991.
181 Greathead, D.J. in Biological Control of Locusts and Grasshoppers
(eds C.J. Lomer and C. Prior), pp. 105-2 1. Proceedings of workshop held at the International Institute of Tropical Agriculture, Cotonou, Benin, 29 April-l May 1991. CAB International, Wallingford, 1992. 191 Hassanali, A. in Abstracts ofAdvances in Tropical Insect Sciences: HYPE’s Accomplishments and Future Prospects, p. 24.24th Annual Conference of the
ICIPE, Nairobi. ICIPE, Nairobi, 1994.
[lo] Popov, G.B. Anti-Locust Bull. 31, l-70, 1958. [l I] Uvarov, BP Grasshoppers and Locusts, Vol. 1. Cambridge University Press, London and New York, 1966. [ 121 Uvarov, BP. Grasshoppers nnd Locusts, Vol. 2. Centre for Overseas Pest Research, London, 1977. [13] Norris, M.J. and Richards, O.W. Entomologia fip. Appl. 6,279-303, 1963. [I41 Roffey, J. and Popov, G.B. Nature 219, 446-50, 1968. [15] Ashall, C. and Hemming, C.F. The Locust Handbook, Supplement 2 (197 1). The
Anti-Locust Research Centre, London, 1996. [16] Roffey, J. in Desert Locust Control with Existing Techniques: an Evaluation of Strufegies (ed. A. van Huis), pp. 5541.
Wageningen Agricultural University, Wageningen, The Netherlands, 1994. 1171 _ _ Reus. J.A.W.A. and Svmmons. P.M.
Bull. Ent. Res. 82.517-520, 1992. [ 181 Hielkema, J.U. in Meeting on Desert Locust Research Defining Future Research Priorities, pp. 61-70. FAO,
Rome, 1988. [19] Popov, G.B., Duranton, J.F. and Gigault, J. Etude 6cologique des biotopes du criquet pblerin Schistocerca gregaria (Forskil, 1775) en Afrique NordOccidentale: mise en evidence et description des unites territoriales Bcologiquement homogbnes. PRIFAS, Montpellier, France, 1991 [20] El-Tom, H.A. in Desert Zocust Control with Existing Techniques: an Evaluation of Strategies (ed. A. van Huis), pp. 81-84.
Wageningen Agricultural University, Wageningen, The Netherlands,l994. [21] Gunn, D.L. Phi.&. Truns. R. Sot. LomzimB. 287,375-86, 1979. [22] Bennett, L.V. Bull. Ent. Res. 66511-52, 1976. [23] van Huis, A. (ed.) Desert Locust Control with Existing Techniques: an Evaluation of Strategies. Proceedings of seminar held in
Wageningen, The Netherlands, 6-l 1 December 1993. Wageningen Agricultural University, Wageningen, The Netherlands, 1994. [24] Courshee, R.J. Int. Pest Control 32, 206-12.1990. [25] Symmons, PM. Crop Prot. 11, 206-12,
1992. [26] Skaf, R., Popov, G.B. and Roffey, J. Philos. Trans. R. Sot. London B. 328.525-38,
1990.
The Desert Locust, Schistocerca
gregaria
(Forskti), has been known from ancient times as a plague of agriculture in the Middle East and north Africa. Such plagues evoke images of huge sky-covering swarms that suddenly invade countries. Such swarms can develop when rainfall is abundant over subsequent breeding areas. Under these favourable conditions locusts are not only capable of increasing rapidly in numbers, but also of changing from a solitarious to a gregarious phase (Figure 1). The phase transformation is triggered when densities pass a critical level. During this process the whole appearance, physiology and behaviour of the locust changes. In 1921, BP Uvarov [l], the father of acridology (locust and grasshopper science), first described this phase transformation in the Migratory Locust (Locusta migratoria migratorioides). Before then the solitarious form of the Migratory Locust was erroneously assumed to be another species, viz. Locusta danica. Phase transformation distinguishes locusts from grasshoppers. Other important locust species are the Red Locust (Nomadacris septemfasciata) in central and southern Africa and the Brown Locust (Locustana pardalina) in southern Africa. Very damaging locust species can also be found in the Far East, Central and South America, and in Australia. Because of the spectacular appearance of swarms, locusts catch people’s imagination. This article will describe how the most dangerous locust species, the Desert Locust, can develop into a plague and what can be done to halt that development.
square kilometre belt of arid and semi-arid land extending from the Atlantic Ocean to north-west India and containing more than 25 countries (Figure 2). About 25 mm of rain in a month is sufficient to provide moisture for reproduction, successful egg development, and enough vegetation to feed and shelter the larvae. The wingless larvae are usually called ‘hoppers’. As the number of locusts rises rapidly, density increases and gregarious behaviour develops. Hoppers start to group, forming marching bands. The adults, which acquire wings during the last moult, may then form swarms and these can undertake long migratory flights. The great mobility of the locusts takes them beyond the deserts into agricultural areas where they can cause rapid and locally devastating crop losses. The invasion area extends over about 29 million square kilo-
metres, putting more than 65 countries intermittently at risk (Figure 2). There have been eight major plagues since 1860, varying in length between three and 22 years (Figure 3). The last plague occurred in large parts of Africa from 1986 to 1989. Twenty-six million hectares in the Sahel region and north Africa were treated with pesticides during this period, at a cost of about US$315 million [2]. Desert Locusts eat approximately their own weight (2 g) in fresh vegetation each day. Swarms are likely on average to contain 50 million individuals per square kilometre [3], so that even a ‘small’ swarm measuring 10 km2 eats some 1000 tons of fresh vegetation daily during migration. Although there are claims of impressive crop losses, hard evidence is lacking and attack is normally very localized [4].
Distribution and importance During recession periods, when density is low, the solitarious form of the Desert Locust generally inhabits a broad 16 million A. van Huis, Ph.D. From 1974-79 worked for the FAO in Nicaragua on integrated pest management in food crops, and this formed the basis of his Ph.D. thesis for the Wageningen Agricultural University in The Netherlands, where he is now Associate Professor in Tropical Entomology, dealing with a number of crop protection projects in developing countries, in particular, Africa.
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Figure 1 Locusts in the gregarious phase: (top) a young immature female; (bottom) a sexually mature male (a female would also be yellow but slightly less bright). 0 1995 Elsevier Science Ltd 0160-9327/95/$09.50
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Biology Life cycle Locusts have three development stages: eggs, hoppers and adults (Figure 4). Both hoppers and adults can cause crop damage. Eggs are laid in the soil in pods containing 50-150 eggs. Rainfall is required not only to incubate the eggs, but also to produce sufficient green vegetation to sustain hopper development following hatching. The period of egg incubation varies from several weeks to three to four months, depending mainly on temperature, while hopper development takes one to two months. The last moult produces winged adults, called fledgelings which then develop into fully adult locusts within a few days. Maturation Sexually immature locusts usually migrate about two to three weeks after fledging but the trigger leading to flight is not known. Solitarious locusts migrate by night and swarm by day, both being displaced downwind between successive breeding areas. Immature adults may mature rapidly, laying eggs within two to three weeks of fledging when conditions remain favourable in the breeding area. New green foliage, normally appearing after rains, accelerates maturation. This has been attributed to high concentrations of the plant growth hormone gibberellin A3, in new leaves [5]. Maturation may also be triggered by essential oils released by buds and flowers of the plant Commiphora myrrhae [6]. This is of functional significance because budding and flowering occur a few
weeks before the onset of the rains. Adult locusts may remain sexually immature, feeding on senescent vegetation, if rain does not fall in the areas to which they migrate. Eventually such locusts will die if there is no rain. Migration The Desert Locust is able to migrate over long distances. The swarms are carried by the wind, which may be why in parts of Africa locusts are called ‘teeth of the wind’. In October 1988 swarms commenced a six-day transatlantic journey from west Africa to the Caribbean, a distance of about 5OOOkm. Many of these locusts drowned in the Atlantic. Fortunately, the survivors failed to
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adapt to the Caribbean island environments and died without reproducing [7]. Natural mortaliw factors Besides drowning and starving, locusts are subjected to other natural mortality factors. Natural enemies like parasitic flies and trogid and blister beetles are able to consume most of the eggs in an egg-pod [8]. Hoppers and adults are preyed on by many vertebrate predators such as small carnivores, insectivores, rodents, reptiles, resident and migrant birds, and even men. Invertebrate natural enemies include predators such as scorpions, ground beetles, sphecid wasps, and parasitic flies. Although parasitoids and
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Major plagues Figure 3
Major plagues of the Desert Locust. (Source: FAO.)
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Larvae
Eggs
Females deposit their egg-pods close to those of other females [lO-121, thus producing high densities following hatching. Egg laying in groups is not caused solely by localized favourable properties of the soil (such as sandiness and humidity), but is also enhanced by a volatile signal from the egg froth [9]. Besides chemical, visual and tactile responses are thought to be responsible for aggregated oviposition [ 131. How do plagues develop? J. Roffey and G.B. Popov [14] described three processes characteristic of the phase transformation: concentration, multiplication and gregarization (Figure 5) [ 151. Roffey [16] gave a detailed description of these phenomena. He was an eye witness of an early upsurge in 1967 in Tamesna, northern Niger.
Adults
The first phase of the transformation process was concentration. Adults migrated from distances probably up to 1OOOkm. They landed on green vegetation, days if not weeks after the rains which led to its development. These adults matured rapidly on the green vegetation. Concentration within the habitat occurred by locusts restricting themselves to the scarce vegetation. It also occurred through adults laying egg-pods in clusters of about five per m2. Even up to 70 egg-pods have been found in areas of only 0.1 m2.
I
Fiaure 4 The life cycle of the Desert Locust: ovipositing female; egg-pod; five larval instars (srages); copulating gdults.
predators reduce the multiplication rate and may hasten population decline, they do not prevent outbreaks. Augmenting these natural enemies is not feasible because the vast numbers required can be produced only in living host insects. Infections with men&hid nematodes and fungi occasionally kill entire swarms. The most prominent fungus isolated from grasshoppers and locusts is Entomophaga ‘gtylli’, a frequent cause of natural epizootics that reduce host density. With other pathogenic organisms, this fungus probably regulates some grasshopper and locust species in humid conditions. Entomopox viruses have been found in grasshoppers in west Africa. Protozoic diseases occur but are of little significance in field populations [8]. Semiochemicals Laboratory research has tried to elucidate the mechanisms responsible for phase tmnsformation and it is believed that semiochemicals play an important role. These are
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compounds that mediate the exchange of information between organisms. When used for communication within a species, they are termed pheromones. Why are gregarious locusts, contrary to solitarious locusts, so synchronized in their behaviour? Scientists at the International Centre of Insect Physiology and Ecology (ICIPE) in Nairobi, Kenya, believe that two distinct pheromone systems are responsible, one for the adults and the other for the hoppers. The chemical components involved have been identified and synthetic blends of these have been used to study their effects. The pheromone blend produced by older gregarious males caused gregarious behaviour in young and older males and females, and accelerated maturation. Hopper pheromones induced aggregation of all hopper stages except the first and retarded maturation of both male and female fledgelings. So, it seems that the pheromone systems of adults and hoppers may be the cause of the synchronous develop ments in gregarious populations [9].
Multiplication concerns the increase of the population from one generation to another. Roffey [ 161 claimed that during the upsurge in the Tamesna three egg-pods were laid with a total of about 400 eggs, which is double the egg production of gregarious females. Egg and hopper mortality was estimated to have been about 92 per cent, which means that the multiplication rate of the population was 16 (one couple giving 32 offspring). Gregarization occurs when there is no further rain and hoppers collect in the remaining favourable vegetation (Figure 6). They encounter each other more frequently in these restricted sites. Avoidance is then replaced by mutual attraction. The degree of gregarization in the adult stage influences the stage of gregarization of the hatchlings in the next generation. From outbreaks to plagues It normally takes several years for a plague to develop. The first phase leading to a plague is called an outbreak, or an early upsurge, and the next phase an upsurge. At the beginning of an upsurge, hoppers are in small patches or groups, while the adults are still scattered. As the population increases, bands and swarms develop, becoming larger and more cohesive each time. The population increasingly occurs in gregarious units and consequently the infested area shrinks. The dimensions and locust numbers in recession and early upsurge swarms are small compared to those characteristic of plagues.
Characteristic plague swarms may reach tens, and sometimes hundreds, of square kilometres at densities ranging from 20 to 150 million locusts per square kilometre [3]. A plausible size of settled swarm invading a country during a plague would be about 1200 km* (this might be composed of about 20 medium-sized swarms) [3]. The size of hopper bands varies greatly, and depends on the larval stage (Figure 4). Bands of newly emerged hoppers (hatchlings) measure several square metres, containing 30,000 larvae per square metre when settled and 250-500 when marching. Bands of last stage larvae vary from one hectare to several square kilometres, containing 1000 hoppers per square metre when settled and 10-30 when marching [3]. Drought, control, and movement of populations to areas unsuitable for breeding and survival are cited as playing a role in ending plagues. However, their relative importance is often not clear, and, in particular, the impact of control efforts is a controversial issue.
SCATTERED LOCUSTS
LARGE-SCALE CONCENTRATION within convergent winds and by settling in green areas
SMALL-SCALE CONCENTRATION occurs when immigrants move to preferred habitats and to restricted laying sites
MULTIPLICATION follows good rains numbers increase and close contact leads to
GREGARIZATION groups form and fuse to become
HOPPER BANDS AND SWARMS \
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Gregarization processes (after Ashalland and Hemming [15]).
Monitoring and forecasting The information required to forecast locust development includes the weather (wind, ram and temperature), and habitat conditions conducive to supporting locust development and the presence of locusts in those habitats. The Food and Agriculture Organization of the United Nations (FAO) in Rome co-ordinates locust monitoring. Reports of the locust situation are received by telex, fax and post from locust-affected countries and regional locust organizations. Reliable information on the locusts’ stage of
Patches of hoppers may occur at the base of or within clumps of vegetation. (Reproduced by kind permission of K. Cressman, FAO.)
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development, population density and degree of gregarization should normally be provided regularly by ground survey teams. Data regarding egg-laying, hatching and fledging are calculated using a computer model developed at the FAO and based on the relationship between temperature and development rate [ 171. The main aim of weather monitoring is to detect when and where it has rained in the Desert Locust recession area. Both daily rainfall data from meteorological services and satellite imagery are utilized. Infrared images received from the Meteosat satellite are analysed for cold clouds that may produce significant rainfall. Cloud-top temperature is translated into rainfall maps using predefined relationships between cold cloud duration and actual observed rainfall. The ARTEMIS (Africa Real Time Environmental Monitoring using Imaging Satellites) system, used by the FAO Remote Sensing Centre in Rome, estimates the number of rainfall days during a lo-day period [18]. Habitat monitoring involves detecting the presence of suitable vegetation in the Desert Locust recession area. The FAO uses satellite data to detect the relative amount of green-leaf vegetation biomass. However, high spatial resolution is required to detect the sparse vegetation in the desert areas where the locust matures and breeds. The technique needs to be further developed if it is to be of reliable use. Apart from satellite imagery, the suitability of biotopes also determines Desert Locust population development. G.B. Popov et al. [19] have described the bio-ecological potential of different biotypes and delimited their area in north-west Africa, west of the River Nile. The study was based on ecological data from the Sahara, the experience of locust survey teams, and historical data on locust outbreaks. J.U. Hielkema [ 181claimed that only approximately 6 per cent of the 16 million square kilometre area inhabited during a recession can support development of Desert Locust populations in terms of soil/vegetation and geomorphologicah topographic conditions. The information on suitable Desert Locust habitats is being combined with the rainfall and vegetation data obtained by satellite imagery in a Geographical Information System (GIS) based at FAO headquarters. This should improve reliability when identifying potential Desert Locust breeding areas. This information can then be used by ground survey teams to select high priority areas for monitoring. In order to help estimate swarm movements a wind trajectory model has just been developed for the FAO by Meteoconsult in Wageningen, The Netherlands. The FAO prepares the monthly Desert Locust Bulletin summarizing the current situation and forecasting likely events for the next six weeks. This bulletin is sent by fax, e-mail and post to locust-affected countries and donors.
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Control strategies Three different strategies of control can be distinguished: preventive control (current strategy), upsurge suppression, and plague elimination. These strategies relate to different times of human intervention during the development from an outbreak (the very early stage) through an upsurge to a plague. Preventive control is the current strategy of locust-affected countries and the FAO. Surveillance and control of the Desert Locust consist of the following three steps: 1. Monitoring of ecological conditions in the potential breeding areas, and delimiting of possible breeding areas using satellite imagery, weather reports and information from nomads. 2. Ground surveillance of potential breeding areas for gregarization of locusts carried out by teams who know the area and who are in close contact with nomads. In the initial stage of an upsurge, the hoppers often occur in patches and adults in groups. 3. Survey teams or special teams may control small infestations immediately, or keep the infestation under observation. In practice, the early stages described above are inconspicuous. Consequently, they are often not detected in time, and gregarization occurs. Moreover, an outbreak may occur in an area where access is difficult for logistical or security reasons. Then the strategy to be followed must be upsurge suppression, or containment. Hopper bands or swarms become larger and increasingly cohesive, and more of the population occurs in gregarious units. These are traced and controlled wherever and whenever possible to prevent upsurges developing into widespread plagues. The strategy may fail, partly because there may be insufficient time available for organizing adequate control. The strategy of plague elimination is aimed at destroying all bands and swarms wherever and whenever they are found. In the past, most of the control has been directed against hopper bands and swarms. Crop protection is carried out as the gregarious locusts move towards agricultural areas. In certain countries, like the Sudan, monitoring and control is often limited only to the agricultural areas because the whole breeding area of almost one million square kilometres is simply too extensive [20], The feasibility of the control strategies Dieldrin dilemma The feasibility of all these strategies may
be questioned, now that the organochlorine insecticide dieldrin is no longer available. The insecticide has been withdrawn from the market because of its persistence and bio-accumulation, and, consequently, its potentially negative effects on the environ-
ment. Dieldrin had been used extensively and effectively in locust control since the 1950s. It is not excreted from or detoxified in the insect’s body when a sub-lethal dose is ingested, but accumulates until a lethal level is reached and the insect dies. Dieldrin was used in ‘barrier’ spraying for hopper control. Vast tracts of land could be sprayed in a short time from the air, using distances between tracks of as much as 5km, with limited amounts of dieldrin (10-20 grams active ingredient per hectare). Control by this technique was very effective and cheap [21]. It had the additional advantage that individual bands did not need to be found before spraying, although the area within which hopper bands were present had to be identified. Contact insecticides
Instead of dieldrin, contact insecticides now have to be used. For logistical reasons, bands and settled swarms are treated with concentrated ultra low volume (ULV) formulations, using wide overlapping swaths. This can be done only by treating blocks which are almost invariably much larger than a single band. The detection and demarcation of blocks with a high percentage of band infestation is very difficult. On the other hand, treating individual bands by vehicle or on foot is time-consuming and only a small proportion of those present could be treated with the limited resources likely to be available. The situation is probably even more difficult with the numerous small gregarious hopper infestations characteristic of the early stages of an upsurge. Bands and swarms become progressively larger and more cohesive during the development of an upsurge. When the 1968 plague developed, locust numbers during two generations increased from two billion to 30 billion locusts, while the area infested decreased from over 100,000 km2 to about 5000 km2 [22]. Therefore, some countries, such as India, claim that only the later stages of upsurges can be contained [23]. Swarm versus hopper spraying Swarm spraying has been argued to be more
efficient than hopper control [24,25]. These authors even claim it to be the only feasible method of achieving general population reduction. PM. Symmons [25] calculated that to prevent the formation of a mediumsized swarm covering 40 km*, an area of something like 1000 km2 might have to be sprayed at the hopper stage. These calculations are based on the assumption that in blocks for ULV treatment, no more than 2 per cent is covered by bands and that 1 km2 of band gives rise to 2 km2 of swarm. Despite this, most campaigns concentrate on hopper control. This is unavoidable in the early stages of an upsurge when swarms are small and perhaps transitory. In late upsurges, swarm control is theoretically attractive. However, swarms may escape control efforts because of their
mobility and the limited time available. Control of swarms requires timely action and excellent organizational and logistic capabilities. The impact of chemical control operations
Theoretical calculations may give some indications about the feasibility of plague prevention by chemical control. By assuming an initial population, different multiplication rates and several generations, the final population can be calculated. For example, let us assume that no control is carried out and that during five generations the multiplication rate is 10 (this is plausible taking into account the rate of 16 claimed during upsurges). The population would then increase from, let us say, 10 million locusts (2 km* swarm) to 106 million, which is a 10s times increase (Figure 7). By controlling 90 per cent of the locusts at each generation, the population would remain at the same level. If control were not possible in one of the five generations, then the control rate in the other four would need to be between 94 and 95 per cent. A multiplication rate higher than five requires more than 80 per cent control. These theoretical calculations show that with a multiplication rate of five, plague prevention is possible only when insecticides can kill more than 80 per cent of the population. This assumes that all the hopper bands are sprayed as well as the scattered hoppers during the early upsurge breedings and it has been argued that such a coverage is practically impossible. Chemical control at high multiplication rates of the Desert Locust would then only slow the development of a plague but not suppress it. Doubts are also cast on efficiency rates of pesticide treatments in locust control. The standard method for the treatment of hoppers is to spray heavily infested blocks using ULV techniques. However, the method is often poorly understood and may seldom be applied correctly (P.M. Symmons, pers. commun., 1994). A replacement for dieldrin?
Testing of the benzoylurea insecticides (diflubenzuron, teflubenzuron) is continuing. These compounds inhibit the synthesis of chitin necessary for the formation of a new cuticle when moulting to a next stage. Death occurs during moults several days to weeks after ingestion of a lethal dose on treated foliage. Because of its action, the compound is active only against the nymphal stages of the locusts. It has few side-effects, as it acts only against arthropods that possess a chitinous exoskeleton. Although their degree of persistence on vegetation is satisfactory, these insecticides do not accumulate in the insect. It is doubtful whether they can effectively and economically replace dieldrin in hopper control. Safer contact insecticides?
International projects also focus on a replacement for the synthetic chemical con-
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Figure 8 Locust infected with the pathogen Metarhyzium anisopliae. (Source: International Institute of Biological Control.) ’ -
tact pesticides in order to avoid their negative side-effects. Fungal pathogens show promise. The fungi Metarhyzium spp. and Beauveria bassiana have lipophilic spores that are easily mixed with oils. Therefore, they can be used by ULV application. The oil penetrates into the thin membranes of the joints’ articulating surfaces, carrying spores to the most vulnerable parts of the cuticle. The spores then germinate and penetrate the cuticle. The fungus develops inside the body, competing with the tissues for nutrients. In one to two weeks, depending on the spore dose and the temperature, the insect dies from physiological starvation (Figure 8). But these entomopathogens do not provide a replacement for dieldrin although they do reduce environmental impact.
Conclusion The Desert Locust has an amazing capability to respond to environmentally favourable conditions. A rapidly increasing population is characterized by a complete change in behaviour, form and colour. Within the uninhabited desert area of millions of square kilometres these gregarious locusts can migrate large distances, often several thousand kilometres. Locust-affected countries and the international donor community are normally much slower in their response to a rapidly developing locust plague. Most locust-affected countries are able to maintain only a very low capacity of survey and control. In plague situations they have to rely on donor assistance for vehicles, aircraft, pesticides, fuel, etc. Also, the lack of trained people is a severe handicap. The 123
high number of countries involved makes international co-ordination difficult. During recession periods national locust control capability almost inevitably declines. Regional locust organizations face similar problems as they depend on contributions from member states. The main constraints~ to preventing upsurges can be summarized as follows: Technical
constraints
1. Excessive resources are necessary effectively to monitor and control early upsurges. 2. Monitoring is not possible due to inaccessibility or insecurity. 3. Detection rates of gregarious and gregarizing locusts at the start of an upsurge. 4. Insecticides are no longer available that are persistent and accumulate in the insect body. 5. Immediate available resources or backup capacity to respond quickly to developing upsurges are lacking. Organizational
constraints
1. Monitoring and control operations are organized on a country basis, making international co-ordination complex. 2. Organizing campaigns which require donor assistance is difficult (delays often occur). 3. There is a lack of trained manpower. 4. Locust-affected countries and donors lack interest during recession periods. Chemical control is still the only way to control locusts. New pesticides such as the insect growth regulators have not yet proved to be an effective replacement for die&in. Fungal spore preparations may replace chemical contact pesticides as an environmentally safe alternative, but they will not solve the logistical and organizational problem of control. Although techniques are improving, the better to identify suitable breeding areas, gregarizing or gregarious locusts still have to be found and controlled in large and remote areas. What is required is a clear strategy in order effectively to carry out control in space (priority areas) and time (at what time during an upsurge, and hopper versus
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swarm control). Methods should be able to ensure that pesticides have a considerable impact on the prevention or containment of plagues and do not simply pollute the environment. Updated studies on the feasibility and impact of different control strategies are still lacking. Donors and locust-affected countries should press for urgently needed studies on the feasibility and impact of diffeent control strategies. However, as J. Hewitt (quoted in [26]) has already mentioned: ‘The real problem facing operations of any kind during years of recession is that governments, while responding to crises, never anticipate them.’ Acknowledgements The suggestions made by K. Cressman, C. Kooyman, L. McCullock, E Meerman and P. Symmons are gratefully acknowledged. References 111 Uvarov, BP Bull. Ent. Res. 12, 135-63, ,011 17LI.
121 Gruys, P Grasshopperand locust campaigns 1986-89 and FAO’srole: a review. Unpublished report FAO-AGPP, 1991. r31 Pedgley,D. Desert Locust Forecasting Manual, Vol. 1. Centre for Overseas Pest Research,London, 1981. 141 Krall, S. in New Trends of Locust Control (eds S. Krall and H. Wilps), pp. 7-22. DeutscheGesellschaftftir Tecbnische Zusammeuarbeit(GTZ), Eschbom, Germany, 1994. I51 Ellis, PE., Carlisle, D.B. and Osborne, D.J. Science 149,546-47, 1965. 161 Carlisle, D.B., Ellis, PE. and Betts, E.J. Znsect Physiol. 11, 1541-58, 1965.
171 Richardson, C.H. and Nemeth, D.J. Geo. J. 23, 349-57, 1991.
181 Greathead, D.J. in Biological Control of Locusts and Grasshoppers
(eds C.J. Lomer and C. Prior), pp. 105-2 1. Proceedings of workshop held at the International Institute of Tropical Agriculture, Cotonou, Benin, 29 April-l May 1991. CAB International, Wallingford, 1992. 191 Hassanali, A. in Abstracts ofAdvances in Tropical Insect Sciences: HYPE’s Accomplishments and Future Prospects, p. 24.24th Annual Conference of the
ICIPE, Nairobi. ICIPE, Nairobi, 1994.
[lo] Popov, G.B. Anti-Locust Bull. 31, l-70, 1958. [l I] Uvarov, BP Grasshoppers and Locusts, Vol. 1. Cambridge University Press, London and New York, 1966. [ 121 Uvarov, BP. Grasshoppers nnd Locusts, Vol. 2. Centre for Overseas Pest Research, London, 1977. [13] Norris, M.J. and Richards, O.W. Entomologia fip. Appl. 6,279-303, 1963. [I41 Roffey, J. and Popov, G.B. Nature 219, 446-50, 1968. [15] Ashall, C. and Hemming, C.F. The Locust Handbook, Supplement 2 (197 1). The
Anti-Locust Research Centre, London, 1996. [16] Roffey, J. in Desert Locust Control with Existing Techniques: an Evaluation of Strufegies (ed. A. van Huis), pp. 5541.
Wageningen Agricultural University, Wageningen, The Netherlands, 1994. 1171 _ _ Reus. J.A.W.A. and Svmmons. P.M.
Bull. Ent. Res. 82.517-520, 1992. [ 181 Hielkema, J.U. in Meeting on Desert Locust Research Defining Future Research Priorities, pp. 61-70. FAO,
Rome, 1988. [19] Popov, G.B., Duranton, J.F. and Gigault, J. Etude 6cologique des biotopes du criquet pblerin Schistocerca gregaria (Forskil, 1775) en Afrique NordOccidentale: mise en evidence et description des unites territoriales Bcologiquement homogbnes. PRIFAS, Montpellier, France, 1991 [20] El-Tom, H.A. in Desert Zocust Control with Existing Techniques: an Evaluation of Strategies (ed. A. van Huis), pp. 81-84.
Wageningen Agricultural University, Wageningen, The Netherlands,l994. [21] Gunn, D.L. Phi.&. Truns. R. Sot. LomzimB. 287,375-86, 1979. [22] Bennett, L.V. Bull. Ent. Res. 66511-52, 1976. [23] van Huis, A. (ed.) Desert Locust Control with Existing Techniques: an Evaluation of Strategies. Proceedings of seminar held in
Wageningen, The Netherlands, 6-l 1 December 1993. Wageningen Agricultural University, Wageningen, The Netherlands, 1994. [24] Courshee, R.J. Int. Pest Control 32, 206-12.1990. [25] Symmons, PM. Crop Prot. 11, 206-12,
1992. [26] Skaf, R., Popov, G.B. and Roffey, J. Philos. Trans. R. Sot. London B. 328.525-38,
1990.