June

Feature article / En vedette
By Emma Despland
Locust plagues then and now
So Moses stretched forth his rod over the The desert locust (Schistocerca gregaria, land of Egypt, and the Lord brought an east Orthoptera: Acrididae) is the world's most wind upon the land all that day and all that notorious insect pest. As these quotes in- night; and when it was morning the east wind dicate, the desert locust has plagued Western civ- had brought the locusts. And the locusts came ilization since the dawn of recorded history. Still up over all the land of Egypt, and settled on today, it constitutes a severe threat to both sub- the whole country of Egypt, such a dense sistence and commercial agriculture across Afri- swarm of locusts as had never been before, ca, the Middle East and Eastern Asia — as testi- nor ever shall be again. For they covered the fied by the swarms currently ravaging Northern face of the whole land, so that the land was darkened, and they ate all the plants in the In the summer of 2003, the right amount of land and all the fruit of the trees which the rain in the right places at the right times led to hail had left; not a green thing remained, excellent locust breeding conditions in the West- neither tree nor plant of the field, through all ern Sahel and along the Red Sea Coast, two noto- the land of Egypt. Then Pharaoh called Mo- rious locust breeding zones that have given rise ses and Aaron in haste, and said, "I have to many outbreaks in the past (Popov 1997) — sinned against the Lord your God, and indeed, Moses' east wind brought locusts from against you. Now therefore, forgive my sin, I the Red Sea coastal plains. Locust populations pray you, only this once, and entreat the Lord grew and gregarised, and raised alarm that out- your God only to remove this death from breaks might be developing. In the winter 2003- me." So he went out from Pharaoh, and en- 2004, the Red Sea locusts were unable to find treated the Lord. And the Lord turned a very suitable breeding conditions, and, instead of mi- strong west wind, which lifted the locusts and grating into Darfur as was feared, they died out, drove them into the Red Sea; not a single dispersed and disappeared. However, in Western locust was left in all the country of Egypt. Africa, the swarms formed in the Sahel migrated north to the Mediterranean coast, where vegeta-tion flourishes following the winter rains. Theirprogeny returned to the Sahel in the summer of2004 where breeding conditions were once again containing thousands of locusts per square metre excellent, and the population exploded. Swarms and covering tens of square kilometres have beenobserved last fall leaving the Sahel, moving Northtowards fruit orchards on the Mediterranean coastand south towards tropical farmland. Each swarm Emma Despland has been an Assistant Profes- can easily contain billions of individuals and eat sor in the Biology Department at Concordia more in a day than the entire human population of University, Montreal, Quebec, Canada, since New York does in a week. This past winter, 2002. Before that, she worked at the University numerous swarms criss-crossed Morocco and of Oxford (UK) studying phase change in the Algeria and raised fears about the onset of a new desert locust. Her research interests involve in- major plague(Enserink 2004; FAO 2005).
dividual behaviour of outbreaking insects, Locust outbreaks are based on a individual- focussing mainly on feeding, social interactions level change from a solitarious to a gregarious and density-dependent phenomena. Contact de- form (Simpson et al.1999). So different are these two forms, that they were long considered to be (514) 848-2424 ext 3426; Web page: two separate species until Sir Boris Uvarov, in the 1920s, showed that an individual locust could Bulletin of the Entomological Society of Canada Volume 37(2), June 2005 En vedette
change from one to the other (Uvarov 1977). At factors at different spatial scales, making locust low population density, locusts are in the solitar- swarms very difficult to predict (Despland et al.
ious form: they are green and timid, don't move much, avoid one another and are barely noticed Control efforts are currently underway across by humans. However, when population density Northern and Western Africa to halt the progres- is high, locusts switch to the gregarious form: the sion of swarms: ground and aerial spraying of most dramatic change is their adoption of bright insecticides, mainly organophosphates, were used black-and-yellow stripes. However, more impor- to treat 2.2 million ha in November 2004, and an tantly, gregarious locusts are much more active additional 880 000 ha in December (FAO 2005).
and are attracted to one another. This change in More environmentally-friendly control agents, behaviour leads them to aggregate and form large including the entomopathogenic fungus Me- groups that migrate together, destroying all vege- tarhizium and insect growth regulators, are being tation in their path. The switch between forms is developed, but have not reached the stage where triggered by contact with other locusts (Simpson they are used operationally. Widespread spraying et al. 2001). So when density is high, contact against locusts raises concerns not only about between individuals increases, locusts start be- risks to non-target organisms including livestock having gregariously and aggregate, further in- and people, but also about its efficacy. Locust creasing contact between individuals. This feed- swarms are very mobile and travel in very remote back loop creates a snowball effect that can spiral areas, and, during outbreak conditions, it is im- further and further, generating plague conditions possible to find, let alone treat, all swarms. Lo- where hundreds of swarms migrate across conti- cust swarms can cover hundreds of kilometers nents, following weather conditions suitable for per day with the right winds, and often cross international borders. Control operations there- Once a locust population has gregarised, it can fore require precise up-to-date information on the seed an outbreak if it remains concentrated and positions of swarms, good infrastructure from multiplies. However, if vegetation in the locust's which control efforts can be mounted wherever desert breeding grounds is too sparse and if to- they are needed, and international cooperation.
pography and winds break up developing swarms, The history of locust control efforts shows the the locusts can be decimated and dispersed, and extent of these logistical challenges.
swarms can simply disappear. Both the gregari- The last big locust plague occurred in 1986- sation of solitarious populations and the further 1989. Gregarious populations appeared in the expansion of these gregarious populations into notorious breeding areas along the Red Sea coast.
swarms depend on a multitude of environmental Pest management was neglected during the Ethi-opian-Eritrean war, and locust populations grewunchecked in breeding areas strewn with land- The locusts lay their eggs and die in like mines. Swarms soon spread to 23 countries.
manner after laying them. Their eggs are Massive control efforts were deployed and 25 subject to destruction by the autumn rains, million ha were sprayed with insecticide at a cost when the rains are unusually heavy; but in of US$ 310 million (Showler 2002). Hindsight seasons of drought the locusts are exceed- now makes it clear that, although insecticide spray- ingly numerous, from the absence of any de- ing saved valuable crops, the plague was brought structive cause, since their destruction seems to an end, not by control efforts, but by natural then to be a matter of accident and to depend conditions. Like in the book of Exodus, swarms were blown out to sea and drowned; others were Aristotle (350 B.C.E.) History of Animals, lost on barren desert sands and poor rainfall im- Past experience thus suggests that it is next to Volume 37(2) juin, 2005 Bulletin de la Société d'entomologie du Canada Feature article
impossible to bring severe plagues under control evaluate the relative value of different control strat- before they run themselves out naturally. Reac- egies. Moreover, the practicality, desirability and tive interventions against large swarms are very effectiveness of locust control strategies remain costly, both financially and environmentally, and controversial. Although we are no longer quite as are not effective at halting plagues, although they powerless before locusts as were ancient peo- can be critical in protecting valuable crops. Alter- ples, Aristotle's observation still holds true, that, native proactive control strategies involve regular during a major plague, "their destruction seems monitoring of populations in high risk areas, and then to be a matter of accident and to depend on spraying gregarious populations when they be- gin to pose a threat but before they spiral out ofcontrol (Showler 2002). These strategies can be References
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afford to spare these resources on insects that are 2004. Landscape structure and locust swarm- not causing an immediate problem, when so many ing: A satellite's eye view. Ecography, 27:381–
other problems (including other pest insects) de- Enserink M. 2004. Can the war on locusts be Some experts therefore advocate abandoning won? Science, 306:1880–1882.
the hope of controlling outbreaks and focussing FAO 2004. Rapport spécial: mission FAO/PAM on protecting high value crops, leaving the lo- d'évaluation des récoltes et des disponibilités custs free rein with other vegetation (Enserink alimentaires en Mauritanie, axé plus par- 2004). Indeed, although locust plagues can be ticulièrement sur les pertes dues aux criquets catastrophic locally, the damage they cause is rel- pèlerins. Tech. report, Système Mondial atively small compared to other agricultural threats such as droughts and endemic pests, and does not have long lasting ecological consequences. A FAO (2005.) Situation update. Tech. rep., Desert landscape can appear totally desolate after the passage of locusts, but desert vegetation is by nature transient and resilient, and grows back rap- idly to its original state if rainfall is suitable. How- Popov G.B. 1997. Atlas of desert locust breeding ever, under this minimalist reactive approach, crops and pasturelands of low financial value are Showler A.T. 2002. A summary of control strat- left to the locusts, and these resources are essen- egies for the desert locust Schistocerca gre- tial to economically vulnerable subsistence farm- garia (Forskål). Agriculture Ecosystems and ers and herders, who then need to be compensat- Environment, 90: 97–103.
ed for their losses. In many cases, food aid may Simpson S.J., Despland E., Hägele B., and Dodg- be required. In 2004, Mauritania bore the brunt son T. 2001. Gregarious behavior in desert of most locust activity, and the crops that remain locusts is evoked by touching their back legs.
are expected to meet only 21% of cereal require- Proceedings of the National Academy of Sci- ments (FAO 2004). Another 20% have been se- ences, 98: 3895–3897.
cured via commercial imports and food aid pledg- Simpson S.J., McCaffery A.R., and Hägele B.
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1999. A behavioural analysis of phase change The true costs of locust outbreaks are therefore in the desert locust. Biological Reviews, 74: 461–
not only financial but also social, political and humanitarian. This makes it particularly difficult Uvarov B.P. 1977. Grasshoppers and locusts, to assess the importance of locust damage and to Bulletin of the Entomological Society of Canada Volume 37(2), June 2005

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