Schistocerca gregaria (Forsskål)
Taxonomic placing: Insecta, Hemimetabola, Hemiptera, Orthoptera, Acridoidea, Acrididae.
Common name: Desert locust.
Geographical distribution: West and North Africa, the Mediterranean region, the Middle East and Turkey, Iran and the Central Asian Republics.
Host plants: Highly polyphagous, feeding on many rangeland shrubs, grasses and on economic crops, during pest outbreaks. Millet {Pennisetum typhoideum (L.) R.Br.} is a preferred plant.
Morphology: The desert locust occurs in 2 forms, solitariform and gregariform, which differ in their morphology. Both sexes of the solitariform are green to light-brown, reflecting their environment, and their pronota bear convex crests. The gregariform male is yellow with reddish markings whereas the female is light brown in color; its pronota are concave or straight. The immature adult locusts are pink. The females are 50-60 mm long, males about 40-50 mm.
Life history: The desert locust usually develops in primary breeding sites, mainly in semi-arid and arid deserts, raising 2-5 annual generations. Females of both forms deposit clutches of egg pods in suitably-wet soils. Solitarious females produce about 60-80 eggs/clutch, gregarious females approximately 40-60. The eggs hatch in about two weeks. Gregariform nymphs (called hoppers) pass through five instars before becoming adults and solitariform nymphs pass through six. Solitariform individuals tend to disperse, whereas gregariform nymphs tend to aggregate, being more active. Under optimal conditions (30°C and enough food) development in the laboratory required about 8 weeks, of which 3 for the nymphs. Under unfavorable conditions development may require several months. The young adults are reproductively immature and their pre-reproductive period is synchronized with rainfall and growth of desert plants in their primary breeding sites. Reproductive maturation usually takes about three weeks when food is available, but can be delayed during drough, when the rainfall is less than 200 mm. Heavier rains in the breeding sites initiate rich plant growth that leads to a greater locust numbers. Within 2-3 generations of growth the increasing population density and the scarcity of food during dry periods induce the transformation to the gregariform phase. These nymphs aggregate and march together in bands, whereas adults fly in dense and highly mobile swarms, migrating actively in the direction of prevailing winds. Their swarms may consist of many hundred million locusts. Their swarms ascend on thermal currents during day to considerable heights, where they may encounter wind directions differing from those found at lower altitudes. A swarm can daily fly 100 km, borne on the prevailing winds, covering areas ranging from 1 to 1000 km2, at a an average density of around 100 individuals/ meter2, but may be much bigger. Reproduction is usually delayed during flight, but as soon as the locusts alight at a new, plant-rich site, they initiate feeding and laying egg pods.
Economic importance: The book of Joel, chapter 1, verses 4-7, states: “What the cutting locust left, the swarming locust has eaten. What the swarming locust left, the hopping locust has eaten, and what the hopping locust left, the destroying locust has eaten”. Plagues of locusts have caused famines and starvation to millions of the world’s humans throughout history by greatly endangering their food security, especially in Africa, but also in the Middle East and Asia. The last major desert locust upsurge in 2004–05 caused significant crop losses in West Africa. Damage caused by this pest is due to its polyphagous nature, the density of the population, its aggregation and swarming and to its ability to fly rapidly across great distances. Each individual gregarious locust can daily consume its own weight in foliage. This partly explains how dense swarms of adults, or marching bands of nymphs, can inflict considerable injury in a short time. The damage caused by gregarious swarming populations occurs in cycles, in most cases afflicting economies in fragile ecosystems that are least able to sustain complete crop loss. The desert locust is a very dangerous pest and its great range requires international cooperation for effective control. The FAO has a special website, http://www.fao.org/ag/locusts/en/info/info/index.html, which broadcasts up-to-date information about the pest, and the monthly Desert Locust Bulletin (last issue June 2015, #440). The FAO also publishes the Locust Technical Series and similar information.
The desert locust problem has nowadays become controversial and had acquired political aspects. This is because swarms migrate between countries, some of which will suffer no harm, the lack of field evidence that preventive measures are effective, that the costs of locust damage are not known, nor whether these costs balance the expense of the prevented damage. However, new data obtained with geographical information systems (GIS), and survey and application techniques using global positioning systems (GPS), may lead to more precision spraying.
Management
Monitoring: Populations are monitored during recession as well as outbreak periods in areas most likely to harbor locust populations that could undergo phase transformation, using remotely sensed images that depict green vegetation. This is continuisly combined with relevant GIS data and historical locust frequency information. Short and medium-term forecasts are prepared indicating potential locust migrations and areas of breeding. These forecasts form the basis of action plans in affected countries. More recently, by using data obtained from remote-sensing MODIS (Moderate Resolution Imaging Spectroradiometer), it became possible to identify habitats where vegetation is drying, signaling loss of habitat attractiveness, which is likely to be abandoned by locusts. Such data would enable control centers to lead to more efficiency in locust control management. Monitoring S. gregaria in remote areas near the Red Sea, in order to pinpoint risk areas and target survey efforts, could further be enhanced by using the association between plant communities (especially millet) and the locust.
Mechanical methods: Digging trenches in front of advancing swarm, swamping them and burying them, scaring the swarms by making noise, fires or other methods. At best these are short-term methods and the swarms may return.
Chemical control: Bran that contains an insecticide may be spread in the path of the migrating hoppers, provided their locality is known. Pesticides that are less persistent, like pyrethroids, insect growth regulators (IGRs) and neem extracts are often effective. Some plant extracts have antifeedant and/or inhibitory effects on the nymphs. However, chemical control is not always possible or efficient. The sporadic breeding sites are spread over wide areas, often in remote locations and difficult to access, security in certain areas is questionable, and resources are limited.
Biological Control: Various natural enemies, parasitoids and predators attack the pest during its development, but none seem to be able to suppress locust populations once they are swarming. These enemies may contribute to the collapse of locust populations after the outbreak had peaked. Recent control efforts with oil formulations of the entomopathogenic fungus Metarhizium anisopliae (Metchnikoff) Sorokin are encouraging, as are trials with the microsporidian Nosema locustae Canning and the egg parasitoids Scelio spp. (Sceliodae).
References
Akman Gündüz, N.E. and Gülel, A. 2002. Effect of temperature on development, sexual maturation time, food consumption and body weight of Schistocerca gregaria Forsk. (Orthoptera: Acrididae). Turkish Journal of Zoology 26: 223-227.
Cressman, K. 1996 Current methods of desert locust forecasting at FAO. EPPO Bulletin 26: 577–585.
Ghazawy, N.A., Awad, H.H. and Abdel Rahman, K.M. 2010. Effects of azadirachtin on embryological development of the desert locust Schistocerca gregaria Forskål (Orthoptera: Acrididae). Journal of Orthoptera Research 19: 327-332.
Ghoneim, K.S., Tanani, M.A. and Basiouny, A.L.F. 2009. Influenced survival and development of the desert locust Schistocerca gregaria (Acrididae) by the wild plant Fagonia bruguieri (Zygophyllaceae). Egyptian Academy Journal of Biological Sciences 2: 147-164.
Lomer, C.J., Bateman, R.P., Johnson, D.L., Langwald, J. and Thomas, M. 2001. Biological control of locusts and grasshoppers. Annual Review of Entomology 46: 667-702.
Renier, C., Waldner, F., Jacques, D.C., Ebbe, M.A.B., Cressman, K. and Defourny, P. 2015. A dynamic vegetation senescence indicator for near-real-time desert locust habitat monitoring with MODIS. Remote Sensing 7: 7545-7570.
Showler, A.T. 2002. A summary of control strategies for the desert locust, Schistocerca gregaria (Forskål). Agriculture, Ecosystems & Environment 90: 97-103.
Showler, A.T. 2013. The Desert Locust in Africa and Western Asia: Complexities of War, Politics, Perilous Terrain, and Development. Radcliffe’s IPM World Textbook, University of Minnesota.
Uvarov, V.B. 1966. Grasshoppers and Locusts, Vol. 1, Cambridge: Cambridge University Press.
Uvarov, V.B. 1977. Grasshoppers and Locusts, Vol. 2, London: Centre for Overseas Pest Research.
Van der Werf, W., Woldewahid, G., Van Huis, A., Butrous, M. and Sykora, K. 2005. Plant communities can predict the distribution of solitarious desert locust Schistocerca gregaria. Journal of Applied Ecology 42: 989–997.
Van Huis, A. 2007. Strategies to control the desert locust Schistocerca gregaria. (In Vreysen, M.J.B., Robinson, A.S. and Hendrichs, J. (eds) Area-Wide Control of Insect Pests, pp 285-29.
Websites:
https://www.youtube.com/watch?v=xMR4Jkyomvw