Locust species, such as the infamous Desert locust, Schistocerca gregaria (pictured above), express a remarkable form of phenotypic plasticity known as phase polyphenism. The expression of phase polyphenism is mediated by changes in local population density and results in changes in a variety of physiological, morphological and behavioral traits, including coloration. High population densities during outbreaks generate gregarious phase locusts that travel in groups referred to as migratory bands and flying swarms.
Based on my discovery of density-dependent warning coloration in Schistocerca lineata grasshoppers, I suspected that a similar color change in the Desert locust might also result in the expression of density-dependent warning coloration, with deterrence to predators mediated by feeding on toxic host plants. Field experiments conducted in the Sahara desert in the West African country of Mauritania demonstrated that Desert locust juveniles do derive toxicity to predators by feeding on native toxic host plants. In addition, their expression of density-dependent color change was shown to function as a shift in anti-predator strategy from crypsis to warning coloration in experiments with native visually-hunting predators (Sword et al., 2000; Sword & Simpson, 2000).
The recognition of density-dependent warning coloration in the Desert locust provided a key insight into its population dynamics; warning coloration at locally high densities can reduce per capita predation and facilitate additional population growth leading to outbreaks. Understanding host plant-mediated deterrence in conjunction with knowledge of local spatial resource distribution patterns can be used in population monitoring programs to assess the propensity of different habitats to generate Desert locust swarms (Babah & Sword, 2004). The ultimate goal of this work is to help locust management organizations such as the Centre de Lutte Antiacridienne (CLAA) [French site | Arabic site] in Mauritania reduce the amount of time, manpower, and pesticides necessary to manage Desert locust outbreaks. Special thanks to Mohammed Abdallahi ould Babah and the CLAA for being outstanding hosts during my visits to Mauritania.
One of my long-term interests is the role of genetic versus environmental factors in locust swarm formation. Ideally, this would be best accomplished by conducting a comparative phylogenetic analysis of the expression of density-dependent traits in related swarming and non-swarming species. To this end, I collaborated on a molecular phylogenetic analysis of Schistocerca using mtDNA (Lovejoy et al. 2006). I also maintain a long running collaboration with Hojun Song (University of Central Florida) who is also keenly interested in these questions. Being back in Texas with lots of Schistocerca species readily available will make working on them much easier.
While I was at the University of Sydney in Australia, we worked on a huge range of projects involving the Australian Plague locust, Chortoicetes terminifera, and the migratory locust, Locusta migratoria. Topics addressed included population genetics, functional genomics, and collective migratory behavior – to name just a few. Although there are no swarming locusts in the U.S., I still maintain an active collaborations with Steve Simpson to keep me involved in locusts in Australia for the foreseeable future.
Chapuis, M-P., Streiff, R. & Sword, G.A. (2012) Long microsatellites and unusually high levels of genetic diversity in the Orthoptera. Insect Molecular Biology 21:181-18.
Chapuis, M-P., Simpson, S.J., Blondin, L. & Sword, G.A. (2011) Taxa-specific heat shock proteins are over-expressed with crowding in the Australian plague locust. Journal of Insect Physiology 57:1562–1567.
Robinson, K.L., Tohidi-Esfahani, D., Lo, N., Simpson, S.J. & Sword, G.A. (2011) Evidence for widespread genomic methylation in the migratory locust, Locusta migratoria (Orthoptera: Acrididae). PLoS ONE 6(12): e28167.
Simpson, S.J., Sword, G.A. & Lo, N. (2011) Polyphenism in insects. Current Biology 21:R738-749.
Buhl, J., Sword, G.A., Clissold, F. & Simpson, S.J. (2011) Group structure in locust migratory bands. Behavioral Ecology and Sociobiology 65:265–273.
Berthier, K., Chapuis, M-P., Moosavi, S.M., Tohidi-Esfahani, D. & Sword, G.A. (2011) Nuclear insertions and heteroplasmy of mitochondrial DNA as two sources of intra-individual genomic variation in grasshoppers. Systematic Entomology 38:285-299.
Chapuis, M-P., Popple, J.A., Berthier. K., Simpson, S.J., Deveson, E. Spurgin, P., Steinbauer, M.J. & Sword, G.A. (2011) Challenges to assessing connectivity between massive populations of the Australian Plague locust. Proceedings of the Royal Society of London B 278:3152-3160.
Chapuis, M-P., Tohidi-Esfahani, D., Dodgson, T., Blondin, L., Ponton, F., Cullen, D.A., Simpson, S.J. & Sword, G.A. (2011) Assessment and validation of a suite of reverse transcription-quantitative PCR reference genes for analyses of density-dependent behavioural plasticity in the Australian plague locust. BMC Molecular Biology 12:7.
Hansen, M.J., Buhl, J., Bazazi, S., Simpson, S.J. & Sword, G.A. (2011) Cannibalism in the lifeboat – collective movement in Australian plague locusts. Behavioral Ecology and Sociobiology 9: 1715-1720.
Bazazi S., Romanczuk, P., Thomas, S., Schimansky-Geier, L., Hale, J.J., Miller, G.A., Sword, G.A., Simpson, S.J. & Couzin, I.D. (2011) Nutritional state and collective motion: from individuals to mass migration. Proceedings of the Royal Society of London B 278:356-363.
Latchininsky, A., Sword, G., Sergeev, M., Cigliano, M.M. & Lecoq, M. (2011) Locusts and Grasshoppers: Behavior, Ecology, and Biogeography. Psyche 578327:1-4.
Cresswell, K.A., Satterthwaite W.H. & Sword, G.A. (2011) Understanding the evolution of migration through empirical examples. In: Fryxell, J., Milner-Gulland, E.J. & Sinclair, A.R.E. (eds.) Migration – A Synthesis. Oxford University Press, Oxford, pp 7-16.
Sword, G.A., Lecoq, M. & Simpson, S.J. (2010) Phase polyphenism and preventative locust management. Journal of Insect Physiology 56:949-957.
Cullen, D.A., Sword, G.A., Dodgson, T. & Simpson, S.J. (2010) Behavioural phase change in the Australian plague locust, Chortoicetes terminifera, is triggered by tactile stimulation of the antennae. Journal of Insect Physiology 56:937-942.
Berthier, K., Chapuis, M-P., Simpson, S.J., Ferenz, H-J., Habib Kane, C. M., Kang, L., Lange, A., Ould Babah, M. A., Rodenburg, K. W., Rogers, S., Torto, B., Vanden Broeck, J., Van Loon, J. J. A. & Sword, G.A. (2010) Laboratory populations as a resource for understanding the relationship between genotypes and phenotypes: A global case study in locusts. Advances in Insect Physiology 39:1-37.
Reynolds, A.M., Sword, G.A., Simpson, S.J. & Reynolds, D.R. (2009) Predator percolation, insect outbreaks and phase polyphenism. Current Biology 19:20-24.
Gray, L.J., Sword, G.A., Anstey, M.L., Clissold, F.J. & Simpson, S.J. (2009) Behavioural phase polyphenism in the Australian Plague locust (Chortoicetes terminifera). Biology Letters 5:306-309.
Simpson, S.J. & Sword, G.A. (2009) Phase polyphenism in locusts: Mechanisms, population consequences, adaptive significance and evolution. In: Whitman, D. & Ananthakrishnan, T. (eds.) Phenotypic Plasticity of Insects: Mechanisms and Consequences. Science Publishers Inc., Plymouth, pp 147-190.
Simpson, S.J. & Sword, G.A. (2008) Locusts. Current Biology 18:R364-366.
Bazazi, S., Buhl, J., Hale, J.J., Anstey, M.L., Sword, G.A., Simpson, S.J. & Couzin, I.D. (2008) Collective motion and cannibalism in locust marching bands. Current Biology 18:735-739 (Cover story).
Chapuis, M-P., Lecoq, M., Loiseau, A., Michalakis, Y., Sword, G.A., Piry, S. & Estoup, A. (2008) Do outbreaks affect genetic population structure? A worldwide survey in Locusta migratoria, a pest plagued by microsatellite null alleles. Molecular Ecology 17:3640-3653.
Chapuis, M-P., Popple, J.A., Simpson, S.J., Estoup, A., Martin, J.F., McCulloch, L., Steinbauer, M. & Sword, G.A. (2008) Eight polymorphic microsatellite loci for the Australian Plague Locust, Chortoicetes terminifera. Molecular Ecology Resources 8:1414-1416.
Sword, G.A. (2008) Gregarious behaviour in insects. In: Capinera, J. (ed.) Encyclopedia of Entomology. Springer, Netherlands, pp. 1733-1740.
Beekman, M., Sword, G.A. & Simpson, S.J. (2008) Biological foundations of swarm intelligence. In: Blum, C. & Merkle, D. (eds.) Swarm Intelligence – Introduction and applications. Springer, Netherlands, pp. 3-41.
Lovejoy, N.R., Mullen, S.P., Sword, G.A., Chapman, R.F. & Harrison, R. (2006) Ancient trans-Atlantic flight explains locust biogeography: Molecular phylogenetics of the locust genus Schistocerca. Proceedings of the Royal Society of London B 273:767-774.
Babah, M.A.O. & Sword, G.A. (2004) Linking locust gregarization to resource distribution patterns across a large spatial scale. Environmental Entomology 33:1577-1583.
Sword, G.A. (2003) To be or not to be a locust? A comparative analysis of behavioral phase change in nymphs of Schistocerca americana and S. gregaria. Journal of Insect Physiology 47:709-717.
Sword, G.A. & Simpson, S.J. (2000) Is there an intraspecific function for density-dependent color change in the Desert locust? Animal Behaviour 59:861-870.
Sword, G.A., Simpson, S.J., El Hadi, O.T.M. & Wilps, H. (2000) Density-dependent aposematism in the Desert locust. Proceedings of the Royal Society of London B 267:63-68.