Locust ecology and evolution

gregaria nymphs


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 behavioural 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.

gregaria adult
One of my long-term goals is to examine the role of genetic versus environmental factors in locust swarm formation 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) and am currently conducting a follow-up analysis in collaboration with Hojun Song (The Ohio State University) using multilocus AFLP markers.

Now that I am in Australia, I will be working on the Australian plague locust, Chortoicetes terminifera. Planned projects to date include a continental-scale population genetics study using DNA microsatellites to examine migration patterns, as well as studies examining the expression and genetics of behavioural phase polyphenism. I suspect that another web page exclusively dedicated to these projects will be coming soon.

Relevant publications

Simpson, S.J. & Sword, G.A. (In Press) Phase polyphenism in locusts: Mechanisms, population consequences, adaptive significance and evolution. In: Ananthakrishnan, T. & Whitman, D. (eds.) Phenotypic Plasticity of Insects: Mechanisms and Consequences.

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. (* equal contributors)

Simpson, S.J., Sword, G.A. & DeLoof, A. (2005) Advances, controversies and consensus in locust phase polyphenism research. Journal of Orthoptera Research 14:213-222.

Babah, M.A.O. & Sword, G.A. (2004) Linking locust gregarization to resource distribution patterns across a large spatial scale. Environmental Entomology 33(6):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(7):709-717.

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.

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.

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