In contrast to the majority of herbivorous insects, most grasshoppers are considered to be generalist feeders. As a result, they are often assumed not to be involved in species-specific host plant associations. However, studies that properly account for developmental, temporal, and geographic variation in host plant use are surprisingly rare in grasshoppers. Understanding these patterns can provide critical insights into the role that ecological divergence plays in genetic diversification and speciation.
My work with the grasshopper, Schistocerca lineata (aka S. emarginata, shown in photo), in central Texas, USA illustrated that when host use studies are properly designed, they can reveal specific grasshopper-plant associations (Sword & Dopman, 1999). Through observation, field, and lab work, I identified previously unknown developmental changes from specialist to generalist host plant use patterns. This insight facilitated the recognition of geographic patterns of host plant use among populations based on juvenile host plant use. Having established an underlying pattern of specific grasshopper-host plant associations, I went on to examine a number of other novel host plant-associated phenomena in grasshoppers. These included: (i) the elucidation of host plant-associated genetic differentiation among S. emarginata populations using mtDNA (Dopman et al., 2002) and more recently in Hesperotettix viridis using AFLP (Sword et al., 2005), (ii) the demonstration of host plant-mediated toxicity to a vertebrate predator (Sword, 2001), and (iii) the discovery of density-dependent warning coloration (Sword, 1999; 2002).
These studies suggest that many more cryptic host plant associations may exist in generalist insect herbivores. Indeed, it was this perspective that led me to become involved in locust research. Knowledge of specific host plant-related ecological interactions may have implications for the development of ecologically-based management strategies. These include cultural practices such as livestock grazing and burning, the effects of which on grasshopper population dynamics I have been investigating in collaboration with with David Branson (USDA-ARS) and Tony Joern (Kansas State University).
Branson, D.B. & Sword, G.A. (2010) An experimental analysis of grasshopper community responses to fire and livestock grazing in a northern mixed-grass prairie. Environmental Entomology 39:1441-1446.
Branson, D.B. & Sword, G.A. (2009) Grasshopper herbivory affects native plant diversity and abundance in a grassland dominated by the exotic grass Agropyron cristatum. Restoration Ecology 17:89-96.
Branson, D.H., Joern, A. & Sword, G.A. Sustainable management of insect outbreaks in renewable grassland ecosystems: An ounce of prevention is worth a pound of cure. BioScience 56:743-755.
Sword, G.A., Joern, A. & Senior, L.B. (2005) Host plant-associated genetic differentiation in the snakeweed grasshopper, Hesperotettix viridis (Orthoptera: Acrididae). Molecular Ecology 14(7):2197-2205 (Cover photo).
Dopman, E.B., Sword, G.A., & Hillis, D.M. (2002) The importance of the ontogenetic niche in resource-associated divergence: evidence from a generalist grasshopper. Evolution 56(4):731-740. (Cover photo)
Sword, G.A. (2001) Tasty on the outside, but toxic in the middle: Grasshopper regurgitation and host plant-mediated toxicity to a vertebrate predator. Oecologia 128:416-421.
Sword, G.A. (1999) Density-dependent warning coloration. Nature 397:217.
Sword, G.A. and Dopman, E.B. (1999) Developmental specialization and geographic structure of host plant use in a polyphagous grasshopper, Schistocerca emarginata (=lineata) (Orthoptera: Acrididae). Oecologia 120:437-445.
Chapman, R.F. and Sword, G.A. (1997) Polyphagy in the Acridomorpha. In: Gangwere, S.K., Muralirangan, M.C. & Muralirangan, M. (eds.) The Bionomics of Grasshoppers, Katydids and Their Kin. CAB International, Wallingford, pp. 183-195.
Chambers, P., Sword, G., Angel, J., Behmer, S. & Bernays, E.A. (1996) Foraging by generalist grasshoppers: dietary mixing in two cryptic species. Animal Behaviour 52:155-165.
Sword, G.A. and Chapman, R.F. (1994) Monophagy in a polyphagous grasshopper, Schistocerca shoshone. Entomologia Experimentalis et Applicata 73:225-234.
Chapman, R.F. and Sword, G.A. (1994) The relationship between plant acceptability and suitability for survival and development in the polyphagous grasshopper, Schistocerca americana. Journal of Insect Behavior 7:411-431.
Chapman, R.F. and Sword, G. (1993) The importance of palpation in food selection by a polyphagous grasshopper (Orthoptera: Acrididae). Journal of Insect Behavior 6:79-91.