Tuesday, August 7, 2007

PS 21-3: Host plant availability and activity patterns among three populations of lubber grasshoppers (Romalea microptera)

Kevin M. Kocot, Jason E. Jannot, and Steven A. Juliano. Illinois State University

Geographic clines in body size within species can indicate spatial differences in selective pressures among populations.  However, selective agents (e.g., temperature, food, predation) responsible for body size clines have rarely been identified.  Lubber grasshoppers (Romalea microptera (Beauvois)) are large, flightless, polyphagous herbivores, with strong chemical defenses that deter predators.  Lubber populations exhibit an east (large) to west (small) adult body size cline at our sub-tropical field sites in south Florida.  Among our sites, climate is relatively uniform; however, elevation and hydroperiod differences result in radically different plant communities among the sites. We predicted that differences in adult body size among lubber populations are due to differences in behavior (particularly feeding behavior), host plant usage, or both.  To evaluate behavior, we examined the daytime activity patterns of three populations across the east-west cline using scan sampling.  Grasshopper behaviors were categorized as resting, moving, feeding, mating, mate guarding, and ovipositing.  We also recorded the plants each individual used as substrate (e.g., perch) or food.  Multivariate analysis of grasshopper behavior indicates a significant site-by-sex interaction (MANOVA Pillali’s Trace p< 0.0001; controlling for time and density effects), suggesting sexual differences in behavior differ across sites.  Univariate follow-up tests (ANOVA) indicated that feeding and resting (associated with digestion) were higher in western v. eastern sites (p< 0.0001).  Multivariate analysis of the types of plants used by lubbers indicates that there are significant differences in plant usage at the three sites (Canonical Correspondence Analysis Pillali’s Trace p<0.0001).  Our data provide evidence that behavior and plant community structure might contribute to geographic clines in adult body size among these populations of lubber grasshoppers.