Spatial heterogeneity and host-plant preferences influence the abundances of an insect herbivore

Background/Question/Methods

Spatial heterogeneity and movement behavior interact at large spatial scales to influence population dynamics. Theory suggests that similar but subtler patterns could arise at scales within habitat patches. In this study, I assessed the influence of host-plant heterogeneity and oviposition behavior on abundance patterns of a gall-making tephritid (Eutreta diana) on its host plant, sagebrush (Artemisia tridentata). I censused galls and plant characteristics in 200 square-meter spatially explicit plots over two years at Valentine Eastern Sierra Reserve, CA. I dissected galls and inspected puparia to identify healthy E. diana emergence, parasitism, or predation. To test movement propensity of females, I censused a 400 square-meter plot, randomly chose 32 host-plants and manually removed their galls before fly emergence. I returned after one generation and counted galls on removal and control plants. To test for host-plant preferences, I reared females in cages and recorded time spent on clipped branches from naturally occurring plants with high or low gall abundances. Finally, I used models of E. diana movement, preference, and oviposition to synthesize these data and test hypotheses explaining the observed spatial patterns.

Results/Conclusions

Host-plants with more flowers had more galls, and gall abundances on host-plants were correlated between years. This suggests plant quality was important for E. diana and consistent in time. There was no spatial autocorrelation in gall abundances, and high-abundance plants were often adjacent to low-abundance plants. There was significant spatial autocorrelation in the temporal change in gall abundances. This suggests there was spatial synchrony in E. diana dynamics among neighboring plants. Plants from which I had removed galls were recolonized and had densities correlated to pre-removal levels. This demonstrates that the spatiotemporal consistency in gall abundances was not caused by lack of female movement. Females in the preference experiment spent significantly more time on branches from host-plants with high gall abundances. This suggests females detect and respond to plant quality and that high- and low-abundance plants differed qualitatively. These results taken together paint a picture of a system in which space influences the distribution and dynamics of the herbivore at a fine scale despite an ability to move and select resources according to quality.