Population-level patterns and host-plant scale processes in a gall-making insect herbivore: density-dependent reproduction, host-plant preferences, and plant chemistry

Background/Question/Methods

Many insect herbivores species exhibit remarkably high variation in abundances among host-plants but low variation within plants through time. Relatively little is known about how insect herbivore demography at the plant-scale influences these population level patterns. This work examines the plant-scale demography of gall-formation and host-plant choice in a stem galling tephritid (Eutreta diana) that attacks big sagebrush (Artemisia tridentata).

I assessed density-dependence in per capita rates of gall formation with an experiment that manipulated adult density in cages on plants with different natural levels of infestation and used model selection to show whether prior infestation predicted density-dependent or -independent gall-formation parameters. I then used choice assays to estimate the strengths of female host-plant preferences. A gall-removal experiment tested the ability of naturally occurring females to find and colonize plants manually stripped of galls prior to emergence.

Results/Conclusions

In the density experiment, total gall formation per host-plant quickly increased to an asymptote as the number of females per host-plant increased. Plants with higher prior infestation levels gave the gall-makers significantly higher recruitment asymptotes, but did not yield different intrinsic rates of increase—a density-independent parameter. The preference assays showed females had strong preferences for foliage from plants with high infestation levels and had strong preferences for wet plants over drought stressed plants. In the gall-removal experiment, females readily colonized plants stripped of galls and returned gall densities on those plants to pre-removal levels.

The saturating density-dependent gall formation, high degree of host-plant preference, and ability to recolonize plants within a generation offer an explanation for the remarkable variation in gall abundances among host-plants and for the consistency in those abundance patterns through time. The effect of plant quality on host-preferences and on the maximum gall recruitment per plant likely structures the variation in gall abundances among plants, while the saturating nature of the density-dependent gall formation serves to stabilize variation in gall abundances through time.