| Plant architecture is expressed through the growth of iterated parts or "modules" such as buds, shoots, leaves and flowers. To investigate the role of plant architecture in the interactions between a bud-galling midge (Rabdophaga sp.) and its host plant, coyote willow (Salix exigua), a modular approach was taken. First, the response of the midge to host plant growth was examined and related to performance parameters of the species at both between and within-plant levels. Gall and egg distribution data and female preference experiments revealed that the midge differs from other gall former species studied in that there was no preference for vigorously-growing plants or plant modules. However, growth vigor was positively related to midge performance parameters such as fecundity and larval survival. The poor preference-performance relationship of Rabdophaga sp. was explained by oviposition and flight behaviors which prevent females from accurately assessing plant and plant module quality for their progeny.;In an attempt to understand the complexities of plant response to herbivory, modular demography was used to assess the mechanisms of Rabdophaga sp. impact on willow growth. By following bud populations over two seasons of growth on galled and ungalled shoots and ramets, I determined that galling causes significant growth and reproductive losses despite plant compensatory responses to herbivory. Ultimately, the majority of lateral shoots that developed in response to galling were lost through natural abscision.;Bud-galling also was found to have a greater impact on willow growth than a grazing form of insect herbivory. In a common garden experiment where the affects of galling were compared to simulated shoot-tip grazing, grazing was found to elicit the stronger growth response from S. exigua, with grazed shoots producing up to 2-fold more lateral shoots than galled shoots. By summer's end, it seemed that grazed plants had over-compensated and galled plants under-compensated for herbivore losses. However, after further bud losses from fall shoot abscision were considered, only grazed plants had compensated for damage. |
