Plant-plant and plant-insect interactions: Direct and indirect effects of plant neighbors

In greenhouse and field experiments using Amaranthus retroflexus and Chenopodium album, I investigated whether competition and artificial leaf herbivory acted synergistically in depressing plant growth (height, diameter, and vegetative mass) and reproduction (reproductive mass and time of flowering). In several experiments, competition and herbivory had a negative synergistic effect on reproductive mass, vegetative mass, and diameter of A. retroflexus, and on reproductive and vegetative mass of C. album.In a field experiment to determine the indirect effects of second order plant neighbors, I grew target plants (either A. retroflexus or C. album) with near or far neighbors, or with both near and far neighbors. Second order neighbors had a positive indirect effect on reproductive mass of A. retroflexus.Using field plantings of A. retroflexus and C. album, which are hosts of the common sooty wing skipper (Pholisora catullus), I studied the effects of varying the relative quality and number of host neighbors on skipper egg numbers. As C. album is the preferred host, I was able to manipulate neighbor quality by changing the neighbor species. Through path analysis, I examined the direct effect of neighbor species and number of neighbors on target plant egg numbers, as well as the indirect effects on egg numbers via changes in plant attractiveness effected by changes in competition environments. For both plant species, proximity to non-conspecifics, as opposed to conspecifics, directly reduced egg numbers. However, competition from non-conspecifics had less impact on the absolute attractiveness of the target plants, thus they indirectly increased egg numbers. An increased number of neighbors of either species reduced egg numbers both directly and indirectly.Additionally, I found that P. catullus larvae consistently had higher survivorship on C. album than A. retroflexus, but only the August broods of adult skippers showed a preference for ovipositing on C. album. The July broods of skippers did not discriminate between species. Host plants close to nectar sources did not receive more eggs than plants farther from nectar sources.U of I OnlyETDs are only available to UIUC Users without author permissio