Influence of Insect Herbivory on Photosynthetic Area and Reproduction in Gutierrezia Species

Abstract

(1) Larvae of the two tortricid lepidopterans, Synnoma lynosyrana and Synalocha gutierreziae, form silk cases that tie together a significant proportion of the photosynthetic stems and leaves of the common rangeland perennial, Gutierrezia sarothrae, which are toxic to cattle. (2) These insects were significantly more abundant in cattle-grazed plots than in ungrazed plots of the Sevilleta National Wildlife Refuge, located in central New Mexico, USA. (3) Using these herbivores as the experimental manipulation of the plant, measurements were made of photosynthetic area and proximate reproduction of plants with both tied and control (non-tied) branches. In within-plant comparisons, tied branches had significantly less photosynthetic area than control branches. The number of flowers/ branch, flowers/capitulum, seeds/branch and seeds/capitulum were all significantly greater for control than for tied branches. However, seeds produced by tied branches germinated more rapidly and at a higher percentage than seeds from control branches over the same time period. (4) The onset of senescence occurred on smaller volume plants in the ungrazed, high larval abundance plots than in the grazed, low larval abundance plots. (5) The results suggest that, at natural levels of tissue loss, Synnoma and Synalocha can seriously affect both the photosynthetic area and the reproduction of individual plants, and can thus potentially affect Gutierrezia population dynamics.