The commercial release of transgenic maize with resistance to rootworms incorporated with glyphosate tolerance has become a reality, and questions have arisen regarding the impact of grassy weed phenology on the biology of the western corn rootworm, Diabrotica virgifera virgifera LeConte. A study to determine the impact of host plant phenology on the survivorship (percentage larval recovery), growth (dry-weight gain), and development (change in head capsule width) of western corn rootworm larvae was conducted in a split-split-plot randomized complete block design experiment under greenhouse conditions. Six host species were evaluated under eight weekly infestations of 15 neonate western corn rootworm larvae that were sampled for larval recovery after 7, 14, and 21 d, each with five replications. Percentage larval recovery, change in head capsule width and weight gain were significantly impacted by infestation time and host species. Other than from maize, Zea mays L., larval survivorship was highest in large crabgrass, Digitaria sanguinalis L. Scop, followed by giant foxtail, Setaria faberi R. A. W. Herrm, witchgrass, Panicum capillare L., woolly cupgrass, Eriochloa gracilis, and green foxtail, Setaria viridis L. P. Beauv. Infestation in weeks 4, 5, and 6 supported western corn rootworm survivorship, growth, and development better than infestation at later times. Alternate host phenology may be an important larval movement factor between grassy weeds and transgenic rootworm-resistant maize.