The impact of a herbivore upon Mirabilis hirsuta, a fugitive prairie plant.

We determined the effects of a host-specific moth, Heliodines nyctaginella (Lepidoptera: Heliodinidae), upon growth and seed production in Mirabilis hirsuta (Nyctaginaceae), by comparing in one season naturally infested plants with experimentally protected neighbors. In addition, we monitored infestation, growth, and yield of a single cohort over ten years. Plants fed upon by H. nyctaginella larvae grew less, and more slowly and produced fewer propagules than did their uninfested neighbors. Propagules from moth-infested plants were smaller and were formed 4-6 weeks later in the season. Thus, because successful colonization of suitable sites by M. hirsuta depends upon plant size and on the number and sizes of propagules produced, H. nyctaginella potentially can exert an enormous selection pressure upon its host. However, periodic escape from the herbivore greatly reduced its impact: plants that escaped infestation for as little as one growing season showed increased growth and propagule production over several years. Large, old plants often partially escape by flowering before the herbivore population becomes dense.