Response of a Host-Specific Herbivore to Resource Density, Relative Abundance, and Phenology

Ovipositing females of the host-specific herbivorous moth Frumenta nundinella Zeller discriminated among horsenettle (Solanum carolinense L.)plants in the laboratory and at five field populations. In laboratory experiments, the number of larvae that developed in high-density plant groups was not significantly greater than the number that developed in low-density groups. Consequently, individual plants in low-density groups were disproportionately affected; twice as many larvae developed on them as on individual plants in high-density groups. Moths also discriminated between groups of pure horsenettle (high relative abundance)and groups of horsenettle plus another naturally co-occurring species (low relative abundance). Individual plants in high relative abundance groups had a tenfold greater infestation than did plants in low relative abundance groups. Natural horsenettle populations varied in density, relative abundance, plant size, and fruit production. Plant size, fruit production, and population density were all positively correlated, so the populations with the greatest density had the largest plants, which produced the most fruits. Despite differences in these plant parameters among sites, the extent of infestation differed little, except for one site that lacked parasitoids that attack F. nundinella. Infestation within field sites was heterogeneous among plants. Virtually all large plants (those with 15 fruits), but only about half the small plants (those with < 15 fruits), became infested (one or more infested fruit per plant). However, a greater proportion of the fruit crop was infested on small plants than on large plants. Large plants remained in flower an average 2.5wk longer than did small plants, so a smaller proportion of their flowers was available during the short period of high oviposition risk. Because of the short oviposition period, plants that flowered earlier or later than the mean flowering time of their populations appeared to have lower infestations than did plants that flowered close to the population means.