Competition between the gypsy moth, Lymantria dispar, and the northern tiger swallowtail, Papilio canadensis: interactions mediated by host plant chemistry, pathogens, and parasitoids

The gypsy moth, Lymantria dispar, and the northern tiger swallowtail, Papil io canadensis, overlap geographically as well as in their host ranges. Adul t female swallowtails are incapable of distinguishing between damaged and u ndamaged leaves, and the opportunities for competition between these two sp ecies are numerous. We designed field and laboratory experiments to look fo r evidence of indirect competition between P. canadensis and L. dispar larv ae. Swallowtail caterpillars were reared in the laboratory on leaves from g ypsy-moth-defoliated and undefoliated trees to explore host-plant effects. We tested for pathogen-mediated interactions by rearing swallowtail larvae on both sterilized and unsterilized leaves from defoliated and undefoliated sources. In addition, we measured the effects of known gypsy moth pathogen s, as well as gypsy moth body fluids, on the growth and survival of swallow tail larvae. Field experiments were designed to detect the presence of para sitoid-mediated competition, as well: we recorded parasitism of swallowtail caterpillars placed in the field either where there were no gypsy moth lar vae present, or where we had artificially created dense gypsy moth populati ons. We found evidence that swallowtails were negatively affected by gypsy moths in several ways: defoliation by gypsy moths depressed swallowtail gro wth rate and survival, whether leaves were sterilized or not; sterilization significantly reduced the effect of defoliation, and gypsy moth body fluid s proved lethal; and swallowtail caterpillars suffered significantly increa sed rates of parasitism when they were placed in the field near gypsy moth infestations.