Opposite Density Effects of Nymphal and Adult Mortality for Periodical Cicadas

In a cohort of 17—yr periodical cicadas, I measured adult density, density of newly hatched nymphs, and density of nymphs after 2 yr. I looked for density dependence by regressing the logarithms of nymphal density upon logs of adult density. Mortality during the adult and egg stages decreased as the density of adults increased, corroborating earlier observations. However, nymphal mortality during the 1st 2 yr was higher where the density of newly hatched nymphs was higher. This density—dependent nymphal mortality more than compensated for the inversely density—dependent mortality during the adult stage; the number of 2—yr—old nymphs produced per adult was greater at sites of lower than of higher adult density. I found no correlation between soil and rootlet traits and number of 2—yr nymphs recovered. Factors responsible for nymphal mortality remain unknown. There may be selection upon adults to disperse away from dense chorusing centers before ovipositing, because nymphs nearest chorusing centers suffer higher mortality during the 1st 2 yr of life. Almost all cicadas that I observed dispersing away from chorusing centers were mated females.