Nymphal engorgement weight predicts sex of adult Amblyomma americanum, Amblyomma maculatum, Dermacentor andersoni, Dermacentor variabilis, and Rhipicephalus sanguineus ticks.

The engorgement weights of laboratory-raised nymphs of five common ticks in the USA, Amblyomma americanum, Amblyomma maculatum, Dermacentor andersoni, Dermacentor variabilis, and Rhipicephalus sanguineus, experimentally fed to repletion on an adult sheep (Ovis aries) were evaluated to determine the sex of molted adult ticks. Length of feeding period of nymphs, molting success and durations, and sex ratios between tick species were also compared. Individual replete nymphs were weighed and allowed to molt to adults in a humidity chamber. Length of feeding duration was different by species (F = 1963.79; P < 0.0001); R. sanguineus nymphs became replete fastest, followed by A. americanum, D. variabilis, A. maculatum, and D. andersoni. Significant difference in molting success was not detected. The mean body weight of engorged nymphs of A. americanum (t = 32.3; df = 662), A. maculatum (t = - 9.70; df = 255), D. variabilis (t = 15.7; df = 751), and R. sanguineus (t = 5.17; df = 560; all P < 0.0001) molting into females was greater than that of nymphs molting into males, whereas heavier D. andersoni engorged nymphs became males (t = 8.71; df = 480; P < 0.0001). Amblyomma maculatum nymphs that molted to females fed to repletion faster (t = 3.33; df = 265; P ≤ 0.001) than nymphs that molted to males and a higher proportion (χ = 48.4; df = 1, P < 0.0001) of A. maculatum and D. andersoni (χ = 8.19; df = 1, P = 0.004) molted to females than males. Our study demonstrated biological and behavioral differences in and between engorging nymphs of five ixodid species. These findings may aid in studies evaluating the role of tick sex in transmission of tick-borne pathogens.