Survival of immature Ixodes scapularis (Acari: Ixodidae) at different relative humidities.

Survival of unfed larvae and nymphs of Ixodes scapularis Say (formerly named I. dammini Spielman, Clifford, Piesman & Corwin) was tested at 27 degrees C for relative humidities ranging from 65 to 100%, which were maintained by various saturated salts or water. The times until half the larvae died (LT50) at 100, 93, 85, 75, and 65% RH were 67.1, 26.6, 8.3, 1.3, and 1.1 d, respectively. Maximal larval survival in the laboratory was 129, 59, 24, 3, and 2 d at the same relative humidities. There was a distinct difference between the mortality curves for larvae at 93% RH and 100% RH, which suggests that larvae were taking in water from the air at the higher RH whereas those at an RH < or = 93% were not. There was little difference between the 100 and 93% RH survival curves for the nymphs. All of the nymphs at 100% RH survived at least 149 d. At 93 and 100% RH, one nymph survived at each RH for 210 and 205 d, respectively. The LT50s for the nymphs of I. scapularis were 169.9 (100% RH), 168.5 (93%), 118.8 (85%), 10.7 (75%), and 3.6 (65%) d. Maximal nymphal survival in the laboratory was 162, 65, and 8 d at 85, 75, and 65% RH, respectively. The inability of the larvae to retain water effectively is probably more important than the ability to imbibe water from subsaturated air in the survival of this stage. Subadult I. scapularis require a relatively humid habitat for survival, although some individuals have the potential to seek a host for many weeks within a mesic environment.