Water balance of the American dog tick, Dermacentor variabilis, throughout its development with comparative observations between field-collected and laboratory-reared ticks

Water balance characteristic data are compiled and determined for each non-fed instar of the American dog tick, Dermacentor variabilis, vector of tularaemia and spotted fever rickettsia, comparing wild-caught and laboratory-colonized populations, to establish how habitat influences D. variabilis dehydration resistance. In contrast to ticks from the field, laboratory-colonized ticks show a progression in percentage body water content from a low of 59% in the egg to a high of 68% in the adult (all instars from the field population have similar values, averaging 63%) and are less tolerant of water stress (2-6% reduction in dehydration tolerance (DT)). We presume that these are colony artefacts as a product of relaxing selective pressure by rearing under optimal conditions of temperature, relative humidity (RH) and host quality. For field-collected and laboratory-colonized populations, larvae survive down to 75-85% RH [critical equilibrium humidity (CEH)], but nymphs and adults require moist air of 85-93% RH, and eggs do not absorb water vapour. Larvae, but not eggs, experience rapid water loss at 30-35 degrees C [critical transition temperature (CTT)], which is 5 degrees C lower than that of post-larval instars. Larvae succumb to dehydration more readily (lower DT limit, eggs have the highest) and are more prone to water loss by having a high net transpiration rate (NTR) as a result of their minute body size and high activation energies for water loss. Comparison of DT with water loss during development for seven tick species reveals that NTR is a species-specific characteristic and species that lose water quickly or slowly as larvae do so for subsequent developmental instars. The CEH, CTT and DT reveal little with regard to habitat suitability and matching a tick to a particular habitat. Ticks operate at the microhabitat, not habitat, level, indicating that ticks can find suitable refuges in many different geographic areas, which accounts for the overlapping distribution of ticks with different NTRs.