Flea fitness is reduced by high fractional concentrations of CO₂ that simulate levels found in their hosts' burrows

Nidicolous ectoparasites such as fleas and gamasid mites that feed on small and medium-sized mammals spend much of their time in their hosts' burrows, which provide an environment for living, and often feeding, to their pre-imaginal and/or adult stages. Thus, these ectoparasites should be adapted to environmental conditions in burrows, including high fractional concentrations of CO₂ (F_CO2). We examined how a high F_CO2 (0.04) affected survival and reproductive success of a hematophagous ectoparasite of burrowing rodents using fleas Xenopsylla ramesis and Sundevall's jirds Meriones crassus. In the first experiment, fleas fed on hosts housed in high-CO₂ (F_CO2=0.04) or atmospheric-CO₂ (F_CO2≈0.0004) air, and were allowed to breed. In a second experiment, fleas were maintained in high CO₂ or CO₂-free air with no hosts to determine how CO₂ levels affect survival and activity levels. We found that at high FCO₂ fleas laid fewer eggs, reducing reproductive success. In addition, at high F_CO2, activity levels and survival of fleas were reduced. Our results indicate that fleas do not perform well in the F_CO2 used in this experiment. Previous research indicated that the type and intensity of the effects of CO₂ concentration on the fitness of an insect depend on the F_CO2 used, so we advise caution when generalizing inferences drawn to insects exposed to other F_CO2. If, however, F_CO2 found in natural mammal burrows brings about reduced fitness in fleas in general, then burrowing hosts may benefit from reduced parasite infestation if burrow air F_CO2 is high.