Factors affecting the seasonal abundance of ground squirrel and wood rat fleas (Siphonaptera) in San Diego County, California.

Abiotic and biotic factors affecting the seasonal abundance of 3 species of sylvatic fleas on their hosts were examined at 7 sites in San Diego County, California. Indices for Oropsylla (Diamanus) montana (Baker) were usually highest on Spermophilus beecheyi nudipes (Huey) when < 18.4 degrees C (October-December), whereas those for Hoplopsyllus anomalus (Baker) were highest when > 18.4 degrees C (July-September). O. montana was affected most by ambient temperature (inversely for the coastal site [134 m] and directly for most mountain sites [> 1,183 m]). O. montana was affected most (directly) by relative humidity at the inland valley site, which concurs with this flea being most abundant in other areas during periods yielding higher relative humidties and moderate ambient temperatures. H. anomalus was influenced most (directly) by ambient temperature at lower sites (< 1,183 m) and by host activity at higher ones (> 1,456 m). Usually nonrandom distribution findings, when indices for each squirrel flea were higher on some hosts, indicate that flea numbers are correspondingly higher in some nests and burrows because of more favorable microconditions. Data also indicated that plague may persist at higher sites in southern California or in other areas where climatic conditions permit increased and prolonged interactions between more abundant O. montana and its host during spring through summer. Whereas, plague may have less chance of persisting at lower sites because these interactions are decreased and shortened during the warmer months. Squirrels may become infected with plague following hibernation and again when reoccupying colonial burrows. The flea Orchopeas sexdentatus was affected most (inversely) by relative humidity at a coastal site, with higher indices occurring on fewer Neotoma lepida Thomas and on more abundant Neotoma fuscipes macrotis Thomas during colder months. Such increased flea/Neotoma fuscipes Baird activity at lower sites may favor plague amplification during the winter which may involve ground squirrels and other rodents in the spring.