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 speci es of sylvatic fleas on their hosts were examined at 7 sites in San Di ego 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 anom alus (Baker) were highest when > 18.4 degrees C (July-September). O. m ontana was affected most by ambient temperature (inversely for the coa stal site [134 m] and directly for most mountain sites (> 1,183 m]). O . montana was affected most (directly) by relative humidity at the inl and valley site, which concurs with this flea being most abundant in o ther areas during periods yielding higher relative humidities and mode rate 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, indicat e that flea numbers are correspondingly higher in some nests and burro ws 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 intera ctions between more abundant O. montana and its host during spring thr ough summer. Whereas, plague may have less chance of persisting at low er sites because these interactions are decreased and shortened during the warmer months. Squirrels may become infected with plague followin g hibernation and again when reoccupying colonial burrows. The flea Or chopeas sexdentatus was affected most (inversely) by relative humidity at a coastal site, with higher indices occurring on fewer Neotoma lep ida Thomas and on more abundant Neotoma fuscipes macrotis Thomas durin g colder months. Such increased flea/Neotoma fuscipes Baird activity a t lower sites may favor plague amplification during the winter which m ay involve ground squirrels and other rodents in the spring.