Post-Drought Rodent Population Explosion in Alameda County

Alameda County Vector Control Services District (ACVCSD) receives ‘requests for service’ (RFS) relating to a variety of potential vectors of disease; one of the major program groups is rodents, which include house mice, deer mice, Norway rats, roof rats, wood rats, tree and ground squirrels, and meadow voles. This is an overview of the events of 2016, and some data from 2017, which began with much-needed rain and Alameda County bloomed. Along with an increase in biomass came an overabundance of rodents. The much-needed rainfall has historical correlation to meadow vole RFS and led to new records of meadow vole requests for services, and then to an unprecedented influx of house mice from open fields into neighborhoods. Our 2009 record-high 31 meadow vole requests for service was dwarfed by our 46 requests for service in 2016, which is almost a 50% increase over the 2009 high and over 4 times higher than the 10-year average of 9 RFS (some years had no meadow vole RFS). Additionally, in 2016 we received an astounding 637 house mouse RFS, an increase of over 73% of any of the previous 10 years which averaged 312 per year; the next-highest house mouse year generated 367 RFS. What seems to have accelerated the skyrocketing house mouse calls for service was one severely-affected Livermore neighborhood’s use of a neighborhood-networking website, which a neighborhood activist used to spur the residents to contact us for intervention. Our first indications of unusual problems for residents living in Livermore, and to a lesser extent in Fremont, followed three days of heavy rainstorms in mid-October produced nearly 10 inches of rainfall in some parts of the Bay Area, which exceeding the average monthly rainfall. We subsequently we received 257 rodent-related service requests.