Relationship between rainfall and stable fly (Diptera: Muscidae) abundance on California dairies

Populations of adult stable flies, Stomoxys calcitrans (L.), were visually estimated by counting flies on the front legs of cattle on southern and central California confined dairy feedlots between late April and mid-June (encompassing the peak stable fly activity period). Fly counts on 45-90 animals (three to six dairies) per weekly sample date were conducted in 1985, 1986, 1993, 2002, and 2003. Average biting intensity (flies per front leg) for the peak fly season was not significantly related to early winter (December-January), late winter (February-March), or total period (December-March) rainfall, but it was strongly related (r2 = 0.726) to March rainfall. March rains probably moistened outside decaying manure habitats and similar substrates that are particularly suitable at that time for stable fly oviposition and larval development. Degree-day accumulations link the timing of significant (> or = 1.3 cm) late winter or early spring rains to peak adult stable fly activity two generations later in May and early June.