Seasonal abundance and survivorship of Culicoides sonorensis Wirth & Jones were examined at a dairy in southern California from January 1995 to December 1997. Insects were collected one to two times per week using five CDC-type suction traps (without light) baited with CO2 at a constant release rate of 1,000 ml/min. Female and male abundance was greatest during late summer and early fall and was directly correlated with mean monthly air temperature. Parity of females was lowest during late summer and early fall. The gonotrophic cycle was estimated to require 3–4 d during hot summer months and up to 14 d during cool winter months. Estimated extrinsic incubation of bluetongue virus (BLU) was 9–10 d during August and September. The estimated daily survival ranged from <60% in the summer to >95% in the winter, resulting in an expectation of life of only 2–3 d in summer and >10 d in winter. The probability of females surviving the extrinsic incubation period for BLU virus, and the expectation of infective life were both lowest during late summer and early fall. During 1997, midge abundance during late summer was not high enough to overcome very low survivorship, and the absolute number of females expected to survive the extrinsic incubation period was relatively low. However, in 1995 and 1996, very high midge abundance compensated for low survivorship during summer and the number of females expected to survive the extrinsic incubation period was relatively high. Although abundance was generally very low during the cool winter and spring, host-seeking females were captured throughout the year, and their winter survival was high. Overwintering of BLU virus by continued transmission of the virus by active midges appears possible.
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Vol. 37 • No. 5