Abstract:
BACKGROUND: Insight into the factors that regulate the circadian host-seeking flight activity of Culicoides vectors
(Diptera: Ceratopogonidae) will be of importance to assess the risk of transmission of Culicoides-borne pathogens.
This study aimed to determine the impact of temperature and humidity on the flight activity of Culicoides imicola
Kieffer, and other livestock associated Culicoides species, under both laboratory and field conditions.
METHODS: Batches of 500 field-collected C. imicola females were acclimatized at a predetermined range of
temperatures (10–29 °C) and relative humidity (34–85%). After acclimatization, these females, prompted by a light
source, were allowed to escape through a transparent plastic funnel into a paper cup, where they were counted
after an hour. Flight activity under field conditions was determined seasonally by hourly light trap collections done
overnight at four sites near cattle.
RESULTS: Experiments conducted at various test conditions in the laboratory indicated that flight activity started at
13 °C. Peak in activity was observed between 16 °C to 18 °C, and temperatures above 20 °C seemingly inhibit flight.
Under field conditions, a peak in numbers collected was observed immediately after sunset. With mean nocturnal
temperatures below 19 °C, more than 74% of the Culicoides were collected within two to three hours after sunset.
With mean nocturnal temperature above 19 °C, the peak in numbers at sunset was sustained until after midnight,
with somewhat higher numbers collected after midnight once temperatures dropped below 20 °C. No peak in
numbers was observed at dawn. Although very low numbers were collected during the day, which partly may have
been a result of the collecting method, Culicoides were present throughout periods of 24 hours. Humidity seemed
to play a minor role in the regulation of flight activity.
CONCLUSIONS: Abundance and species diversity results as obtained in this study indicated a high level of risk of virus
transmission in the first hours following sunset. A strong relationship was found between host-seeking activity, and
hence trap efficiency, and within limits, temperature. Light traps primarily measure flight activity and may as such
underestimate adult abundance of C. imicola if deployed at temperatures outside thresholds of 16–20 °C.
