Culicoides subimmaculatus Lee and Reye breeds in the intertidal zone of estuarine environments from northern Queensland to South Australia. The population dynamics of this pest species were studied at Tingalpa Creek, Brisbane, and aspects of the life cycle studied in the laboratory.
The density of adult populations was assessed by sampling with a light trap, truck trap and human bait. Meteorological observations were also made, and multiple covariance analysis used on light trap data to determine the effect of five variables (windspeed, temperature, time, saturation deficit and cloudcover) on flight activity. Partial regression coefficients were derived for the three variables which significantly influenced activity, enabling samples to be corrected to standard conditions. Thus comparative population estimates could be made from samples taken under different conditions.
Windspeed had the greatest influence on activity, followed by temperature and time after sunset. Activity was related to temperatures up to 22.5 C, but was independent at higher temperatures (up to 25 C). Moonlight increased numbers taken in the truck trap but had little effect on light trap catches. Activity was independent of saturation deficit and cloudcover.
An age-grading system was devised for female C. subimmaculatus, enabling the age structure of populations to be determined. Seven grades (newly emerged; gravid; uniparous; bloodfed; maturing second or subsequent batch; gravid with second or subsequent batch; multiparous) were established, which could be recognised without dissection.
At Tingalpa Creek, the three sampling methods gave similar estimates of population trends. During the first year of sampling, light trap samples were also taken at Boggy Creek, 11 km from Tingalpa Creek. Populations of C. subimmaculatus followed similar trends at both sites.
Twenty-four hour sampling sessions revealed that maximum activity occurred at sunset and sunrise, and generally continued at lower levels throughout the night. Little activity occurred during the day. Peak activity at sunset was correlated with rate of change of light intensity.
The main aims of the study were (i) to determine the relationship between emergence of C. subimmaculatus and the tidal cycle, and (ii) to examine seasonal abundance. Twenty-two other species of Culicoides were collected, seven of which were examined in some detail.
Immature stages of C. subimmaculatus were sampled at Tingalpa Creek. Their vertical distribution was determined in relation to tide levels, with maximum numbers occurring 1.94 m above datum, i.e. just above Mean High Water Neaps. At this level larvae were covered by at least one tide per day except at neap tides, when the area could remain exposed for several days.
Larval data proved particularly useful in interpreting seasonal abundance. C. subimmaculatus populations overwintered as 4th instars. Four generations were completed each year with the generation length varying from 10 to 12 weeks, excluding the overwintering population.
C. subimmaculatus matured its first egg-batch autogenously, but required a bloodmeal for subsequent batches. The first reproductive cycle was studied in laboratory-reared females, and required 2h days at ambient temperatures (26°C - 27°C). Field caught females were bloodfed in the laboratory to study the second cycle, which lasted 5 days (22 C - 25 C). Gonotrophic harmony did not occur in this species.
Emergence of C. subimmaculatus was closely related to the tidal cycle and generally occurred at neap tides. Laboratory studies were performed to determine the factors which linked emergence to the tidal cycle. Emergence was not affected if pupae were subjected to tidal coverages of up to 9 hours per day. The time of pupation and emergence of field-caught 4th instar larvae was not altered by various artificial tidal cycles. Evidence was presented to support the suggestion that neap tides synchronized ecdysis of 3rd to 4th instars, resulting in emergence peaks one month later.
Eggs from field-caught females were reared to adults in the laboratory. Development rates of immature stages were determined at four temperatures (18°C, 22°C, 25°C and 28^C). At 25°C development of immatures was completed in 46 days.
Advantages of autogeny and the intertidal zone as a breeding habitat were considered in relation to the success and survival of C. subimmaculatus as a species.