The bio-ecology of the blue tick (Boophilus decoloratus) in the central Free State

[摘要] English: Boophilus decoloratus is the most dominant and economically important tick that infestscattle in the central Free State. Very little research on this tick in the central Free State has,however, been conducted. The broad objectives of this study were to investigate aspects ofthe oviposition and reproduction, tolerance of engorged females to sub-zero temperatures,and microhabitat selection, survival and seasonal abundance of non-parasitic larvae, and theseasonal dynamics of B. decoloratus parasitic on cattle in Botshabelo, central Free State.Engorged B. decoloratus females exposed to 10°C did not lay eggs whereas those kept at 15,20, 25, and 30°C, respectively laid eggs. The shortest (3.5days) mean pre-oviposition periodwas recorded at 30°C and 75% RH and the longest (13.2 days) at 15°C and a RH of 35%.The developmental zero temperature for pre-oviposition was calculated at 9.138°e. Theoviposition period was more extended at lower temperatures. The shortest mean ovipositionperiod (8 days) was recorded at 30°C and 35% RH. Incubation was also more extended atlower temperatures. The developmental zero temperature for incubation was calculated at4.73°C. This value was lower than expected and probably due to a smaller sample size orlack of values for eggs incubated between 10 and 15°C. Relative humidity did not affect theoviposition period or the pattern in daily egg production. Peak egg production (533) wasreached within five days from the start of oviposition. B. decoloratus females lay averagesized eggs. Females weighing more than 200mg produced an average of 11.4 eggs per mg ofbody mass. The average conversion efficiency index (Cêl) values recorded in this study recorded nutrient index (NI) values were 10-15% less than the recorded values for specificweight classes of B. microplus.Females exposed during May 1998 to naturally fluctuating conditions died during latewinter (August) without laying eggs. Eggs which were laid during April and May did notaccumulate sufficient temperature for eclosion to take place. Larval survival periods wereinversely dependent on increased temperature accumulation.The results of this study have shown that engorged female ticks do not dig into the soil. Theticks seek refuge in cracks or openings in between gravel particles, or underneath debris.B. decoloratus is able to withstand exposure to sub-zero temperatures. Mortality, however,increased with an increase in exposure time. At an exposure temperature of -10°C and anexposure time of 8 hours, 40% of the females still managed to survive and lay eggs whichhatched normally. The results further indicated that direct chilling injury could occur at <-8°C, which may also be indicative of the supercooling point for engorged B. decoloratus. Investigations on the longevity of larvae under laboratory conditions have shown thattemperature to a great extent affected survival as manifested by a relatively short survivalperiod (2 days) of larvae kept at 30°C. In general the survival periods for the larvae were lessthan those reported in the literature. The differences are believed to be related to differentmethodologies that were used. The results of this study also indicated that B. decoloratus displayed an appetence behaviour by preparing an ambush for its hosts. The larvae migratedvertically and showed a preference for longer (45cm) compared to shorter (5 and 25cm)questing substrates. The larvae formed clusters at the tips of the questing substrates.B. decoloratus ticks were present on cattle all year round. The numbers were, however, lowduring July to September. Peak adult numbers were collected from cattle during April andMay, respectively. Nymphs and larvae displayed peaks during February and April,respectively. It was surmised that B. decoloratus can complete three generations per year inthe central Free State. The overall sex ratio of the ticks collected from cattle did not differfrom unity. This did not conform to the general pattern of male dominance observed forother metastriate ticks. B. decoloratus was the most abundant larvae collected from the drags. The off and on hostseasonal abundance of B. decoloratus larvae correlated well. Drag sampling provided a fairlyaccurate indication of, not only the seasonal abundance of questing larvae, but also the speciesconditiosition in the Botshabelo area.

[发布机构] University of the Free State