Development and survival of Rhipicephalus Appendiculatus Neumann, 1901 (Ixodidae) in laboratory and field
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Abstract
This study was financed by the Ministry of Overseas Development, and was carried out in two parts. The first part involved a year of preliminary work in the Department of Zoology, at the University of Edinburgh, and covered the first exploratory enquiries into standards of development and survival under laboratory conditions. The second part was conducted while the writer served a 30-month tour of duty at the East African Veterinary Research Organisation, Muguga, Kenya, and during this time the initial tre x of the studies was followed up while making a concurrent examination of development and survival under near - natural conditions at two contrasting outside sites of typical East African character.
The studies carried out on the engorgement of Rhipicephalus appendiculatus on rabbits, cattle and various other hosts revealed no evidence of host specificity. In only two aspects were variation in feeding performance directly attributable to factors within the tick. First, a lack of vigour, produced by delaying application to the host for prolonged periods after emergence, resulted in a decline in the proportion of successful engorgements yielded by a given number of ticks applied. Second, the completion of engorgement by adult females was shown to be completely dependant upon mating. An initial impression that a greater degree of infestation resulted in a shortening of the engorgement period was not substantiated, and it was concluded that this impression arose from a pre-attachment elimination of the slower-to-attach proportion of the ticks applied. The character of individual host response appeared to be one of the principal factors in the variation in tick feeding performance.
It was established that successive applications of R. appendiculatus to rabbits produced a prolongation of the engorgement period; a decline in the number of successful engorgements and in the degree of repletion; and an increasing severity in host reaction.
The rate of development was shown to be determined by temperature and that, provided that the ticks survived, the level of humidity had no influence on this aspect. There was no evidence of any diapause mechanism in any stage of R. appendiculatus. No development of any sort was recorded at 9°C or below, and sustained temperatures of 4°C were lethal to engorged instars. Observations at the two outside sites showed that only one life cycle could be completed each year at Muguga, whereas the rate of development at Kedong would permit two life cycles to completed each year in conditions of high humidity.
Survival was shown to be governed by water relations. In the unfed instar, this was a straightforward relationship with saturation deficiency, so that the effects of low humidity were more rapidly lethal at higher temperatures. Thus, at 20% RH, all unfed instars survived for roughly twice as long at 18 °C as they did at 25 °C. There was some indication that, in proportion to their size, unfed adults were more adapted to dry conditions than immature instars. Eggs were very sensitive to aridity, and no hatching was observed in egg batches which had been exposed to 40% RH for periods longer than 14 days - at any temperature. Unfed ticks were shown to regain moisture from the atmosphere. Though low humidities were lethal to engorged instars, the relationship with saturation deficiency was by no means as clearly defined as with unfed instars.
Observations at the two outside sites emphasised the important influence that the physiognomy of grass cover has upon the survival of ticks. It was apparent that the life cycle of R. appendiculatus could be completed at Muguga no ratter at what season of the year it was commenced. At Kedong, on the other hand, it was apparent that the survival of a R. appendiculatus population would depend upon whether or not conditions were moist enough to allow the hatching of wet-season eggs. A comparison between the survival of ticks free on the herbage at Kedong and the survival of ticks confined in containers within the same grass cover indicated the crucial influence which transpirational microclimates may have on the survival of the species in such marginal areas. This indication was to some extent reinforced by results obtained with an environmental model constructed for studies in the laboratory.
The findings of the study are discussed in relation to observations made during previous field experience. An example is given of a R. appendiculatlis population increasing under a system of improved grassland management because the timing of grazing rotations coincided favourably with the development rate of the species. Some aspects of traditional grass- burning regimes are discussed in conjunction with associated vegetational successions.
Although not in fact used for the purpose for which it was devised, a successful and useful system of labelling ticks with the radio-isotope Cerium-144 is described in an appendix to the study.
The studies carried out on the engorgement of Rhipicephalus appendiculatus on rabbits, cattle and various other hosts revealed no evidence of host specificity. In only two aspects were variation in feeding performance directly attributable to factors within the tick. First, a lack of vigour, produced by delaying application to the host for prolonged periods after emergence, resulted in a decline in the proportion of successful engorgements yielded by a given number of ticks applied. Second, the completion of engorgement by adult females was shown to be completely dependant upon mating. An initial impression that a greater degree of infestation resulted in a shortening of the engorgement period was not substantiated, and it was concluded that this impression arose from a pre-attachment elimination of the slower-to-attach proportion of the ticks applied. The character of individual host response appeared to be one of the principal factors in the variation in tick feeding performance.
It was established that successive applications of R. appendiculatus to rabbits produced a prolongation of the engorgement period; a decline in the number of successful engorgements and in the degree of repletion; and an increasing severity in host reaction.
The rate of development was shown to be determined by temperature and that, provided that the ticks survived, the level of humidity had no influence on this aspect. There was no evidence of any diapause mechanism in any stage of R. appendiculatus. No development of any sort was recorded at 9°C or below, and sustained temperatures of 4°C were lethal to engorged instars. Observations at the two outside sites showed that only one life cycle could be completed each year at Muguga, whereas the rate of development at Kedong would permit two life cycles to completed each year in conditions of high humidity.
Survival was shown to be governed by water relations. In the unfed instar, this was a straightforward relationship with saturation deficiency, so that the effects of low humidity were more rapidly lethal at higher temperatures. Thus, at 20% RH, all unfed instars survived for roughly twice as long at 18 °C as they did at 25 °C. There was some indication that, in proportion to their size, unfed adults were more adapted to dry conditions than immature instars. Eggs were very sensitive to aridity, and no hatching was observed in egg batches which had been exposed to 40% RH for periods longer than 14 days - at any temperature. Unfed ticks were shown to regain moisture from the atmosphere. Though low humidities were lethal to engorged instars, the relationship with saturation deficiency was by no means as clearly defined as with unfed instars.
Observations at the two outside sites emphasised the important influence that the physiognomy of grass cover has upon the survival of ticks. It was apparent that the life cycle of R. appendiculatus could be completed at Muguga no ratter at what season of the year it was commenced. At Kedong, on the other hand, it was apparent that the survival of a R. appendiculatus population would depend upon whether or not conditions were moist enough to allow the hatching of wet-season eggs. A comparison between the survival of ticks free on the herbage at Kedong and the survival of ticks confined in containers within the same grass cover indicated the crucial influence which transpirational microclimates may have on the survival of the species in such marginal areas. This indication was to some extent reinforced by results obtained with an environmental model constructed for studies in the laboratory.
The findings of the study are discussed in relation to observations made during previous field experience. An example is given of a R. appendiculatlis population increasing under a system of improved grassland management because the timing of grazing rotations coincided favourably with the development rate of the species. Some aspects of traditional grass- burning regimes are discussed in conjunction with associated vegetational successions.
Although not in fact used for the purpose for which it was devised, a successful and useful system of labelling ticks with the radio-isotope Cerium-144 is described in an appendix to the study.
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