Causes of spatial variation in parasite and pathogen pressure in insects

• Main Author: Pastok, Daria
• Published: University of Liverpool 2015
• Subjects: 570; QL Zoology
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.677530

The reproduction of the two-spot ladybird, Adalia bipunctata, is inhibited by a sexually transmitted ectoparasitic mite Coccipolipus hippodamiae that sterilizes female hosts, and a range of heritable microbes that kill male hosts during embryogenesis for instance Spiroplasma. Historical sampling indicated these two parasites were present commonly in A. bipunctata in the south of Sweden but were absent or rare in northern populations. In this thesis, I first established that the distribution of the mite was still stable over time, with a boundary to mite presence at 61˚N, as found 10 years previously. The incidence of male-killing Spiroplasma, in contrast, showed a small northward shift in its presence. I then examined the causes of mite presence/absence, and concluded ladybirds from northern populations were competent to carry and transmit mite infection, but that host phenology prevented its persistence in natural populations. I then explored the hypothesis that mite presence might select for increased reluctance of females to mate by comparing mating behaviour between mite present/mite absent populations. I observed that whilst rejection behaviour was protective against mite transmission, it was not more commonly observed in ladybirds derived from mite present populations. Finally, I examined whether the presence of male-killing Spiroplasma might affect mite epidemiology, and whether the mite itself might impact on the dynamics of the male-killing Spiroplasma. I observed that Spiroplasma did not affect individual competence to acquire and transmit mite infection, and did not protect its host against mite induced sterility. However, Spiroplasma was observed to mildly prolong host longevity, which may enhance the capacity of the mite to pass from overwintered to new generation cohorts of its host. Further, population sex ratio biases induced by Spiroplasma were predicted to influence mite epidemiology, through altering mating rate and per contact mite transmission probability. I therefore conclude first that mite incidence can be explained through host phenological variation, and that sex ratio distorting symbionts may impact on the dynamics of the mite infection. Future studies should explore the causes of high prevalence Spiroplasma infection in A. bipunctata in southern Sweden, and the features that prevent the establishment of infection in the north of the country.