Graduate Thesis Or Dissertation
 

Biological studies of Eriosoma pyricola with particlar emphasis on morph production

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  • Various aspects of the biology of the woolly pear aphid Eriosoma pyricola were studied in order to further understand its annual cycle. It was already known that. E. pyricola produces alate sexuparae in response to the onset of dormancy in its secondary host, a pear. This study of E. pyricola on detached root pieces showed that alatae were also produced in response to crowding. The crowding stimulus appeared to be necessary for elate production and could act alone, without any seasonal stimulus from the host plant, as the root pieces were cut from live plants throughout the year. The crowding stimulus followed a delayed density dependent pattern when densities were high, which was not apparent when densities were low. Aging cultures produced greater numbers of alatae. The crowding stimulus did not act maternally, but on the second and third instar and occasionally on the fourth instar young. The alate fourth instar young were found to be gregarious, a factor enhancing the crowding effect once the first alatae appeared so that a large proportion of the population would become alatiform within a short time. The population remaining on the roots continued as the apterous morph until densities built up again. Neither the crowding nor the host plant stimulus affected the whole population, some individuals remaining in the apterous form. It was considered that sporadic crowding during the summer determined which individuals would respond to the host plant stimulus.
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