Overwintering and low temperature biology of cereal aphids

The aphids Sitobion avenae F., Metopolophium dirhodum Wlk. and Rhopalosiphum nadi L. are important pests of cereal crops. More knowledge of their biology during autumn, winter and spring is needed to improve the control of summer outbreaks. The work described in this thesis was undertaken to investigate the species' low temperature biology and overwintering ecology. All three species can survive the winter as eggs (holocyclic overwintering) or as viviparae (anholocyclic overwintering). Most of the work in this study investigated anholocyclic overwintering. It concentrated particularly on the central role played by low temperatures in winter development, reproduction and survival.The host plant in most of the experiments on viviparae was winter wheat, the crop on which cereal aphids cause most damage. Environmental and intrinsic factors affecting low temperature mortality were investigated in the laboratory. As extensive winter sampling of natural populations in Hampshire was being undertaken by S.C. Hand, fieldwork on anholocyclic overwintering in the present study involved experiments on artificially established populations. In these the processes of mortality, development and reproduction were analysed separately, which enabled relatively precise mathematical relationships to be determined. Mortality due to weather factors was quantified in a field experiment which also allowed comparisons between low temperature mortality under laboratory and field conditions to be made. Detailed monitoring of clip-caged S. avenae viviparae on winter wheat from September to March enabled relationships between development rate, reproduction rate and temperature and between survival, reproduction rate and age to be determined accurately. A less detailed investigation of differences in winter performance between S. avenae and R. padi on winter wheat and perennial rye grass was also carried out.Nearly all the work on holocyclic overwintering concerned M. dirhodum on its primary hosts, Rosa spp. Population surveys in spring and autumn in a west Hampshire study area were undertaken to estimate population densities and characterize favourable overwintering sites. The surveys were done in collaboration with S.C. Hand, who undertook the great bulk of the analysis involved. Colonies of N_. dirhodum on selected rose bushes were monitored to determine the timing of the holocyclic cycle and obtain information on mortality factors. The monitoring and surveys allowed the importance of holocyclic overwintering in M. dirhodum to be assessed. A small amount of laboratory work on holocyclic life-stages was also undertaken.