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Aspects of the population biology of the cyst nematode parasites of oilseed rapeInvestigation of the host-parasite relationship between oilseed rape, Heterodera cruciferae and H. schachtii has shown that the rate of hatching, development and reproduction is strongly influenced by temperature, two possible generations occurring on an autumn-sown crop. H.schachtii preferred warmer temperatures, hatched and reproduced more than H. cruciferae. Comparisons between newly-formed eggs in cysts and egg sacs showed that their different hatching responses were related to their physiology; cyst-bound eggs hatched poorly whereas juveniles hatched readily from egg sacs and facilitated the early establishment of a second generation. Multiplication of both species varied greatly between cultivars and differences in hatching and multiplication were attributed to the effects of plant growth and intrinsic differences between cultivars. Plant age influenced the hatching activity of root diffusates and nematode development. Multiplication rates of single and mixed species populations declined with increaSing initial population density indicating that intraspecific competition and root damage limited population growth. Nematodes multiplied synergistically in concomitant infestations suggesting that interspecific competition was less important. In a damage assessment test, root and shoot growth of nematodeinfested plants was reduced and the increased accumulation of calcium in their shoots indicated that they used water less efficiently than uninfested plants. These effects were density-dependent and H.schachtii was more damaging than H.cruciferae. Tolerance to nematode attack was attributed to good root establishment. The rate of decline of H.cruciferae populations varied with time, soil depth and between populations; low soil moisture and temperature favouring nematode survival. The role of weeds as ~maintainer hosts' of H.cruciferae was assessed but considered negligible. Nematode population dynamics were simulated using a computer model. Population densities fluctuated considerably under typical crop rotations but large populations had generally declined to less damaging levels before a host was cropped again. It was indicated that a long run of non-hosts or nematicide use would achieve better control of H.schachtii than H.cruciferae.