1 The fluctuations in the density and composition of populations of Pemphigus trehernei, an aphid that lives on the roots of the halophyte Aster tripolium, were followed on an area of saltmarsh on the north Norfolk coast between 1971 and 1973.
2 The aphid has a typical Pemphigus life-cycle, but the emphasis is on the non-sexual anholocycle. Many diapausing aphids overwinter on the marsh and relatively few alates fly to or from it.
3 The aphids are confined to the top 11 cm of soil and are highly aggregated: the dispersion of the population can be fitted to the negative binomial. The absolute density of the populations is relatively low, with a mean value of about 2000 aphids/m2 of available marsh area.
4 Temperature, host-plant growth and fungal attack are probably the most important factors affecting seasonal population fluctuations. Up to 50% of the populations may be infected with the fungus Metarrhizium anisopliae, which is widespread in the soil.
