Aphids are small plant sucking bugs that have prodigious rates of increase with an individual potentially capable of producing in the order of 1011descendants in the course of a season, which is considerably greater than the reproductive potential of other similar sized insects. It is of interest to ask: how have aphids achieved such high rates of population increase? A partial answer is that when conditions are favourable populations consist entirely of females that reproduce without males, i.e., they are parthenogenetic, and all individuals are capable of founding colonies, which rapidly increase in abundance. Thus one would expect aphids consistently to be serious pests of agriculture and forestry. However, not all years are outbreak years and it is therefore of practical importance to ask: what factors prevent aphids achieving pest status each year? This paper attempts to address these questions by taking a closer look at the reproductive biology and host plant relationships of aphids.