Population dynamics

Abstract

Aphids can become very abundant, noticeably so on agricultural crops. A hectare of field beans can produce 4000 million alatae of the black bean aphid (Aphis fabae) (Way and Banks, 1967) and the cereal aphid, Metopolophium dirhodum, can achieve population densities of 220 per tiller or 1000 million per hectare of wheat. Although aphids are small these numbers are equivalent in mass to an elephant and a cow respectively. Insect predators feeding on these aphids can also reach impressive numbers. In England in 1976, 280 000 seven-spot coccinellid adults, Coccinella septempunctata, matured per hectare of wheat after feeding on the cereal aphid Sitobion avenae, which nevertheless achieved a peak population of 200 million per hectare. The estimated 24 000 million seven-spot ladybird beetles produced in the county of Norfolk combined with some produced in other counties in 1976 gave rise to a ladybird ‘plague’ that drove people off the beaches in parts of East Anglia.