A population of G. viridula (Chrysomelidae) on Rumex obtusifolius was studied for 4 yr. The population is divided between a river bank and a shingle island. The part of the population on the shingle bank was subject to flooding, which eliminated the subpopulation if it occurred in the larval stage, and much reduced other stages. The probability of flooding at different times of the year was calculated and related to the life cycle of the beetle. Re-invasion of the island from the river bank depended on the rate of dispersal of the beetle and the timing of dry periods when the channel between the island and the mainland dried up, in relation to the dispersive phase. Local extinctions of this nature are common in G. viridula as a result of cutting of meadows or roadside verges and development of wasteland.