Practical Application of the Stable Age Distribution: Analysis of a Tetranychid Mite (Acari: Tetranychidae) Population Outbreak

Abstract

Stage-structured counts of tetranychid mites (eggs, immatures, and adults) on 360 whole cotton plants were made during a single growing season in California, These counts were analyzed by using their departure from a stable stage distribution (65% eggs, 25% immatures, and 10% adults) implied by the birth and death rates from a previous life history study of the twospotted spider mite. By using these stable distributions as base lines for analysis, several questions were addressed, including: (1) What was the possible influence of migration on age structure? (2) How did the birth and death rates in the field populations compare with those determined in the laboratory? and (3) How did birth and death rates differ among the three growth phases? It is concluded that in general the stable age distribution is useful in demographic field studies because it focuses attention on age structure, any observed departure from it demands an explanation, and any explanation must take into account collective changes in proportions of stages intercoupled with commensurate shifts in growth rate.