Life-history evolution of spider mites

Abstract

A literature review of data on life histories of spider mites (Acari: Tetrany-chidae) under standardized conditions (25 ± 2°C) reveals a large amount of interspecific variation. The intrinsic rate of population increase (r m ) varies from c. 0.1/day to 0.3/day. This variation may well be caused by (phylo) genetic factors, host-plant related factors and/or (co-)evolutionary interactions with other organisms (competitors, predators, pathogens). All these may act as constraints to evolutionary change of one or more life-history components. One way to reveal their importance is to assume that constraints are absent, and to test hypotheses on how life-history components evolve under r selection. Analysis of available data shows that life-history components (excluding sex ratio among offspring) strongly covary; an increase in r m arises from a combination of faster development, higher rate of oviposition and higher fecundity. It is when fecundity and oviposition rate enter the range of high values that progress in development rate becomes slower and slower, suggesting that there may be a constraint to speeding up development. Why sex ratio among offspring does not bear a relation to any of the other life-history components, is an open question. In an attempt to provide an explanation, a new hypothesis is discussed stating (1) that spacing and aggregating within a web are two alternative ways of decreasing predation risk; and (2) that these strategies have different consequences for the population mating structure, and hence also for sex-ratio evolution.