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The relative efficiency of three methods of selection (index, independent culling levels and tandem) is compared in terms of genetic gains in economic units. The comparison covers cases where variances, heritabilities and economic weights are unequal, while the case of two correlated characters is also examined. Various factors may influence the relative efficiency, including selection intensity, the number of traits under selection, the relative importance of those traits (in terms of a factor λ, which is the product of economic weight, heritability and phenotypic standard deviation), and the correlations between them.The conclusions are:(i) In all circumstances the index is never less efficient than independent culling levels, though in some cases it is no more efficient. Independent culling is, in turn, never less, but in some cases no more efficient than tandem selection.(ii) The superiority of the index over other methods increases with an increasing number of traits under selection, but decreases with increasing differences in relative importance, its superiority being at a maximum when the traits are of equal importance. The superiority of the index over independent culling levels decreases with increasing selection intensity, but its superiority over tandem selection is independent of intensity.(iii) The superiority of independent culling over tandem selection increases with increasing selection intensity or an increasing number of traits under selection, but decreases with increasing differences in relative importance.(iv) The relative efficiency of the index over other methods is much affected by the phenotypic correlation between traits when the traits are of equal importance, the relative efficiency of the index being higher when the phenotypic correlation is low or negative. The effect of genetic correlation is only apparent when the traits are of unequal importance and its influence on relative efficiency changes with changes in other parameters.(v) The relative efficiency of selection methods may be changed by their use for special purposes.

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Control of sex ratio in the progenies of mated female spider mites was investigated in two laboratory experiments. In Experiment I, a single strain of Tetranychus urticae Koch was reared in four different environments. In Experiment II, 4 different geographic strains of T. urticae were reared in a single environment.Results indicated that spider mite females were able to control the sex ratio of their progeny at two levels: an initial control of mean population ratio according to ovipositional environment. In addition, a secondary control resulted in an approximately uniform daily ratio. Although each parental female produced a varying number of eggs from day to day, a more or less constant fraction of those eggs were fertilized every day, It is concluded that both of these characteristics are probably important to the colonizing ability of the species.