Evolution of the host range in spider mites

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Phytophagous mites of the genus Tetranychus are highly polyphagous and occurs on a variety of plant species. However, the host adaptability of the mites, as expressed by their survival and oviposition rates, varies greatly among the plant species, and some species such as chrysanthemums are very unfavorable to them. This study elucidated the mechnisms that determine the host range of the mites, with special emphasis on the two-spotted spider mite, T.urticae, one of the most important agricultural pests. The results of the study are summarized as follows. Genetic, intraspecific variations exist in the host adaptability of T.urticae, and the ecological performance on unfavorable hosts is enhanced very rapidly by an artificial selection that may occur also in agricultural ecosystems. However, whether or not the mites can stay and settle on an unsuitable host is largely affected by their previous experiences on the host through feeding experience and also by the experience of the odur that is emitted by the plant. Even if the mites experienced only the odur, their acceptability of the unfavorable host was greatly increased. Thus, the mites have high behavioral plasticity in adapting to new hosts. Unmated females have much higher plasticity than mated ones, to unfavorable hosts. The analysis using RAPD method suggested that genetic differentiation is not unlikely to occur among different host species in polyphagous spider mites, such as T.urticae. We conclude that the host acceptability of T.urticae, which has a very low dispersal ability, is enhanced by the non-genetic, behavioral change of the mites, and it is then accerelated by genetic selection pressure that will act on the mite population.