MAINTENANCE OF DIAPAUSE VARIABILITY IN THE 2-SPOTTED SPIDER-MITE, TETRANYCHUS-URTICAE, IN A HETEROGENEOUS AND STOCHASTIC ENVIRONMENT

A simple nongenetic mathematical model analyzed the processes responsi ble for the variations in the diapause percentage among populations of the two-spotted spider mite, Tetranychus urticae Koch. This model inc orporates the following assumptions. 1) Mites have diapause (DD), non- diapause (NN), and ''plastic'' (DN) populations (plasticity exists in the phenotypic expression of diapause in response to habitat condition s at the time of diapause induction). 2) A heterogeneous mite habitat consists of microhabitat L, in which all the non-diapausing mites die during the winter due to the lack of winter host plants, and microhabi tat O with winter hosts capable of supporting some of the non-diapausi ng mites overwinter. 3) Temporal fluctuation of winter conditions whic h affect the survival and reproduction of non-diapausing mites. Using these assumptions, we compared the fitness functions of the three popu lations and analyzed the conditions under which each population is fav oured over the other two, thereby elucidating the processes involved i n the maintenance of variability in diapause. Our analysis revealed: 1 ) frequent mild winters are of primary importance for the non-diapause trait to be maintained, 2) the existence of winter hosts is also impo rtant for the nondiapause trait to be favoured, and this importance de pends greatly on the degree of the adaptive diapause expression in the DN mites in response to habitat conditions, i.e., the better the phen otype environment matching in DN, the higher the probability that DN w ill be favoured, 3) The combined effect of the temporal and spatial va riation enhances the maintenance of variablity in the diapause trait o f the mites.