Habitats of Twospotted Spider Mites (Acari: Tetranychidae) During Winter and Spring in a Cotton–Producing Region of Australia

Abstract

Winter and spring habitats of the twospotted spider mite, Tetranychus urticae Koch, were investigated to determine the source of mites infesting seedling cotton crops. The two possibilities were that mites may overwinter in diapause in the soil or litter of cotton fields or they may overwinter on suitable vegetation. Few mites were found in litter or soil collected from fallow cotton fields in winter. Significant numbers of diapause mites were found in litter at only 2 of 15 sites. At one of these sites the abundance of mites in litter declined dramatically following cultivation. The cotton crops at these two sites were late maturing and, therefore, more mites may have been induced to diapause because of cooler temperatures, shorter days, and possibly lower food quality than at other sites where more typical, earlier maturing crops were grown. The contribution of mites that overwinter in the soil or litter of cotton fields to infestations on the subsequent cotton crop is not likely to be significant. Vegetation from within and from the borders of fallow cotton fields and from sites remote from cotton (>1–km distance) were examined for presence of mites in winter and spring. Remote sites were further classified as adjacent to permanent water (wet) or not (dry). Nondiapausing mites, actively feeding and reproducing, were found on a wide range of plant species (37) in the vicinity of cotton fields in winter and spring. Natural senescence of these plants in spring probably forces mites to disperse, often onto young cotton seedlings nearby, resulting in a pronounced edge effect in cotton fields. Mite abundance on weeds was far lower at dry remote than at cotton sites, possibly caused by the lack of a continuous sequence of hosts. Mite abundance at wet remote sites was intermediate between cotton and dry remote sites, probably because they provide a continuity of hosts for mites. Management of the weeds on cotton farms through winter/spring could potentially reduce levels of mite infestation of cotton seedlings in the following season.