VARIABILITY IN RESPONSE OF LABORATORY-REARED AND FIELD-COLLECTED POPULATIONS OF TETRANYCHUS SPP (ACARI, TETRANYCHIDAE) TO HEXYTHIAZOX

Variability in hexythiazox (Savey) toxicity to the eggs of spider mites (Tetranychus spp.) from six laboratory colonies and 26 field collections (5-6 each from cotton, almonds, pears, roses, and strawberries) was determined using a leaf disk bioassay method. The difference in the response of T. urticae Koch and T. pacificus McGregor to hexythiazox was 10-fold when contact and residual methods were compared. Intraspecies differences in susceptibility were found with laboratory colonies of both T. urticae and T. pacificus, indicating that a low level of cross-resistance to other compounds may exist. Variation in susceptibility of field-collected spider mites to hexythiazox within and between crops was also found for both species. Field collected T. urticae were significantly more susceptible to hexythiazox than were field-collected T. pacificus. Few significant differences were found when responses of field-collected spider mites tested on their original host plant leaves were compared with responses on cotton cotyledons in the laboratory. The use of laboratory-reared cotton had far more advantages than the use of host leaves. These results suggest that continued monitoring of hexythiazox susceptibility, determination of cross-resistance patterns, and the implementation of resistance management tactics are warranted before and after this compound is registered for use.