EFFECT OF STRAWBERRY PLANT PHYSIOLOGICAL STATUS ON THE TRANSLAMINAR ACTIVITY OF AVERMECTIN B-1 AND ITS EFFICACY AGAINST THE 2-SPOTTED SPIDER-MITE (ACARI, TETRANYCHIDAE)

Plant physiological status determines translaminar movement of avermec tin B-1 in strawberry leaves, and the resulting noncontact residual ki ll of twospotted spider mite, Tetranychus urticae Koch. Significant T. urticae mortality was obtained upon contact with avermectin B-1 with and without the addition of dormant oil, and with propargite. Nonconta ct residual kill of T. urticae by avermectin B-1 was greater on fruiti ng summer-type plants than on plants in winter semidormancy. When aver mectin B-1 was applied only to the top surface of leaves, addition of dormant oil enhanced T. urticae mortality on lower leaf surfaces of pl ants in winter semidormancy. T. urticae mortality was not affected by the addition of oil when plants were in either a spring-type vegetativ e growth state or a summer-type sexual state. The results of this stud y suggest an explanation for inconsistencies observed in field applica tions of avermectin B-1 for control of T. urticae.