Responses of Susceptible and Resistant Tetranychus Urticae to San 831A, 1990

Studies were conducted on a susceptible reference strain (S) and a highly Apollo®/Calibre® (hexythiazox/clofentezine) resistant strain (QRP) of T. urticae. Neither strain had been previously exposed to SAN 831 A. Strain S and QRP were maintained on potted French bean, Phaseolus vulgaris L., at room temperature, under constant illumination for at least 6 months prior to testing. Strain QRP was pressured weekly with Apollo SC (500 g/liter suspension concentrate of clofentezine) and was highly resistant (>1000 fold) to Apollo SC and Calibre 100 WP (100 g/kg W of hexythiazox). Adults were tested by transferring 22-25 young females to 25 mm diameter French bean leaf discs. Serial dilutions of SAN 831 A (200 g/kg W) were prepared in an aqueous suspension. A 2 ml aliquot of liquid was then sprayed onto 2 discs per concentration using a Potter spray tower producing a deposit of 1.6 mg/cm2. Each test was replicated 3 times and a water only replicate was included as a control. After spraying, the discs were maintained on moistened cotton wool at 28 ° 1°C under constant illumination. Mortality, defined as an inability to walk when prodded, was assessed after 48 h. Ovicidal/larvicidal activity was assessed by placing 12-15 females on each leaf disc and maintaining them for 18 h at 28°C under constant illumination. The females were removed leaving only newly laid eggs and the discs were immediately sprayed with serial dilutions of SAN 831 A. The method was as previously described except 4 ml of liquid was sprayed giving a deposit of 3.2 mg/cm2 to improve coverage. The eggs were counted and discs then held for 8 d at 28°C under continuous illumination. Egg hatch was then assessed visually. To calculate larval mortality, the number of young adult survivors was counted and subtracted from the number of eggs. Log dose-probability curves (ld-p) were calculated for all tests. LC50 values were calculated using probit regressions after correction for control mortality.