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• W1444294515 abstract "Effect of an induced resistance in soybean on the susceptibility of twospotted spider mites {Tetranychus urticae Koch) to two acaricides was studied under greenhouse conditions. Expression of induced resistance was strongest in reducing spider mite fecundity and this observation was consistent with previous workers. As a consequence, significant differences and trends were detected only with egg populations. The interaction was elucidated via probit regression analysis. The results indicated that, while the egg reduction achieved by the combination of inducible resistance and acaricide was maximum at ca. 10 ppm, the interaction peaked at ca. 75 ppm. These maxima were the same for both acaricides. More statistically significant differences were detected with dicofol than with dimethoate. While the actual differences detected were slight, they have implications with respect to a reduction of pesticide usage for spider mite control. Induced resistance to twospotted spider mites has been mostly studied in cotton plants (Karban and Carey, 1984; Karban, 1985, 1987). Recently Hildebrand et al. (1986) found that populations of the twospotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), feeding on soybean plants {Glycine max L.) previously exposed to mite feeding, increased at a significantly slower rate when compared to those on unexposed soybeans. Nurdin (1987), also working with soybeans, found no significant effect on mite weight, longevity, or ovipositional preference between mites reared on undamaged leaves of previously exposed versus unexposed plants. There was, however, a significant reduction in fecundity among female mites feeding on plants which had experienced mite exposure. This finding corroborated that of Hildebrand et al. (1986) who found that the effect of induced resistance was most obvious in the reduction of egg production. If it is assumed that reduction in fecundity is indicative of stress or nutritional deficiency for the mites as opposed to the plant, then one may ask whether this might also alter the mites' susceptibility to pesticides. If this is true, then it could suggest an important application for inducible resistance in mite management. Materials and Methods Induced resistance procedure: The methods used to induce resistance followed those of Nurdin (1987). All experiments were conducted at the Acarology Lab oratory of the University of Kentucky. Mites were obtained from a laboratory colony maintained on horticulture dwarf beans (Phaseolus vulgaris L.). Three soybean seeds (var. 'Williams') were planted in each of 80, 18-cm diameter pots in a greenhouse on June 1, 1987. After the soybeans emerged, plants were thinned to one plant/pot. Once the plants reached the cotyledon stage, when the cotyledons 1 The investigation reported in this paper (No. 89-7-169) is in connection with a project of the Kentucky Agricultural Experiment Station and is published with approval of the Director. 2 Department of Agronomy, University of Kentucky, Lexington, Kentucky 40546. Accepted for publication 14 November 1989. This content downloaded from 40.77.167.54 on Thu, 06 Oct 2016 04:10:18 UTC All use subject to http://about.jstor.org/terms VOLUME 63, NUMBER 2 219 had opened but unifoliate leaves were still folded, half were randomly selected for mite exposure and were infested with ten Tetranychus urticae mated adult females. The control group comprised the remaining seedlings which were not exposed to mites. Each experimental and control plant was enclosed in a cylindrical acetate cage, 58 cm high by 18 cm in diameter to prevent mite migration. Five days after infestation, both experimental and control plants were sprayed with a solution of 400 ppm dicofol which killed all mites (Karban and Carey, 1984). No true leaves had expanded at the time of dicofol treatment, and the acaricide produced no direct or interactive effect on subsequent mite populations on cotton (Karban, 1985). Pesticide exposure test: Two weeks after mite removal, exposed and unexposed plants were divided into eight different treatments consisting of four concentra tions of dicofol (0, 10, 100, 400 ppm) and four concentrations of dimethoate (0, 10, 100, 400 ppm). These two chemicals were selected for this study because they are the most widely used miticides on soybeans in Kentucky. The exposed and unexposed plants which had been sprayed with the same concentration and pes ticide were physically kept as close as possible. Each treatment was replicated five times. A few hours later, after the leaves had dried, five T. urticae adult females (< 1 day old) were placed on the most recently expanded new leaf, V4 stage (Fehr and Caviness, 1977), of each plant and the acetate tube was reinstalled on the pot. Fourteen days later, all leaves were removed and the number of eggs, immatures, females, and males counted. A split-plot design, with concentration as main plots and induced and non induced resistance as sub-plots, was used in this experiment. An analysis of vari ance (ANOVA) and protected least significant different test (LSD) (Fisher, 1935) were performed on the number of eggs, immatures, males, and females to test for significant differences (P < 0.05). A probit regression analysis was used to analyze the interaction between acaricides and induced resistance. In order to perform this analysis, each replication of each acaricide concentration was compared to the corresponding replication of the unsprayed controls in the same exposure category (i.e., exposed or unexposed). This approach assumed that, had the acar icides not been applied, each treatment replication would have produced the same number of eggs as the corresponding control replication. The SAS PROBIT (SAS Institute, 1985) procedure was used for this analysis. This program requires input data in the form of the number of individuals and the number which responded to that treatment. For the number treated (in each replication), we used the number of eggs in a given control replication. For the number that responded, we used the difference between the control and the treatment. For example, replication 1 of the exposed dicofol control (0 ppm) had 1142 eggs while replication 1 of the exposed, 400 ppm dicofol treatment had 24 eggs. Replication 1 of the 400 ppm dicofol treatment was then considered to have 1142 individuals treated of which 1118 responded. Results and Discussion Females, males, immatures and eggs were generally more numerous on the control (0 ppm) plants than on the plants sprayed with either acaricide (Tables 1, 2). However, the number of mites across all stages was lowest on plants This content downloaded from 40.77.167.54 on Thu, 06 Oct 2016 04:10:18 UTC All use subject to http://about.jstor.org/terms 220 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY Table 1. Mean number ? SE of different stages of T. urticae on exposed (E) and unexposed (U) plants when sprayed with dicofol. Cone, (ppm) Males Immatures Eggs Total mites 0(E) 27 ? 17(a)1 0(U) 10(E)" @default.
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• W1444294515 title "Interaction of chemical pesticides and induced resistance in soybean against Tetranychus urticae Koch." @default.
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