Evaluation of bacillus thuringiensis (var. kurstaki), spinosad and cypermethrin for the control of diamondback moth (plutella xylostella l.) on cabbage.
Legwaila, Mosimanegape Mitch
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An experiment was conducted al the Botswana College of Agriculture (BCA), Gaborone, Botswana (24° 55’S, 25" 95’E) at an altitude of 998m above sea level to evaluate the effectiveness of Bacillus thuringiensis(Bt), spinosad and cypermethrin for the control of diamondback moth, Plutella xylostclla (L.) on cabbage (Brassica oleracea var. capitula L.). The insecticides were applied against DBM eggs and larvae on plants enclosed in cages in a greenhouse. The insecticides were formulated at five concentrations: Bt at 2, 4, 6, 8 and 10g/l; spinosad at 0.12, 0.36, 0.6, 0.84 and 1.0 g/1; and cypermethrin at 1.2, 1.6, 2.0, 2.4 and 2.8g/l. The recommended dosage level of each insecticide (4.0g/l for Bt, 0.6g/l for spinosad and 2.0g/l for cypermethrin) was one of the concentrations used. The results were analysed using probit analysis where logio dose was plotted against probits. LDjo and LD<« values found with each insecticide were estimated on different assessment periods (24, 48, 72, 96, 120 and 144 hours) after application and used to compare effectiveness of the three insecticides and also determine when the recommended dose achieved effective control of DBM under the experimental conditions of 30±5"C that occurred in the greenhouse. LDoo values were also used to compare effectiveness of the recommended dose of each of the three insecticides against larvae and eggs. The LDto values for cypermethrin against DBM larvae were 1.82 and 1.19g/l while those against eggs were 1.63 and 1.40g/l respectively therefore lower doses of cypermethrin can be used to control DBM eggs and larvae. LDgo values achieved after application of spinosad to DBM larvae were 0.82 and 0.78g/l while those achieved after application to eggs were 0.55 and 0.51 g/1 respectively showing that spinosad was relatively more effective when applied against eggs than against larvae. Similarly, the LDW values achieved after Bt application to DBM larvae were11.02 and 10.22g/l and those after application to eggs were 6.94 and 6.24g/l respectively therefore higher doses of lit would be required to achieve effective control of larvae and eggs. The LD90 values and slopes of the probit lines were used to compare relative effectiveness of the three insecticides against larvae and eggs. The LD50 values for spinosad against larvae and eggs were much lower than those of cypermethrin and Bi therefore spinosad was relatively more effective than Bt and cypermethrin. Slopes of the probit lines for spinosad, Bt and cypermethrin against DBM larvae were 3.810, 0.910 and 1.445 respectively; while slopes of probit lines against DBM eggs were 1.315, 1.246 and 0.814 respectively therefore spinosad had the most rapid change in mortality of eggs and larvae with increase in pesticide dose than Bt and cypermethrin. When the period it takes for the insecticide to kill 90% of the pest was used to compare the relative efficiencies of the insecticides at the recommended rales; 90% larval mortalities occurred 6 days after application with Bt and spinosad and only 3 days following cypermethrin treatments. 90 to 100% egg mortalities occurred 4, 3 and 2 days after application of Bt, spinosad and cypermethrin respectively. Comparison of the level of protection achieved by each of the three insecticides showed that cypermethrin sprays using concentrations lower than the recommended dosage (1.2 and 1.6g/l) and the recommended dosage of spinosad achieved effective protection of the crop. However, Bt required application of dosages higher than the recommended dose of 4g/l to achieve adequate protection of cabbage from DBM damage. Since the DBM population in this study did not show any signs of resistance to cypermethrin, continued use of cypermethrin is recommended. Spinosad has shown potential as an alternative to cypermethrin for the control of DBM under Botswana conditions. However, further research and field testing is necessary to confirm that lower dosages of cypermethrin and spinosad can be used without affecting their effectiveness against DBM larvae and eggs and without lowering
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