Effectiveness of Spinosad against Diamondback Moth (Plutella xylostella L.) Eggs and Larvae on Cabbage under Botswana Conditions

The efficacy of spinosad against the diamondback moth (DBM) on cabbage was studied at Botswana College of Agriculture, Gaborone, Botswana in 2011. Using five concentrations of spinosad: 0.12, 0.36, 0.60, 0.84 and 1.08 g/L, bioassays were conducted against DBM eggs and second instar larvae at 30°C ± 5°C. Each treatment was replicated three times. Probit analysis was used to determine LD(50) and LD(90) values for the treatments against eggs and larvae. When the treatments were assessed at 72 and 96 hours, LD(90) values against larvae were 0.74 and 0.59 g/L, whereas they were 0.35 and 0.32 g/L against eggs. This indicated that spinosad was more effective against eggs than against larvae. The slopes of the probit lines for larvae assessed at 48, 72 and 96 hours after application were 3.519, 3.810 and 3.427, while those against eggs were 1.725, 1.316 and 1.086. This indicates that there was a more rapid change in larval mortality with increase in pesticide dosage than in egg mortality. The study shows that spinosad can achieve effective control of DBM eggs and larvae under Botswana conditions.