The quadratic-orthogonal-rotation-combination design was used to determine the effects and interactions of the main components (leaf powder of Chinese cabbage (Brassica chinensis), malt powder, sucrose, casein and spirulina) of the semi-synthetic diet of the diamondback moth (Plutella xylostella L.), and a model of the main components and the rate of pupation was constructed and analyzed. The results showed that proportions of Chinese cabbage leaf powder, casein and malt powder influenced the rate of pupation significantly (p = 0.05). The optimum proportions of the components were: malt powder 5 g, sucrose 3.5 g, Chinese cabbage leaf powder 5 g, casein 3 g and spirulina 0.75 g in 100 g of diet.
When diamondback moth larvae were reared on this diet at 24-26°C, 60%-70% RH and a 13L∶11D photoperiod, the developmental periods of the larvae and pupae were 8.45 days and the 4.5 days respectively and the duration of development from newly-hatched larvae to adult emergence was 12.95 days on average. The rate of pupation was 74.7%, average pupal weight was 5.14 mg, average adult emergence was 94.9%, average longevity of the female adult was 9.5 days, and females laid 135.67 eggs on average. Compared with the control in which the moth was reared on seedlings of Chinese cabbage, rates of pupation, developmental of the larvae, adult emergence and the longevity of the adult were not significantly different, but the pupal weight and the averaged eggs laid by females were significantly greater. This diet is cheap of prepare, and the preparation protocol and application of the diet is simple and convenient. The diet does not to be changed during rearing of the larvae and pupae.