Effects of intercropping Cabbage with Alliums and Tomato, on the incidence of the Diamondback moth, plutella xylostella (L.)

In assessing the efficacy and economic value of intercropping cabbage as a way of managing Diamondback moth (DBM), Plutella xylostella L., six cropping cultures were evaluated. The trial was conducted at the National Irrigation Research Station, in Mazabuka, Zambia, in 1995/96. The cropping cultures consisted of cabbage monocrop and five other crop combinations. These were cabbage + garlic, cabbage + leek, cabbage + onion, cabbage + shallots and cabbage + tomato. Cabbage monocrop had a significantly (P>0.05) higher incidence of DBM immatures in the early growth stage (3"* and 4* weeks after transplanting), than garlic, leek and tomato. There were, however, no significant differences (P>0.05) in the incidence of the pest in subsequent weeks of sampling and in its overall incidence for the whole sampling period. Cabbage monocrop recorded significantly (P<0.05) higher overall leaf damage index (%) for the whole sampling period than leek, shallots and tomato, though weekly observations were not significant (P>0.05). Observed treatment differences for the cabbage mean total yields (t/ha) and cabbage mean marketable yields (t/ha) were insignificant (P>0.05). Cabbage monocrop, however, had significantly (P<0.05) higher percentage yield loss associated with pest leaf damage than other treatments, except for cabbage intercropped with onion. Cotesia plutellae Kurdjumov, a parasitoid, of DBM larva was found to occur in all the six cropping cultures. There were no significant differences (P>0.05) in the incidence of the parasitoid between the six cropping cultures. Actual parasitism levels of DBM larvae by the parasitoid were also insignificant (P>0.05). Overall, the simple linear Correlation Coefficient (r = + 0.87) between mean parasitoid cacoons and DBM larvae was significant (P<0.05). There was thus a positive linear relationship between the overall mean number of parasitoid cacoons and the overall mean number of DBM larvae. The simple linear Correlation Coefficient (r = + 0.96), between overall DBM larvae incidence and overall leaf damage was significant (P<0.05). A positive linear relationship between the parasitoid and the pest was thus confirmed. In terms of profitability, the cabbage + leek combination had the highest positive net returns of + K 11 757 600 00, while the cabbage + tomato combination had the lowest negative net returns (- K 3 059 576 00).