A study of the parasitism efficiency of Cotesia Vestalis (Haliday) on Diamondback Moth, Plutella Xylostella(L.),(Lepidoptera:Plutellidae)
The diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae) is an insect pest of cruciferous crops in the world including Zambia. It causes 52 – 92% loss in marketable produce in India. Control of P. xylostella has mainly depended on use of insecticides. However, the pest has developed resistance to pesticides used against it. One option is biological control. In Zambia, an attempt to control the pest using biological control agents was done between 1977 and 1984 when Cotesia vestalis and Diadromus collaris from Pakistan were released together with indigenous parasitoid Oomyzus sokolowskii. The results were not successful probably because Cotesia vestalis was not efficient hence the motivation to investigate the parasitism efficiency of C. vestalis on P. xylostella under laboratory conditions. Colonies of P. xylostella and C. vestalis were established from larvae of P. xylostella collected from cabbage fields in Chilanga, Kafue and Lusaka districts. Completely randomized design experiment with three replications was set up in the laboratory. Plutella xylostella were exposed to C. vestalis at 24, 48, 72, 96 and 120 hours after emergence of C. vestalis at ratio levels of 2:1 (P2) and 4:1 (P4). In this study, the field parasitism percentage was 34.85 and contributed 73% to total larval mortality. In the laboratory, parasitism rates of 55% and 50% were observed in the first 24 hrs of exposure at P2 and P4 levels respectively from this study. There was significant difference in Searching efficiency of parasitoids (p < 0.01) with exposure time in treatment ratio P2. Significant difference in damage score were observed to cabbage plants treatment levels P2 (p < 0.001) and P4 (p < 0.003). There was negative correlation between % parasitism of P. xylostella and damage score of cabbage. As parasitism increased from 0 in the control to 55% and 50% in treatment ratios P2 and P4 respectively, damage score decreased from 6 in the control to 1 and 2.5 in vii Treatments ratios P2 and P4 in that order. These results therefore, show that C. vestalis has the potential to be used as a biological control agent against P. xylostella in cruciferous crops in Zambia. Parasitism efficiency needs to be investigated under field conditions and where insecticides were not applied.
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