Assessing and Enhancing the Impact of Cotesia plutellae in Management of Diamondback Moth Plutella xylostella on Kale Brassica Oleracea var. acephala in Semi-Arid Areas of Kenya
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Diamondback moth (DBM, Plutella xylostella L.) is a major pest of crucifers which can cause yield losses of up to 100% if left unmanaged. Classical biological control of DBM was adopted in Machakos and Kitui County through Cotesia plutellae imported from South Africa in 2002 and released in March 2004 in farmers’ fields where kale Brassica Oleracea var. acephala is one of the major leafy vegetables. Preliminary studies in 2004- 2006 showed that the parasitoid had not established in the release sites and augmentation releases were done in 2006. Monthly surveys were conducted (May 2012 to April 2013) to assess the spread and contribution of C. plutellae in the management of the DBM. Brassica oleracea var. acephala fields in twenty five randomly selected farms were sampled for number of DBM larvae and pupae, damage, parasitism and parasitoid guild. The data was recorded from 20 plants in each selected farm. The DBM larvae and pupae were placed in plastic containers and taken to the laboratory for DBM or parasitoid emergence. Results revealed that the mean number of DBM/plant in Kitui and Matuu ranged from 0.4 to 2.5 and 0.3 to 3.5, respectively. Mean damage score ranged from 1.2 to 1.6 in both regions, which was positively correlated to the number of DBM on B. oleracea var. acephala. Hymenopteran parasitoids recovered from DBM included Cotesia plutellae, Diadegma semiclausum, D. mollipla, Apanteles sp., Chelonus sp., Oomyzus sokolowskii and Itoplectis sp., while the hyper parasitoids included Mesochorus sp., Pteromalus sp., Notanisomorphella sp., Eurytoma rosae and Eurytoma sp., Brachymeria sp. and Pediobius sp. Cotesia plutellae was the most abundant parasitoid followed by D. semiclausum. Diversity of parasitoids varied between months and study sites. Cotesia plutellae had established and spread beyond the release sites contributing between 40 and 90% parasitism while the indigenous parasitoids Itoplectis sp., Apanteles sp., and D. mollipla contributed less than 5% parasitism. The results show the spread and establishment of C. plutellae in the study sites. In conclusion, there is need for continuous training and educating the farmers to conserve the parasitoids that have contributed in the reduction of the DBM and damage on the crop. This has contributed in the reduction on pesticides use, spraying regime and eventual reduction of cost of production and residues on the produce.