Evaluation of a Granulovirus for the management of diamondback moth, plutella xylostella L. (Lepidoptera: Plutellidae) on Kale at Kabete and Thika, Kenya
Ogutu, Walter Okello
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Diamondback moth (DBM) Plutella xylostella L. is a major pest of brassica crops such as kale and cabbage in Kenya. Farmers generally use inorganic chemical pesticides to manage this pest, a control method that not only is expensive and cause high risk to applicators, farmers and consumers, but have also been noted to be ineffective. There is therefore, a great need to develop alternative control methods in order to reduce the damage caused by this pest. The use of naturally occurring pathogens such as viruses, bacteria and fungi has been known to reduce pest populations. The current studies endeavoured to evaluate a naturally occurring baculovirus, which can be incorporated in the Integrated Pest Management (IPM) programmes for the management of DBM. Laboratory and field studies were conducted at CABI-ARC (ICRAF Complex) and at two Kenya Agricultural Research Institutes (KARI) centres, the National Agricultural Research Laboratories, Kabete (1o 15´ S, 36o 46' E), and the National Horticultural Research Centre, Thika (Oo, 59' S, 37o 05' E), respectively. These studies assessed the persistence of the Plutella xylostella granulovirus (PlxyGV) inoculum in the field, the effect of adjuvants on the efficacy of the PlxyGV inoculum, chemical insecticide mixtures and application techniques used to improve the efficacy of PlxyGV against DBM. Field experiments were conducted for three seasons, the 2001 long rains (April-September), the 2001/2002 short rains (October 2001- January 2002), and the 2002 long rains (January-April). One month old kale seedlings were transplanted in the experimental plots at each site then subjected to the normal agronomic practices such as fertilization with DAP and CAN at planting and 2 weeks and 8 weeks after transplanting respectively, weeding and harvesting. Whole plant, bioassay method was used to assess the persistence of PlxyGV in the field. Leaf-dip and leaf-disc bioassay methods were used to compare the effect of different adjuvants and a synthetic chemical insecticide, which is specific to aphids (Pirimor®) on the efficacy of PlxyGV against DBM larvae in the laboratory. DBM infection levels by PlxyGV as high as 90% were observed immediately after the spray, but reduced to less than 40% on the plants exposed to sunlight after 1 day. Molasses and neem formulation increased the efficacy of an aqueous suspension of PlxyGV against DBM by 100 fold in the laboratory and also gave a yield up to 40% higher than unformulated virus in the field. In the laboratory, a mixture of PlxyGV inoculum and Pirimor® caused a 30% reduction in the efficacy of PlxyGV against DBM, whereas in the field this effect was not observed. Spray application using a V-shaped lance was found to give significantly higher infection of DBM than conventional lance, but did not affect the final marketable yield of kale. PlxyGV was not seen to infect other lepidopteran pests or any beneficials. These results show promising potential for the use of PlxyGV on the management of DBM, but conclusive recommendations cannot be made at this stage, because of the low numbers of DBM in the field during the study period. Further research is required including studies to develop formulations and to improve shelf life, persistence and optimise infectivity of PlxyGV.
- MST-Zoological Sciences