Assessment of yield loss caused by the African white rice stem borer (maliarpha separatella rag (lepidoptera: pyralidae) at Mwea irrigation scheme, Kirinyaga county, Kenya
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Information on losses caused by the African white rice stem borer, Maliarpha separatella rag, which is an important rice pest in Kenya is scanty. The development and implementation of effective pest management strategies relies on accurately defined Economic Injury Levels (ElL) for that pest. Investigations were, therefore, conducted to determine yield losses caused by M. separatella and economic injury level of the pest. The experiment was conducted in an insect proof screen house at Kenya Agricultural and Livestock Research Organization, Mwea station. The experiment was arranged as a (2×6) factorial design and each treatment replicated three times. First factor was time of infestation at two levels, early and late which was 3 and 6 weeks after transplant date, respectively. Second factor was infestation rate at six levels (0, 1, 2 4, 6 and 8 egg masses). Results indicated that infestation levels of 1, 2, 4, 6 and 8 M. separatella egg masses at early infestation resulted in grain yield losses of 59.8, 83.2, 84.8 90.2 and 90.9%, while losses of same infestation levels at late infestation was 34.3, 52.1, 63.4, 81.8 and 80.8%. There was a strong positive relationship between yield loss and M. separatella population levels, (y = -0.214.29x+ 1693.4, r2 = 0.8416). On the basis of cost benefit ratio, the economic injury level was 6 and 8 egg masses per square meter in the early and late infestation, respectively. The action threshold for early infestation was 4 egg masses and 6 egg masses for late infestation.
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