Effects of Insecticide Applications on Sunflower (Helianthus annuus L.) Pollination in Eastern Kenya
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Two insecticides (Dimethoate and Lambda cyhalothrin), commonly used in eastern Kenya to control sunflower pests, were assessed to determine their use-effect on the crop pollination and its subsequent yield Sunflower, cv Hybrid 8998, was planted in plots of 4x4 m in a Randomized Complete Block Design experiment The insecticides were applied at two levels: at pre- flowering period and flowering period. Another two levels of insecticide treatment involved timing of spray applications: in the morning and in the evening. Unsprayed were maintained as control. The experiment was done in two consecutive growing seasons in 2004 and 2005. The results show that there was a significant difference in the productivity of sunflower across the different treatments. Insecticide-treated plots had lower number of developed seeds per sunflower head than the unsprayed plots. Likewise, the number of seeds per head from plots that were sprayed at pre-flowering stage had significantly higher number of seeds than from plots sprayed while at flowering stage. The sunflower-head size and the seed weight per head had similar trend as the number of seeds. The role of pollination was confnmed in the number of observed bees on each plot Insecticide-treated plots had significantly lower number of foraging honeybees (Apis mellifera L.) compared with unsprayed plots. Among the insecticides, Dimethoatesprayed plots had the least number of bee foragers. The findings suggest that insecticide application on a blooming sunflower reduces the efficiency of bees to pollinate, which results to lower yields. It is suggested that farmers should manage sunflower pests at earlier growth stages and avoid pesticide use at flowering period but instead they should use other control options that are friendly to bees.