Effect Of Insecticide Use On Sunflower (helianthus Annus L.) Pollination And Yield, In Makueni District, Eastern Kenya
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Sunflower variety 8998 and insecticides; Dimethoate and Karate were used to evaluate the effect of insecticide on pollinators visit and consequently on sunflower yield. Effect of time of spray on seed set was also evaluated. Dimethoate and Karate were sprayed at the recommended rates in the morning and evening before and after flowering and the number of dead honeybees and foraging honeybees were noted for each treatment for the first five days. At harvesting, the head diameter, seed number, seed weight, number of deformed seed were noted. Dimethoate treated plots was not significantly different from Karate treatments and gave lower seed number, seed weight, and head diameter with higher number of deformed seeds than did plots where no spray was used. Morning before flowering treatment and evening before flowering treatment were not significantly different from plots with control - no application treatments. After flowering applications in the morning and evening were significantly different and gave higher seed number weight, head diameter compared to evening applications. There was an average of 36.45 % increase in seed number, 33.4% increase in seed weight and 46 % reduction in number of deformed seeds when control - no application was used. In addition, an increase of 20 % in seed number, 8.4 % in seed weight and reduction of 20.6 % in number of deformed seeds were realized in the evening insecticide application compared to morning applications. Before flowering insecticide applications had 31.4 % increase in seed number, 19.3 % increase in seed weight and 33.8 % reduction in number of deformed seeds when compared to after flowering applications in the long and short rains. This yield reduction maybe as a result of pesticide toxicity on honeybees and that yield gain from pollinator activity may often offset the loss from seed feeders.