Effect of pollinators on sunflower seed yield helianthus annus l.) in Makueni district, eastern Kenya.
A field experiment was carried out twice in the long rains, 2004 and the short rains, 2005 In Makueni District, Eastern Kenya to establish the diversity of sunflower pollinators and their effect on the seed yield. Four treatments were used in a randomized complete block design with plots sizes of 4 x 4 m replicated four times. The treatments included, bagging at night and opening during the day for free aecessibity by daytime pollinators, bagging during the day and opening at night for free accessibility by nocturnal pollinators, bagging throughout the flowering period to exclude all pollinators, and no bagging for free access by all pollinators. The pollinators were collected and identified and the honeybees' diurnal behaviour noted. Head diameter, the number of seeds per head, weight of seeds per head, number of deformed seeds per head and the % oil content determined. A total of 14 insects species were observed visiting sunflower floral heads. Other visitors included, the nOI1- apis bees; Plebeina denoiti, Ceratina sp, Heriades sp, Pseudoantliidium sp, two (2) species of Diptera namely; Rhynchomydaea sp and Phytomia ill cisa , six (6) species of Lepidoptera; Danaus chrysippus, Belenois aurota, Junonia oenone, Byblia ilithyia. Junonia hierta, Cephonodes hylas and one (I) species of coleoptera, Merylis flavipes Apis mellifera was the most abundant and important sunflower pollinator recording the highest pollination efficiency index (7606). Peak foraging periods for the honeybees and the non-apis pollinators was observed between lOam and 12 pm .The length of visitation was not significantly different with time of the day. Number of seeds per head and seed weight per head increased by two to three times, and number of deformed seeds reduced by 35 % while the oil content or seeds per head increased by 66.7 % in the plots exposed to pollinators compared to the plots bagged to exclude pollinators. Two insecticides namely dimethoate ( Dimethoate 40 % EC) and L -cyhalothrin (Karate 1.75 % EC) were evaluated for their effect on sunflower pollinators and seed yield. Five treatments in a randomized complete block design with plot sizes of 4 x 4 m replicated thrice were used. The treatments were spraying in the morning and evening after flowering for each of the insecticides and a control (untreated). Numbers of foraging honeybees for five days alter treatment were noted. The head diameter, number of seeds per head, weight of seeds and the numbers of deformed seeds were recorded. Untreated plots gave higher numbers of seeds per head, seed weight per head and lower umber of deformed seed per head compared to the treated plots. However, the weight of seeds from Dimethoate and Karate treated plots were not significantly different. Insecticides application in the morning had higher number of 'deformed seeds per head compared to the evening applications. On average seed number per head was reduced by 52 %, seed weight per head by 38 % and the number of deformed seeds per head increased by 63 %. lt was evident that lower yield were attributed to reduced honeybee visits due to pesticide toxicity.