| Alfalfa(Medicago sativa)is one of the most important leguminosae forage in the world. It is hermaphroditic flower and belongs to the typical allogamy,so its seed production maily depends on the process of insects pollinating.To study the relationship between Medicago sativa and its pollinators is one of the important content of pollination biology of leguminosae plants,and it is also valuable for alfalfa seed production based on the exploration the mechanisms of the reproductive traits of alfalfa.In this paper,we investigated the biodiversity of pollinating insects in Hexi corridor,Northwest China,the principle pollinators were selected for further studies on the relationship between environmental factors and floral syndrome,respectively.The results are as below:1)Visiting insects and foraging behaviors of principle pollinators were studied in Zhangye station,Hexi corridor.There were insect visitors from 22 insect taxa that included 4 orders and 11 families.Five native bee species Megachile abluta,Megachile spissula, Xylocopa valga,Andrena parvula and Anthophora melanognatha were principal pollinators.The diurnal foraging activities of these 5 wild bee species of alfalfa showed that the diurnal foraging activities of M.abluta,M.spissula and X.valga displayed the unimodal type with diurnal foraging activities on 11:30-15:30,while A.parvula and A. melanognatha were instead of the double unimodal type with diurnal foraging activities on 9:30-11:30 and 16:30-18:30,respectively.The results above indicated a complementary relationship among them.The tripping frequency and landing times showed significant difference among the principle pollinators,and foraging behaviors were correlated with their body sizes.2)In order to discover the effects of flower-visitor's contribution to pollination,several field experiments were designed to examine the diurnal movement patterns of wild bee species and their inter-specific relationship with environmental factors in Hexi corridor, Northwest China.The correlation analysis between the wild bees and environmental factors indicated that diurnal foraging activities of pollinators significantly correlated with environmental factors,and the unimodal type bees(M.abluta,M.spissula and X.valga) showed a linear model,while the double unimodal type bees(A.parvula and A. melanognatha)showed a parallel quadratic model.The effects of these environmetal factors on foraging behavior of the unimodal type bees were temperature(r = 0.87,P < 0.01)> light intensity(r =0.86,P < 0.01)> relative humidity(r = 0.82,P < 0.01),while the effects on the double unimodal type bees were light intensity(r =0.92,P < 0.05)> relative humidity(r =0.9,P < 0.05)> temperature(r = 0.89,P < 0.05).The results above indicated that the foraging behavior were related to the environmental factors,one is due to the body size and the energy consumed of the of these pollinators,and another possible explanation is that the dynamic changes of the rewards providred by alfalfa under different environmental conditions.3)The floral syndrome of 9 varieties belong to Medicago spp was analyzed,which including the numbers of primary branch,the diameter of calyx,the length of corolla,the numbers of florets/rachis and the numbers of pollens/flore,and the results showed that there was significant difference of these reproductive characters among 9 varieties(P < 0.01).The correlation analysis between the numbers of wild bee visiting flowers and the floral syndrome indicated that foraging behavior significantly correlated with calyx diameter,calyx length,corolla length and weight of floret(P < 0.05),which led to the differences of pod rate among 9 populations of Medicago spp.The correlation between the rewards(pollen and nectar)and the numbers visiting alfalfa flowers were analyzed too.It found that numbers of pollen/floret was positively correlated with numbers of wild bees (Pearson correlation coefficient is 0.438**),while the nectar was positively correlated as well(Pearson correlation coefficient is 0.314*).The consistence between diurnal secretion of nectar per floret of 9 Medicago spp.and diurnal foraging activities of wild bees indicated that the rewards somewhat influenced the foraging behavior of their pollinators. The different tripping frequency observed in 9 Medicago spp.varieties with different color also indicated that color was another factor affecting visitation.Therefore,it is important to investigate the mechanisms between floral morphological variations in Medicago spp.and the foraging behaviors of their pollinators,and this may provide insight for selecting breeding materials to improving the seed set.All in all,this study indicated that solitary wild bees such as M.latreille,M.spissula,X. valga,A.parvula and A.melanognatha are the potential pollinators in Hexi Corridor.And an understanding of the environmental factors and biological traits that affect the behaviors of different wild bees foraging alfalfa are basic to the utilization of solitary wild bees in a practical way for the increased pollination,or more consistent pollination of alfalfa seed crops.
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