Toxicity of Mimosa tenuiflora pollen to Africanized honey bees (Apis mellifera L.)

Background: Mimosa tenuiflora (Willd.) Poir. (Mimosaceae) is a xerophilous plant which is very common in degraded areas in the Brazilian semi-arid region, including areas of beekeeping. The ingestion of leaves from M. tenuiflora by ruminants is responsible for malformations. Chemical analyses of this plant revealed that it contains several secondary compounds including three triterpenoidal saponins, designated Mimonosides A-C, four flavones, five flavanones and four chalcones. Several plant species contain secondary compounds in nectar and pollen that could be toxic to pollinators, including bees. Experimental approaches are necessary to identify plant species that produce pollen that is toxic to bees. The present study aimed to determine the toxic potential of M. tenuiflora (Willd.) Poir. (Mimosaceae) pollen to Africanized honey bees (Apis mellifera L.). Materials, Methods & Results: The plant species utilized in this study was Mimosa tenuiflora (Willd.) Poir. (Mimosaceae) and a voucher specimen was deposited (no. 9591) at the Dardano de Andrade-Lima (MOSS) Herbarium, Universidade Federal Rural do Semi-Arido (UFERSA), Mossoro, RN, Brazil. Pollen samples were collected near Mossoro city, RN, in northeastern Brazil (5 degrees 11'15"S and 37 degrees 20'39"W). Honeycombs that contained pupae of africanized honey bees (Apis mellifera) were collected from the apiary of UFERSA. Newly emerged forager bees, identified on basis of their body size and coloration, were used for the experiment. All the bees used were of the same age. Groups of twenty bees were put into wooden boxes (11 x 11 x 7 cm). The boxes were kept in an acclimatized chamber (BOD) at 32 degrees C and 70% humidity. M. tenuiflora pollen was added to the food (5 parts of sugar mixed to one part of honey) at doses of 0 (control group; n=195), 2.5% (n=178), 5.0% (n=186) and 10.0% (n=186). The bees were observed daily until the last one died. Statistical analyses were performed using a statistical software (GraphPad Prism v. 4 for Mac). Median survival times, with 95% confidence intervals, were estimated using Kaplan-Meier survival analysis. Differences in the time distributions between groups were tested for statistical significance using the log-rank test. The median survival times of the bees were four days for all groups. Kaplan-Meier survival analysis revealed a significant difference (P=0.0041) between the survival curves, but the log-rank test did not show statistical difference (P=0.2825). The survival curve of controls differed (P=0.001) from the survival curve of the group fed 2.5% M. tenuiflora, but did not differ from the other groups. Discussion: Several plant species contain secondary compounds in nectar and pollen that could be toxic to animals that consume them. Bees collect large amounts of pollen grains very efficiently, making these grains generally unavailable for pollination. Thus, presence of secondary compounds in pollen grains could be a strategy designed to restrict the loss of pollen to bees. Even though it contains several secondary metabolites, the results of the present study indicate that the ingestion of the pollen of Mimosa tenuiflora by Africanized honey bees (Apis mellifera L.) at conditions of the present study did not promote toxic effect.