Interactions between sublethal doses of thiamethoxam and Nosema ceranae in the honey bee, Apis mellifera

Both Nosema ceranae and insecticides are adversely affecting honey bees. How both factors affect gene expression and honey bee survival are well studied. However, only a few studies dealt with the interactions of the two factors. Here we studied the effects of both, when alone and in combination, on honey bee survival and expression changes of immune and detoxification genes. Newly emerged bees were randomly assigned four different treatments: control bees, bees exposed to thiamethoxam, bees exposed to thiamethoxam and then infected with Nosema, and bees infected with N.ceranae only. When combined with Nosema, thiamethoxam caused a significant reduction in survival from the control, but this reduction is not significantly different from N. ceranae alone. Exposure to thiamethoxam caused significantly increased expressions compared to the control for the genes of abaecin, apidaecin and hymenoptacin, but significantly decreased expression for the defensin gene. When the two factors were combined, the expression patterns were similar to N.ceranae infection alone (apidaecin, defencin and hymenoptaecin), except abaecin, which was increased compared to Nosema infected bees. For detoxification genesCCE8 and CYP315A1, thiamethoxam caused significantly increased gene expression but Nosema caused significantly decreased expression. When combined, the two factors did not show a further increase in Mortality compared to Nosema infection alone. Our study shows a clear combined effect of thiamethoxam and Nosema, but mainly in worker mortality and very little effect on gene expression. This study highlights the importance of observing mortality when studying the effect of insecticide on gene expression.