Plants and pollinators are mutually beneficial: plants provide nectar as a food source and in return their pollen is disseminated by pollinators such as honeybees. Some plants secrete chemicals to deter herbivores as a protective measure, among which is caffeine, a naturally occurring, bitter tasting, and pharmacologically active secondary compound. It can be found in low concentrations in the nectars of some plants and as such, when pollinators consume nectar, they also take in small amounts of caffeine. Whilst caffeine has been indicated as an antioxidant in both mammals and insects, the effect on insect immunity is unclear. In the present study, honeybees were treated with caffeine and the expression profiles of genes involved in immune responses were measured to evaluate the influence of caffeine on immunity. In addition, honeybees were infected with deformed wing virus (DWV) to study how caffeine affects their response against pathogens. Our results showed that caffeine can increase the expression of genes involved in immunity and reduce virus copy numbers, indicating that it has the potential to help honeybees fight against viral infection. The present study provides a valuable insight into the mechanism by which honeybees react to biotic stress and how caffeine can serve as a positive contributor, thus having a potential application in beekeeping.

Summary Baculoviruses Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) and Bombyx mori nucleopolyhedrovirus (BmNPV) have highly similar genome sequences but exhibit no overlap in their host range. After baculovirus infects nonpermissive larvae (e.g., AcMNPV infecting B. mori or BmNPV infecting Spodoptera litura ), we found that stored carbohydrates, including hemolymph trehalose and fat body glycogen, are rapidly transformed into glucose; enzymes involved in glycolysis and the TCA cycle are upregulated and produce more ATP; adenosine signaling that regulates glycolytic activity is also increased. Subsequently, phagocytosis in cellular immunity and the expression of genes involved in humoral immunity increase significantly. Moreover, inhibiting glycolysis and the expression of gloverins in nonpermissive hosts increased baculovirus infectivity, indicating that the stimulated energy production is designed to support the immune response against infection. Our study highlights that alteration of the host's carbohydrate metabolism is an important factor determining the host specificity of baculoviruses, in addition to viral factors.