Juvenile hormone enhances viral activity in bumble bees.

Bees are susceptible to viral infections that can compromise individual and colony health. As viruses depend on host cellular resources for replication, we investigated the influence of a key insect regulator, Juvenile hormone (JH), which enhances reproduction and metabolic rate on viral activity. We hypothesized that elevated JH levels would promote viral replication in a tissue-specific manner. To test this hypothesis, we used the bumble bee, Bombus terrestris, and the Israeli acute paralysis virus (IAPV). In our first experiment, we compared IAPV replication in pre- and post-diapause queens, which naturally have low and high JH levels, respectively. Post-diapause queens showed significantly increased viral loads in the fat body and ovaries, but not the brain, supporting our initial hypothesis. To establish causality, we treated pre-diapause queens with JH before infection. Queens that received JH exhibited substantially higher viral transcript levels in the fat body and ovaries compared to controls, confirming that JH enhances IAPV replication in these tissues. A third experiment examined whether supplemental JH could further boost viral replication in post-diapause queens already possessing high endogenous JH levels. No additional increase in viral titers was observed, indicating a "ceiling effect." Our findings demonstrate that IAPV replication is enhanced by JH, particularly in metabolically active tissues such as the fat body and ovaries. To investigate the involvement of the immune system in the effect, we measured the expression levels of three major immune genes in pre and post diapause queens. No changes in the immune gene expression were found, suggesting that the JH effect is not driven by changes in the immune system but rather other effects like increase in the metabolic rate. The research uncovers a critical link between host hormones and viral pathogenesis, offering new insights into disease progression in bees.