An inert pesticide adjuvant synergizes viral pathogenicity and mortality in honey bee larvae

Authors: FINE, JULIA - Pennsylvania State University; Cox-Foster, Diana; MULLIN, CHRISTOPHER - Pennsylvania State University

Submitted to: Scientific Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/7/2016
Publication Date: 1/16/2017
Citation: Fine, J.D., Cox-Foster, D.L., Mullin, C.A. 2017. An inert pesticide adjuvant synergizes viral pathogenicity and mortality in honey bee larvae. Scientific Reports. doi: 10.1038/srep40499.

Interpretive Summary: Honey bees are highly valued for their pollination services in agricultural settings, and recent declines in managed populations have caused concern. Recent colony losses following almond pollination have been characterized by brood mortality and eventual colony loss weeks later. In this study, we demonstrate that these symptoms can be produced by chronically exposing brood to an organosilicone surfactant adjuvant (OSS) commonly used on almonds and to exogenous viral pathogens, simulating a horizontal transmission event. Organosilicones are widely used in many cropping systems and in industrial applications. In almonds, the use of OSS has increased significantly since 2003 with OSS become a commonly applied compound, used in tank mixing for pesticide sprays. In our study, we found that the level of viral infections increased significantly when larvae were fed OSS after being exposed to viruses. This increase was accompanied by a decrease in an immune gene needed for anti-viral immunity. The virus that was primarily affected was Black Queen Cell Virus and this virus is widely distributed throughout the United States and found in other pollinator species. Our study identifies for the first time OSS as a factor in bee mortality and opens the way for additional research towards understanding its impact on colonies and bee health.

Technical Abstract: Honey bees are highly valued for their pollination services in agricultural settings, and recent declines in managed populations have caused concern. Colony losses following a major pollination event in the United States, almond pollination, have been characterized by brood mortality with specific symptoms, followed by eventual colony loss weeks later. In this study, we demonstrate that these symptoms can be produced by chronically exposing brood to both an organosilicone surfactant adjuvant (OSS) commonly used on many agricultural crops including almonds and exogenous viral pathogens by simulating a horizontal transmission event. Using an in vitro rearing method, honey bee larvae were grafted into 96 well plates and randomly assigned to treatments. Treatments included controls (fed normal artificial diet), OSS treatment (fed artificial diet plus a field relevant exposure to Organosilicone adjuvant), Virus treatment (low level virus added on first day to diet), and OSS + Virus treatment (viral exposure plus OSS in diet). Multiple replicates were repeated over time from bees taken from three colonies. Survival, symptoms at death, and development time were measured for each larva through adult emergence. On day 6 of larval rearing just prior to pupation, larvae were randomly selected from each treatment in each replicate and frozen for molecular measures of viral infections, select representative immune genes, and actin. Observed synergistic mortality occurred during the larval-pupal molt. Using Q-PCR techniques to measure gene expression and viral levels in larvae taken just prior to observed mortality at metamorphosis, we found that exposure to OSS and exogenous virus resulted in significantly heightened Black Queen Cell Virus (BQCV) titers and lower expression of a Toll 7-like-receptor associated with autophagic viral defense (Am18w). These results demonstrate that organosilicone spray adjuvants that are considered biologically inert are toxic to honey bee larvae in combination with viral exposure, and guidelines for OSS use may be warranted.