Can sublethal pesticides exposure in honeybee colonies with subclinical infections by Paenibacillus larvae favour the development of American foulbrood in clinical form?

Stress factors may interact with additive or synergic effects and a multi-stress condition is the most probable cause for honeybee decline. We evaluated how the combination of different stresses (sublethal dose of pesticides, and electromagnetism) affect survival and make bee susceptible to Varroa attack and viral diseases. Experimental hives, from the same origin and health status, were equally and randomly placed on 5 April 2017 in 2 exposure sites (ES1-2) and in 1 control site (CS) The ESs were inside an experimental farm where chemical treatments are applied in orchards (ES1 sublethal exposures to pesticides), and where there is also a high-voltage electric line (ES2 chemicals as in 1 plus electromagnetism). The CS was far from agricultural fields and from human settlements. Clinical inspections and debris collection from the hives for mite checkup were weekly performed. Monthly, sugar treatments have been also made to check the mite infestation level. During a routine check in mid-June, unexpectedly, one case of American foulbrood (AFB) caused by P. larvae genotype ERIC II was diagnosed in ES1. In the following days AFB was diagnosed again in 1 colony in ES1 and in 2 colonies in ES2. No hive in CS resulted affected. After the diagnosis of AFB, sugar and debris previously harvested and stored have been examined for P. larvae detection (culture method). The sugar collected at the end of May in ES ranged from 174,000 to 5,000,000 CFU/g in diseased colonies and from 60 to 17,000 CFU/g in the asymptomatic ones. In CS it was <20 CFU/g (detection limit). The debris of three diseased colonies collected on 28 April showed already a high number of CFU. Since the bacteriological examination always underestimates the number of spores, we hypotheses that low contamination was present at the beginning of the trial in all the colonies. The presence of stressors in ES1-2 has probably favoured the development of the infection by sporadically giving rise to the AFB disease.