Observation of colony development and mortality in front of honeybee colonies and chemical residue analysis of dead bees

Abstract

This study presents the results of the monitoring of apiaries in the vicinity of fruit production areas in South Tyrol from spring to autumn (March-October) between 2017 and 2020. According to the results in previous studies regarding honey bee colony development, dead bees and contamination of pollen pellets with plant protection products harmful to bees we were interested in a more detailed analysis of dead bees accumulating in front of bee hives and the colony development. In this context, our investigations focused especially on colony development and mortality. At those points in time when mortality increased rapidly and abnormally, dead bees were analysed in a laboratory to search for plant protection product residues. In each apiary, increased mortality was observed whenever residues of at least one plant protection product harmful to bees was detected on the dead bees. Most of the peaks in mortality were observed in spring and the highest number of products harmful to bees were also found during this period. In spring, products primarily used for apple production and, to some extent, cherry production were detected. Residues originating from vineyards were identified only in autumn. At the same time, our investigations observed colony development and to our knowledge this is the first time such observations have been made from March to September in the area of concern. Comparing the results on colony development with those on mortality, we noted that a higher number of dead bees in front of the hives (expressed as a % of the colony strength), corresponded to fewer bees present in colonies in June, when the population generally peaked. However, no effect on colony development was observed due to the contamination of pollen pellets with plant protection products harmful to bees, based on the calculated daily pollen hazard quotient (PHQ).