There is a great interest in finding sustainable alternative plant protection products to safeguard biodiversity and our ecosystem. Entomopathogenic nematodes (EPNs) have received much attention as an alternative biological-control agents to conventional synthetic agrochemicals. However, as nematodes are considered natural enemies, national authorities often approve commercial products based on limited or no data. Here we show for the first time that foliar exposure to a commercial EPN product can significantly reduce honeybee survival and that the nematodes can successfully replicate within the carcasses of adult bees. Following an adjusted EPA protocol for assessing the toxicity of chemical residues on foliage, we exposed newly emerged adult honeybees (Apis mellifera) and larvae of the greater wax moth (Galleria mellonella) for up to 96h to a low (0.25 Mio/m2) and medium (0.5 Mio/m2) field realistic concentration of Steinernema carpocapsae. Daily mortality was recorded over four days and nematode reproduction was assessed in all dead individuals. The date revealed that, S. carpocapsae exposure significantly reduced wax moth larvae survival (both p's < 0.001), resulting in an increased mortality rate of 80% across both treatment groups compared to controls. Likewise, both the low and high concentrations of S. carpocapsae significantly reduced honeybee survival (both p's < 0.001). However, the data revealed a dose-dependent effect wherein the high exposure lead to a significantly higher mortality rate (55%) than the low (43%) when compared to the controls. Nematode reproduction was significantly higher in the wax moths compared to the honeybees (p<0.001), yet no significant difference was observed between the low and high treatment groups for either species (p>0.56). Average nematode reproduction per individual wax moth and honeybee was 126'695 and 4'370, respectively; representing a 29-fold increase in the wax moth compared to honey bees. The data provide clear evidence that S. carpocapsae exposure can impose negative lethal effects as well as reproduce in honeybees. Considering the vast lack of data regarding potential adverse effects of EPNs on non-target pollinating insects, our results underline the urgent need to act cautiously when considering foliar application of EPNs on crops. Further research is required to adequately address the potential risk of EPNs to bees and other NTA`s pollinating insect species regarding foliar and soil application.
